Copyright by Louis Burrell Carrick

Home , Cyclops (copepod), Epidermis (zoology), Hydra oligactis, Out of Water, Paul Schulze, Paul Schulze (zoologist)

1957 A STUDY OP HYDRAS IN LAKE ERIE Contribution toward a Natural History of the Great Lakes Hydridae

DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of philosophy in the Graduate School of The Ohio State University

LOUIS BURRELL CARRICK, B. A., M. S.

•SKHHKHS-

The Ohio State University 19 £6

Approved by:

Advisor / Department of Zoology and Entomology TABLE OP CONTENTS

INTRODUCTION ...... 1 I. PROBLEMS OP TAXONOMY ...... 7

1. CRITERIA FOR THE GE N E R A ...... 7

2. METHODS USED FOR SPECIES DETERMINATION .... 11

3. IDENTIFICATION OP THE. LAKE ERIE SPECIES . . . 16 (a) Hydra ollgactis Pallas, 1776 ...... 17

(b) Hydra pseudollgaotis (Hyman, 1931) .... 20 (c) Hydra amerlcana Hyman, 1929 21

(d) Hydra littoralls Hyman, 1931 ...... 23 (e) Hydra came a L. Agassiz, l8jp0 .... 2I4.

II. HABITATS AND DISTRIBUTION IN THE GREAT LAKES . . 27

1. ABIOTIC HABITAT FACTORS AFFECTING DISTRIBUTION ...... 29 2. DISCUSSION OF HABITATS AND DISTRIBUTION- RECORDS ...... 32 (a) Aggregations on N e t s ...... 34

(b) occurrence in the Plankton ...... 47

(d) Vegetation Zones ...... 54

(e) Lake Erie Island Ponds ...... 60 (f) Wave-swept S h o r e s ...... 65

- ii - III. COMMUNITY INTERACTIONS WITH SPECIAL REFERENCE TO H. LITTORALIS...... 67 1. METHODS USED IN THE STUDY A R E A ...... 69

2. SEASONAL ABUNDANCE ...... 8l (a) The Annual C y c l e ...... 86 (b) Reproductive Potential under Culture C o n d i t i o n s ...... 95

3. CHANGING AGGREGATIONS IN THE MICROHABITATS. . 114). (a) The Swift-Water Community of the Block Rubble ...... 11+6

(b) The MyriophyHum-Leaf Community..... 153 (c) Hydras as Epizoites of Molluscs..... 156

i+. FOOD-CHAIN RELATIONS ...... 161+

(a) Predators of H y d r a ...... 165 (b) Feeding Reactions and Availability of P r e y ...... 171

5. PARASITES AND COMMENSALS ...... 185 (a) Amoebic Infestations ...... 186

(b) Ciliate Commensals ...... 192

6 . THE HYDRA N U I S A N C E ...... 212 (a) Destruction of Fish F r y ...... 215 (b) Injury of Fishermen’s S k i n ...... 22k

- iii - IV. SEXUAL REPRODUCTION ...... 227

1. SEXUAL PERIODS, GONAD DEVELOPMENT, AND SEX R A T I O S ...... 228 (a) Sexuality in H. littoralis...... 229

(b) Hermaphroditism in H. am e r i c a n a ...... 2I4.O (c) H. oligactis and H. pseudoligactis Compared ...... 250

2. INFLUENCE OF EXTERNAL FACTORS ON GONAD FORMATION ...... 257 3. SURVIVAL, DISPERSAL, AND HATCHING OF THE EGGS ...... 27k

k» SPECIATION PROBLEMS ...... 289

LITERATURE CITED ...... 295

APPENDIX: TABLES I- I X ...... 308

AUTOBIOGRAPHY ...... 318

- iv - LIST OP PIGORES

1. The four types of hydra neraatocysts drawn to scale from Lake Erie sexual specimens...... 15

2 . Hap showing location of collecting stations in Fishery B a y ...... 71

3. Photograph showing slide-rack and stone-anchor collecting rig exposed on bottom at Oak Point bar, station 1|., during seiche of December 10, 1953 ...... 75 Ij.. Photograph of station 1 location between the two large dolomite rocks (at left) split off from Gibraltar Island cliff face ...... 75

5. Diagram of the annual hydra cycle in western Lake E r i e ...... 93 6 . Photographs of the cladoceran Anchistropus minor parasitic on h y d r a s ...... 203

7. Photograph of hydras attacking fry of the Erie whitefish Coregonus clupeaformis latus .... 218

8 . photograph of hydras attached to eggs of the yellow pikeperch Stizostedion vitreum vitreum and stinging newly hatched fry ...... 221

- V - LIST OP TABLES

(APPENDIX)

I. Mean sizes in microns of the species of Hydra from Lake E r i e ...... 309

II. Nematocyst measurements in microns from L. H. Hyman’s descriptions of species of H y d r a ...... 310

III. Mean numbers of hydras per square meter of rubble bottom, Gibraltar Island shore, Fishery Bay, 1951-1952, as determined by Britt’s concrete block-chemical bath sampling m e t h o d ...... 311

IV. Summary of 1952 seasonal data from stone- anchor collections: numbers of hydras (H. littoralls) colonizing'stone anchor at rubble-bottom stations (la, lb, lc, 2 , 3 , Ij., 6 ), Fishery B a y ...... 312

V. Summary of 1952 seasonal data from slide-rack collections: numbers of hydras (H. littoralis) colonizing 18 slides at rubble-hottom stations (la, lb, lc, 2, 3, Ij., 6 ), Fishery B a y ...... 313 VI. Summary of 1953 seasonal data from slide-rack collections: numbers of hydras (H. littoralis) colonizing 18 slides at rubble-bottom' stations (1, 2, 3, I4.), Fishery B a y ...... 31I4.

VII. Summary of 195^ seasonal data from slide-rack collections: numbers of hydras (H. littoralis) colonizing 18 slides at rubble-bottom stations (1, 2, 3, It-, 6 ), Fishery B a y ...... 3l£

VIII. Number of buds per hydra during spring pulse as determined by counts from samples of 50 specimens from mixed H. oligactis-H. pseudoligaotis population at station 5 ...... 316 IX. Occurrence of gonadal individuals of H. littor alis in collections from slide-rack and stone- anchor rigs made during sexual period, 1952, Fishery B a y ...... 317

- vi - INTRODUCTION

That huge aggregations of hydras occur in Lake Erie has been known for over a quarter of a century. Attention of biologists was directed, to this phenomenon by Wilbert A. Clemens (1922), who reported the observations he made during the summer of 1920 while staying at a pound-net fishery located on the north shore of Lake Erie midway between Pelee Point and Rondeau Harbor.

A conception of the extent of the hydra settlement on the twenty pound nets, which were operated in strings of five spaced for about nine miles along the shore, is best gained from Clemens' own words:

All of the nets when lifted, in late July and early August were loaded with a; very conspicuous brownish-orange growth in addition to the bright green algal growths. At first sight diatomaceous ooze or a bacterial production was suggested but microscopic examination showed it to be composed of innumerable living Hydras* The nets were lifted into the characteristic flat-bottomed pound-net boats and brought to the dock. The boats were anchored 100 to l£0 yards from the dock and the nets dragged through the water to oars on the dock in order to wash off some of the loose material, especially mud. In addi tion to the mud many Hydras were washed off and these gave to the water a brownish-orange color quite dis tinct from the lighter color of the mud. The bottoms, seats, etc., of the boats were covered with Hydras to the depth of from l/8 to l/lf inches and a quart jar was quickly filled simply by running a hand along the seats. A fisherman eight miles to the west and another seven miles to the east reported Hydra in apparently equal abundance. This means a distribution of at least fifteen miles along this part of the shore. The beach is sandy to gravelly with some large stones. Very - 1 - little life was found on the bottom out as far as one could wade. However, out beyond the region of strong wave action there must be places of attachment for the Hydras other than the nets In order to account for the existence of the species from one fishing season to another, since in 1920 they had not reached sexual maturity by the first week in December when the nets were removed for the season.

The immense population manifesting Itself on the nets appears to have been composed primarily of individuals belonging to the species Hydra ollgactis Pallas. In the absence of gonads, however, no absolute determination could be made by Professor Prank Smith to whom Clemens submitted specimens. Smith at the time, had provided the first synop sis of American hydras in Ward and Whipple's "Fresh-Water

Biology" (Smith, 1918). Even without benefit of Paul Schulze's monograph on the genus Hydra (1917), he was un doubtedly able to recognize the species so often misidenti- fied by other American workers. Consequently, I am accept ing Clemens' report of the occurrence of H. ollgactis as the only reliable species record for Lake Erie published when the present investigation was undertaken in 1951. A primary objective of the study has been to establish the identity of the hydras inhabiting western Lake Erie, the

"key area" (Langlois, 191*8) in this most productive of the

Great Lakes. Working year,-round from the summer of 1951 through the summer of 1951* at Ohio State university's Franz

Theodore Stone Laboratory at South Bass Island, I have been able to observe the life histories of the local hydras and obtain specimens in the sexual state. A study of the taxonomy of these hydras has made definitive determinations of the species possible. It can be reported, therefore, that at least four species of the known hydras live in the waters of Lake Erie. These are:

Hydra ollgactis Pallas, 1766.

Hydra pseudoligactls (Hyman, 1931).

Hydra americana Hyman, 1929. Hydra llttoralls Hyman# 1931. The green hydra, Chlorohydra virldisslma (Pallas, 1766), was not collected in Lake Erie proper. It was found along with Hydra carnea L. Agassiz, l8£>0 in a pond on Pelee Island; also in Fischer's Pond on Middle Bass Island.

Undescribed species of hydras may exist in Lake Erie as the extent of our collections in so large a body of water was necessarily limited.

One other coelenterate was found during the closing period of the investigation - the rarely encountered hydrold stage of the fresh-water jellyfish, Craspedaousta sowerbil

Lankester, 1880. Neither medusae nor polyps have been pre viously collected in the Great Lakes, but it appears their hydroid generation, commonly referred to in the literature as Microhydra ryderl, has established itself in the region of the Bass Islands. Observations of this form will be reported in a separate paper.

Finding where the species of hydras live in the waters of Lake Erie has been a second objective of this study.

- 3 - The habitats and the distribution of the species are dis cussed in the second part of this report. Records of hydras occurring in the other Great Lakes are appraised.

No pretense at analysis of the dynamics of hydra popu lations in so extensive an ecosystem as Lake Erie is i n tended in this study. Quantitative data accrued from yearly collections in a restricted area are presented as an indica- ( tion of seasonal abundance. Both the asexual and sexual reproductive phases of hydra populations have been compre hensively treated, however; and the related literature has been subjected to extensive review.

An attempt is made at reconstimeting the nature of the microcommunities in which the hydras live. Some observa tions on food-chain relations, role as prey and host, preda tory action on fish fry, nuisance to commercial fishermen, are offered toward a solution of the central problem in the ecology of hydras — the position of these organisms in the biotic community (i.e., the ecological niche in the Eltonian sense).

Execution of the field and laboratory work Incident to the investigation has been essentially a one-man operation.

The frequent collections in the selected study area of

Fishery Bay at South Bass Island were made from a row boat, or through the ice when the lake froze. Collections in the open waters were made from the Laboratory power-boats, "Bio- lab" or "Gibraltar," with the assistance of their skippers,

-k- Mr. Roy Thompson and Mr. Paul Webster. Collections of hydras from the string of pound nets at Pelee Island were made

possible through the cooperation of Mr. William Lamb, owner

and operator of the fishery. Especially appreciated are the many services of Mr* Ernest Miller, superintendent of the

Ohio State Fish Hatchery, during conduct of experiments

dealing with effects of hydras on fish fry. The facilities

of the Cranbrook Institute of Science were made available to me through the director, Dr. Robert T. Hatt, during the last year of the work when collections of hydras from Lake St.

Clair were started.

Among ray colleagues at the Laboratory, I am most in debted to Dr. N. Wilson Britt. His practical knowledge as resident limnologist and entomologist was Invaluable in many ways: at the start of the field work he made data accessi ble by permitting me to collect from his concrete-block sta tions with him; he took the photographs and made the prints of Anchistropus minor Birge parasitic on hydras which are used as illustrations herein. For photographs of hydras attacking fish fry and for data from his otter-trawl collec tions I thank Dr. Edward C. Kinney.

Dr* T. H. Langlois, resident director of the Laboratory during the period of the study, suggested the investigation and served as ray academic advisor. I wish to express my appreciation for counsel and encouragement I have received from Professor Langlois while completing this monograph,

- 5 - which it is hoped will stimulate research on the ecology of the Hydridae in the Great Lakes. Also, 1 want to thank Dr.

Clarence E* Taft, Professor of Botany, for checking this manuscript for accuracy of the botanical information, and

Dr* N. Wilson Britt for checking it on matters pertaining to his field of specialty; their editorial criticisms have been very much appreciated. Finally, I am grateful to Dr. Libbie Hyman, of the American Museum of Natural History; without her technical guidance I would never have succeeded in arriving at definitive determinations of the hydras of Lake Erie.

- 6 - PART I

PROBLEMS OP TAXONOMY

The species of hydras found In Lake Erie represent four of the ten species of the genus Hydra thus far known to occur

In North America (Hyman, 1931b, 1938} Hadley and Forrest,

191*9). The taxonomy of American species has been so completely and clearly presented by Hyman In the series of studies cited in the bibliography that only a few points bearing on problems of identification and recent changes in nomencla ture need be dealt with here. Careful consideration is given to matters of taxonomic detail, however, in view of the tendency still prevalent among field and laboratory workers to slight the fundamental question of the identity of the species.

CRITERIA FOR THE GENERA

Prior to the publication of Schulze»s monograph (1917), all fresh-water tentacled Hydrldae were grouped into a sin gle genus Hydra. Schultz proposed splitting the hydras into three genera. The essence of Schulze's criteria are summa rized as follows:

Chlorohydra: symbiotic zooehlorellae present in the gastrodermis, making the body color characteristically green; embryonic theca with polygonal, spineless plates. - 7 - pelmatohydra: body with a distinct stalk; only two tentacles arise simultaneously at first on the bud; buds in a spiral arrangement ascending from the boundary of the stalk with the stouter distal part of the column.

Hydra: body without a distinct stalk differentiating the column into a slender proximal region and a stout distal region; more than two tentacles always arise simultaneously on the buds (i.e*, the origin of the first two tentacles is successive); buds positioned alternately or in whorls. Schulze's classification of the hydras gained rapid recognition among European systematists; but it was not until after Hyman (1929) had acquainted American workers with Schulze's work that the revised nomenclature was adopted in this country. Hyman (1929, p.2ij3; 1930, p. 322; 1938, p. 1) has always been a little dubious about the validity of the genus Pelmatohydra P. Sch. In a personal communication

(19S>2), she says: ” ... you really can't tell when a hydra has a sufficiently distinct stalk to justify putting it in Pelmatohydra. There is no doubt that ollgactis and pseudoligactls look very much alike and they have a general appear ance different from our other species, but putting this dif ference into a generic diagnosis appears impractical."

In a review of the Hydridae, Ewer (I9I4.8 ) presents evi dence that the genus pelmatohydra erected by Schulze (1917)

Is not valid and that the genus Hydra should be extended to include the forms previously classified as pelmatohydra.

- 8 - It will be noted from the designation of the Lake Erie spe cies that I am in complete agreement with Ewer. His evi dence Is convincing. He reviews the characters of all described species, points out that workers with hydras have used only the first of Schulze's characters (I.e., the pres ence or absence of a stalk) to differentiate the species, and that Schulze's other two characters are not valid. In discussing the first character, he mentions that Hyman (1938) in naming Hydra cauliculata, although classifying it in the genus Hydra, describes it as possessing a slender stalk. With respect to the second character, he cites Inherent taxonomic contradictions among North American species as follows: H. oregona, Griffin and Peters, 1939, which has no stalk, but whose buds are born spirally In the manner de scribed by Schulze as characteristic for pelmatohydra;

H. carnea, L. Agassiz, and H. littoralls, (Hyman, 1931a;

1931b) are both stalkless, whereas the tentacles on the buds arise successively. In advocating suppression of the genus

Pelmatohydra, Ewer summarizes the problem as follows (pp. 231-232):

The possession of a stalk is thus a variable character, and intermediates exist between fully stalked and stalkless forms. In addition, all the other characters used to distinguish species occur in both stalked and stalkless forms (see Table I). The stalked species cannot therefore justifiably be placed in a separate genus, and should be included in the genus Hydra. The genus Hydra, thus extended to include the stalked forms, is then distinguished from Chlorohydra by two characters: (1) symbiotic - 9 - zooF^uorellae are absent from the genus Hydra, but oii&sent in Chlorohydra. (2) In Hydra the ein.bryoth.eca be either spined or smooth, but never consists of jt polygonal plates; in Chlorohydra the embryotheca is jr composed of polygonal plates, and Is not spined.

There appear to be no questions concerning the validity of the genus Chlorohydra P. Schulze. Only one species, C. virldissima (Pallas, 1776), has been described so far.

Tbis species, commonly known as the green hydra, is cosmo politan, and is undoubtedly that used by Trembley (17kb) in his pioneer regeneration experiments. Since naturally occurring symbiosis with zoochlorellae is unknown in any of the other hydras, it can also safely be assumed that subse quent experimentalists, who designated the animals they used by the synonyms H. vlridis and H. virldissima, were working with one and the same species.

Identification of species belonging to the genus Hydra, on the other hand, is admittedly difficult. Hydras have been studied properly only in relatively recent years. Error and confusion, resulting from inaccurate determina tions by experimentalists and unwarranted naming of new species by systematists, makes much of the older literature of dubious value (Hyman, 1929; Ewer, 191+8).

- 10 - METHODS USED FOR SPECIES DETERMINATION As Hyman (1929; personal communications, 1952) has emphasized, a hydra can be identified to species only by an ensemble of characters. These Include: the shape of live healthy specimens, the length of tentacles relative to the column, the attitudes in which the tentacles are held, the order of formation of the tentacles on buds, whether the animals are hermaphroditic or dioecious, the shape of testes and the form of the embryonic theca, the sizes of the nematocysts and the manner of coiling of the thread inside the holotrichous isorhizas (large glutinants).

Measurements and examination of the nematocysts are made under oil immersion from hydras squashed in their own cul ture water.

All these characters except the theca have to be ascer tained from fresh material. Fixation distorts the shape of the testes. Sexual specimens are necessary in most cases to complete the diagnosis.

It is obvious from the foregoing that a specialist is not in a position to make determinations for field or lab oratory workers unless he has facilities and time for cul turing hydras. Living hydras, moreover, often become

"depressed" while in transit. Preserved material is worth less. Trying to study nematocysts in the hydras or any of the other Cnldaria in macerated preparations, as Weill says

- 11 - (1934, pp.10-11), Is Impossible. The Interior of the Intact nematocysts and the detail of the exploded mechanism are ruined. Nevertheless, the use of dissociation fluids for the examination of nematocyst structure is still recommended in current treatises (pennak, 1953* P« 109).

I made all observations on fresh material. The cul turing methods described by Hyman (1930, 1941) were effec tive in the cases of C. virldissima, H. carnea, H. amerlcana,

H. ollgactis, and H. pseudoligactls. I had little success, however, in getting clone cultures of H. littoralls from individuals taken from Lake Erie. Hyman (1931b) mentions difficulty in cultivating H. llttoralls from the swift-water habitats of the type locality, and did not obtain thecated embryos for completion of the species description until several years later (Hyman, 1938).

Specimens of C. virldissima and H. carnea became sexual in battery jar cultures at room temparatures. H. ollgactis and H. pseudoligactls cultures sometimes produced sexual

Individuals when the temperature was gradually lowered to

5° to 7°0. Again, sexual individuals would suddenly appear in a culture kept at room temperature. Like other workers with hydras, I can offer no explanation for these occurrences.

Completion of diagnoses of H. llttoralls and H. amerl cana was made from male and female individuals collected in the lake. After studying the sexual organs, such individuals were squashed for nematocyst examination, or tentacles were

- 12 - amputated and examined for nematocyst structure. In this way, comparison of the nematocysts of the gonadal Individ uals with the agonadal Individuals could he made, and defi nite diagnoses reached for the local species.

Some of the structures of the nematocysts are so small that they lie at the limits of visibility. It Is necessary, therefore, to obtain the best optical conditions possible.

Koehler or critical illumination is essential for proper definition to determine the form and shape of the thread (i.e., the tube) in the undischarged holotrichous isorhizas.

I used a research illuminator and a standard binocular com pound microscope with 12.£x compensating oculars, providing good definition and a magnification of 12l£.£ diameters under oil. immersion.

Measurements of the major and minor axes of the undis charged nematocysts were made with an ocular micrometer to the nearest micron. All four types of nematocysts were measured in fresh squashes under oil immersion. Measurements were made from at least thirty different individuals of a species. Mean values of the major axes for each type of nematocyst in these individuals were computed. Mean size and range of the holotrichous isorhizas and stenoteles based upon measurements from several different individuals are especially valuable in differential diagnosis. Consequently, thirty each of these types were measured per individual, whereas only fifteen of the other two nematocyst types were

- 13 - measured. In an effort to get an unbiased sample of the nematocyBt population in an individual, measurements were taken from different areas of the squash, and, in a traverse of a field, only those Images which accidentally coincided with the calibration markings of the micrometer were meas ured. Means computed from averages of major axes measure ments are given in Table I. Ranges of nematocyst size in specimens scaled are also given for comparison with Hyman’s ranges compiled in Table II from her species descriptions. The four types of nematocysts found in all species of hydras are designated according to Weill's classification

(193i|-, PP* 37-38) as: holotrichous isorhizas (large or streptoline glutinants of Paul Schulze and the German workers); atrichous Isorhizas (small or stereoline gluti nants); stenoteles (penetrants); and desmonemes (volvents). The four kinds of nematocysts, drawn to scale from sexual specimens of Lake Erie hydras, are shown undischarged in

Figure 1. Excellent illustrations of the discharged hydra nematocysts are to be found In Toppe (1910) and Schulze

(1917,1922b). Weill's monograph contains 208 figures, illus trating the detailed structure of nematocysts based upon a study In 113 species belonging to all groups of the Cnldaria.

Hyman's treatise (19l|0, pp. 382-392) contains a lucid expo sition of nematocyst structure and function. Attention Is called to an obvious error in Pennak's treatise (1953, Fig.

59A, p. 102) where the tube in the undischarged atrichous

- Ik - Fig* 1. The four types of hydra nematocysts drawn to scale from sexual Lake Erie specimens: A and B» stenoteles of H. ollgactis and A., amarloans: C, D, and E, holotrichous Isorhizas of H. ollgactis. H. llttoralls. and H. amerlcana: F, atrichous Isorhlza of H. carnea: G, desmoneme of H. pseudoligactls. - 1 5 - isorhlza is shown with stout transversely coiled loops in stead of the delicate, longitudinally-wound loops charac teristic of this type of nematocyst.

IDENTIFICATION OF THE LAKE ERIE SPECIES No key to the local species is offered here. A well- illustrated key covering all North American species of the fresh-water Hydrozoa which have been adequately described in the literature is now readily accessible (pennak, 19!>3»

Ch. !(.). I found it necessary, however, to make constant reference to the papers of Hyman and other authors of

American species. Ewer’s review of the Hydridae (19lj.8) is also valuable. It contains summaries of descriptions of the known species.

In the following resume of the local species, emphasis is placed upon nematocyst characters as a means of making provisional identifications. The species become sexual in the fall in Lake Erie (see Pt. IV, Sect. 1); but few males and even fewer females with thecated embryos are found. Since culturing and gonad induction are frequently unsuc cessful, ecologists in the area may flnd.it necessary to resort to tentative identification. The possibility of undescribed species in Lake Erie and the ponds of the islands should not be overlooked, however.

Other data of interest with reference to species descriptions and Lake Erie specimens are also presented in the ”comparative notes.” No mention is made of color.

■ . - 16 - The color In hydras, as will be shown later, depends upon

type of food Ingested and state of nutrition. Except in the

case of Chlorohydra, color therefore has no taxonomic sig

nificance. Habitats and distribution are treated in a sepa

rate section. Reference should be made to Tables I and II for nemato cyst sizes. The relative and absolute sizes of the holotri

chous isorhizas and stenoteles appear to be good species characters. Contemporary-workers are invited to employ the

sampling methods used in this study. Hone of the species

descriptions are specific on this matter.

* *•

Hydra ollgactis Fallas, 1776

Descriptions in Schulze (1917,1927) and in Hyman (1930) Synonyms (from Ewer, 1914-6) r Hydra fuse a Linnaeus,

1767? H* rhaetlca Asper, 1879?; H. roselll Haacke, l880;

H. vulgaris Jickeli, 1882; H. monoecia Heffeman, 1902;

£• Beardsley, 1902; H. corala Elrod and Ricker,

1902; H. dioecla Downing, 1905>; Pelmatohydra oligactis

Schulze, 1917. This frequently encountered species can be distinguish ^ from all other hydras by the lengthwise loops of the thread in the holotrichous Isorhizas (Fig. lc). No other hydra has this constant character. H. umfala Ewer, 19I4.8 , found in

Africa, has a variable thread pattern, ranging from trans verse coiling to predominantly longitudinal looping (Ewer, - 17 - 191*8, p. 288 and Fig. 2). But H. oligaotia never exhibits typical transverse coiling.

A second distinguishing character is the comparatively

small size of the stenoteles (Fig. la). In the hundreds of

stenoteles measured during routine examination of collec tions, none were found exceeding a length of 13 microns.

For rough separation of species in a collection from a mixed population, the distinct stalk is also useful.

Although presence or absence of the stalk is not a valid genus character, it is a good species character for H. ollgaotis and H. pseudoligactis. Even in contracted specimens of H. ollgactis, the stalk region is usually evident. The tentacles are also longer than the contracted column, and more thread-like, than in contracted specimens of the other species. (See Hyman, 1930, pi. 36; Schulze 1917* Figs. 57, 58 for attitudes or "Habitusbild.11)

Comparative Notes. Column, in fully extended specimens, rarely exceeds 15 mm.; tentacles about three times column length. (Hyman, 1930, p. 321*, records column length as attaining to 20 ram., with tentacles extended to 60 to 80 mm.;

Steche (1911, p. 8l) reports tentacles as long as 250 ram.) Tentacles usually four to six, fewer than in other species, but subject to seasonal variation. Buds originate in spirals distally from juncture of oral stouter column with aboral narrow stalk. Tentacles on buds arise in successive pattern

- 18 - as diagrammed by Schulze, 1917, Fig* 61+, and are noticeably uneven In length as figured by Hyman, 1930, PI. 36, Figs.

7-U* All sexual specimens found were dioecius. Testes without nipples, low and rounded. They are numerous, 10 to

2*5, and distributed over the whole body region from the stalk distally. Fertilized eggs spherical, thecae thin without distinct spination, as shown in Hyman, 1930, PI, 37,

Fig. 19* Schulze, 1917* P* 106, states: "Das abgelegte El 1st kugelrund und mit kurzen Httckern besetzt (Fig. 68)."

His optical section of the embryonic theca shows distinct knobs. Hyman, p. 326, does not say that the embryotheca is "spineless," as Ewer (191+8, p. 235) reports, but that "the fertilized eggs which I have seen of this species exhibit practically no spines. The theca is merely wavy." The whole question of the constancy of the character of spines on the theca within a single species needs proper study based on statistical analysis of collections from the field in Europe and America. (For conflicting evidence see:

McConnell, 1935; Hyman, 1938; Griffin and Peters, 1939; Ewer,

191+8; Hadley and Forrest, 191+9.) Mean size of holotrichous isorhizas nearly as long as stenoteles, but much smaller than mean size of 12x5 miorons given by Ewer. Shape (Fig.

1-c) as in Schulze (1917, Fig. 61; 1927, Fig. 11+), only rarely "kidney-shaped" or "navy-bean shaped" as described by

Hyman (1930, p. 325, p. 329 and drawn in Figs. 17-b and 18,

Pi. 37). Atrichous isorhizas at lesser extreme of range

- 19 - given by Hyman. # * •*

Hydra pseudoligactia (Hyman, 1931)

Description in Hyman (1931b)

Synonyms Pelmatohydra pseudollgaotla Hyman, 1931

This species resembles H. oligactls very closely in external characters. It occurs in the same habitat with oligactls. The two species can be separated from others in mixed populations by the distinct slender proximal region and the stout distal region of the column. Only by examina tion of the nematocysts, however, can pseudoligactls be separated from oligactls. The thread of the holotrichous isorhiza is transversely or obliquely coiled (for variations see Hyman, Pi., 29, Pig. 2) as in the pattern shown in Fig. 1-d, whereas oligactls has a longitudinally-looped thread

(Fig. 1—c)• Moreover, the stenoteles are larger than those of oligactls, ranging up to 15> microns in the local speci mens and having a mean size exceeded only by H. americana and H. carnea. The atrichous isorhizas far outrank those of the other local species in length. The size of the desmonemes is exceeded only by those of H. americana.

The sexes are separate as in H. oligactls, but mature males possess pumpkin-shaped testes with nipples (Hyman,

Pi. 29, Pig. 6). The fertilized eggs are similar in appear ance to those of H. oligactls, but the theca Is completely

- 20 - spineless (Hyman, PI. 30, Pigs. 7 and 8). Comparative Notes. Tentacles arise successively.

Arrangement of buds not studied, still to be ascertained for species (Ewer, 19l|-8, P- 236). Gonads confined to body region as stated by Hyman, p. 303 . Developing ovaries in so-called ”amoeboid stage” produce distinct enlargement of body region of column (Hyman, p. 303; PI. 29, Pig. £). Spiral markings indicative of tube amature are distinct in undischarged desmonemes (Fig. lg).

Hydra americana Hyman, 1929

Description in Hyman (1929; 1931a; 1931bJ. Individuals of this well-defined species can quite easily be recognized by the short tentacles arranged like the ribs of an open umbrella (Hyman 1929, Pi. 30, Pigs. 7 and 8). It Is the smallest hydra found in Lake Erie, the slender stalkless column measuring about 6 mm., and the tentacles always being shorter than the column.

The stenoteles (Pig. 1-b) are unusually large, varying

In individuals from 13 x 11 up to 22 x 16 microns. Of even more diagnostic value is the shape of the holotrichous f Isorhizas (Pig. 1-e), They are pyriform while those found in the other Lake Erie species are cylindrical. (Compare with Figs. 1—c and 1-d.) The beak-like apical end is also characteristic, and the short length is reflected in the mean of 9*3 microns. The atrlchous isorhizas are exceeded

- 21 - in size by the desmonemes, which are the largest in the local species.

Comparative Notes. Bryden (19^0, p. 82) states:

"Hyman (1929) has suggested that the American species commonly identified as Hydra vulgaris is not quite the same organism as that described by European workers and has given it a new name, Hydra americana. This change has not been generally accepted, and the name Hydra vulgaris is used in this report." He presents no evidence to justify his posi tion, however. Only habit and color are utilized by Bryden in his key to the Central Tennessee hydras. The ensemble of other characters, especially the morphology of the nematocysts, is disregarded by Bryden both in this survey and in his ecological study (19J?2, p. l+5>). Contemporary systematists, while they recognize H. americana as a well- defined species, do not report Hydra vulgaris Pallas, 1776 as occurring on this continent (pennak, 195>3, pp. 108-113;

Hadley and Forrest, 191+9; Ewer, 191+8; Hyman, 1938; Self, Teague and Bragg, 1937; Bragg and Self, 1937). My own study of the Lake Erie specimens convinces me that they are dis tinct from H. vulgaris as fully described by Schulze (1917» pp. 78-83), from Hydra stellata Schulze, 1911+, which is incompletely described (Ewer, 191+8, p. 239; Hyman, 1929, p. 21+9; Schulze, 1922, p. 22), and from the closely-related American species Hydra hymanae Hadley and Forrest, 191+9.

- 22 - It should be noted that Hyman (1929, pp. 21f8-2l|.9) in de

scribing the new American species mentions that she at first thought the form coincided with H. vulgaris and designated it as such in the paper on grafting and regeneration experi ments performed with Child (1919). She also points out that her identification of it as H. stellata in a paper dealing with reproduction and budding (Hyman, 1928) was erroneous, and that the experimental animals were actually the new species, H. americana* H. americana, as found in Lake Erie, agrees with Hyman's

description in all respects: tentacles on buds arise almost simultaneously; separation of sexes variable— dioe- clus or monoecius; testes conical, with nipples; theca spherical, never plano-convex as in H. hymanae or H. utahensls Hyman, 1931; spines long, but not branched as in H. vul garis (compare Hyman, 1929, PI. 30, Figs. 11 and 12 and

Schulze, 1917, Figs. $1, 5>2, p. 82); nematocysts closely resembling those of H. hymenae and H. utahensls in form, but reflecting differences in size in the mean values.

Hydra llttoralls Hyman, 1931 Descriptions in Hyman (1931b; 1938). The populations of the rocky, wave-sWept shores of Lake

Erie appear to be predominantly composed of this endemic

American hydra. It is stout, with a high conical hypostome, of moderate size, (column about 12: mm.), and stalklesB.

- 23 - The tentacles extend up to about 1^ times the column length, but they are not thread-like as in H. oligactls and H. pseudollgactis. One tentacle, the one which arises first on the bud, is longer than the others. In attitudes observed in cultures, the tentacles are usually not extended beyond the column length. Their posture is characteristic of the spe cies, however. They radiate obliquely upwards and then droop downwards (Hyman, 1931b, pi. 31, Pig. Xif.).

The holotrichous isorhizas (Pig. 1-d) are distinct from those of H. oligactls and H. americana. Unfortunately, however, they resemble those of E. pseudoligac1 1 s and H. carnea very closely. Diagnosis cannot be completed without sexual specimens and careful comparison of all characters with those of allied forms.

Comparative Notes. Sexes separate. Testes appear in immature specimens as low ridges, as figured by Hyman (1931b, pi. 32:, Pig. 27), but become distinctly mammiform with stout nipples in mature males (Hyman, 1938, Pig. 6 ). Females and fertilized eggs scarce. Spines of thecae longer than those of carnea or americana (Hyman, 1938, Pig. 7).

Hydra carnea L* Agassiz, l8£0

Description in Hyman (1931a).

Determinations were made from specimens collected from a shallow pond at Pish Point, Pelee Island, Ontario on

October li|., 1933. The species may occur in Lake Erie, and

— 2I4. — Is treated accordingly with the other species taken in Lake

Erie habitats (Tables I and II).

The species is hard to differentiate from other similar

forms. Only the large pointed atrichous isorhizas are dis tinctive (Fig. 1-f). The larger size of the stenoteles, as reflected in the mean, is useful in separating it from

H. littoralis, with which it might be confused in the asexual

state.

Comparative Notes. Completion of the diagnosis was made from specimens which became sexual in a laboratory culture ten weeks after collection at temperatures fluctu

ating from 16.0° to 23.6°C. Water temperature was 15.1° at time of collection, but sexual specimens were not found. No hermaphroditic Individuals were observed in the culture.

Hyman (p. 2ij.) noted a high degree of protandry, and mentions that the species may be tending toward dioeciousness. Sexual organs as described by Hyman: testes few, distal, and highly conical with prominent nipples (Pi. £>, Fig. 11); ovaries proximal In position; fertilized eggs with thecal

spines shorter than those of H. americana (pi. £. Fig. 13).

Column without stalk, from 7 to 10 mm. Tentacles longer than column, up to three times column length; five to seven in number. Most frequent attitude, as figured by Hyman in

PI. ij., Fig. Jj., with column somewhat contracted and tentacles extending upwards parallel to each other. Origin of tentacles

- 2£ - on buds successive$ the oldest being the longest. Buds are produced on the column in opposite and alternating pairs.

- 2 6 - PART II

HABITATS AND DISTRIBUTION IN THE GREAT LAKES Collections indicate that the most frequently occurring and widely distributed hydras in Lake Erie belong to two species: H. llttoralis and H. oligactis. Both are occupants of the littoral-zone habitats. H. littoralis is found on rocks of wave-swept shores as a member of the typical swift- water community. H. oligactis is found most abundantly on rooted aquatic plants ubmerged in the stiller waters. Often associated with H. oligactls in this community is the similar appearing H. pseudoligactis. These two long-tentacled spe cies can also live in the deep waters of the lake where they are found attached to the shells of molluscs. H. americana appears sporadically in mixed hydra populations attached to rubble in the shallow-water interzone between wave-swept shore and vegetation beds. Hydra carnea and Chlorohydra vlridlssima, not collected in the lake proper, may be con fined to pond habitats.

Hydras, wherever found, are essentially members of the periphyton in the sense of the term as defined by Roll (1939, p>. 65) — ”all© festsltzenden Organismen auf irgendeinem substrat"— and adopted by Young (1914-5 )• The polyps are part of this assemblage of organisms growing upon free surfaces of objects submerged in water. Sedentary and semi-sessile, they are ’’Aufwuchs” or MBewuchsH organisms. Only by accident do - 27 - they occur as members of the truly benthic or plankton!c

communities.

Evidence for these summary statements is introduced im

discussion of collection data in this section. The changing nature of the microhabitats of the nstanding crop” population

is treated in the section devoted to discussion of seasonal abundance as studied in the limited area of Fishery Bay, South Bass Island (see part III).

Identifications of specimens collected outside this area were made from asexual individuals by comparing characters, especially those of the nematocysts, with the completely identified specimens taken in the Bay. In a strict sense, therefore, except possibly in the case of H. oligactls and

H. pseudollgactis, these species identifications must be regarded as provisional.

The few hydra records for Lake Erie and for the other

Great lakes found in the literature are undoubtedly reliable with respect to genus identification. Species names are given as reported, but the accuracy of the species identifi cation must be questioned in most instances. Identifications prior to the beginning of Hyman's studies (1929) were made on very flimsy grounds. The habit of applying names of

European species to American animals superficially resembling them was widespread among both field and laboratory workers. The resultant confusion has contributed to the paucity of our knowledge of the biology of hydras.

For general distribution of the American hydra species, - 28 - the papers of H^man and the key in Pennak (195>3, pp. 109-113) should he consulted. Herein is summarized only distribution in Lake Erie and the other Great Lakes in discussion of gen eral habitats where hydras have been found. Designation of collection points is only approximate. The places mentioned can best be located in the series of charts published by the

U. S. Lake Survey, Corps of Engineers, U. S. A m y , Detroit,

Michigan.

ABIOTIC HABITAT FACTORS AFFECTING DISTRIBUTION

Morphometric, climatic and edaphle features of the western end of Lake Erie are well known, mainly as a result of a series of year-round investigations at the Franz Theo dore Stone Laboratory begun by Chandler In 1938 (Limnological studies I-V, literature cited). The role of these groups of factors In the lake ecosystem is brought into perspective by Langlois in his recent book (19^). Turbidity is interpreted as the limiting factor in the productivity of the key area of the lake.

In view of the constancy of the chemical factors as they have been shown to prevail in the lake, routine measure ments of them in this investigation seemed superfluous. The waters are vertically quite homothermous• Even after periods of summer calm, the maximum temperature difference measured between surface and bottom In the island region, where the deepest waters are about 1 2 meters, has been approximately i o q. C. Seasonal trends In temperature were carefully noted, - 29 - however, for abundance of hydras in lake habitats appears to

vary during the year with temperature. Water temperatures

for the period of the study are graphed in Figure 5.

(Temperatures are designated in degrees Centigrade throughout

the text, unless otherwise specified.)

Hydras, apparently show a wide range of tolerance to

chemical factors such as dissolved oxygen, pH, free carbon dioxide, and alkalinity. Bryden (1952) in his thorough study

of the population dynamics of H. oligactis in Kirkpatricks

Lake, Tennessee, was unable to demonstrate any effect of this group of factors on the composition and distribution of

the population. He presents evidence, however, that hori

zontal distribution is related to type of bottom and kind of vegetation. He concluded that differential vertical distri

bution of the population on plants, submerged stems of trees,

and artificial supports was brought about by wave action and low transparency of the water. That this species responds

positively to light, except direct sunlight, was conclusively demonstrated by Welch and Loomis (1921).). Experiments made in Douglas Lake, Michigan by Welch and

Loomis proved that H. oligactis can tolerate dissolved oxy

gen deficiency down to 0 . 3 cc. per liter (0 .l).3 p.p.m.) at a o o temperature of ij.8 F. (9 C.), pH of 7.0, and depth of 60 feet

(18 m.). The optimum pH for hydras under experimental condi

tions is reported as 7.8-8.0 by Threlkeld and Hall (1928). The range of 6 .8 -8.1). was found suitable for Pelmatohydra in

- 30 - experiment s by Threlkeld and Reynolds (1929)* Lake Erie measurements of 7 • it-— 8• U- indicate optimal hydrogen-ion con centrations for hydras. Waters of the lake are noted for

their high oxygen saturation.

In his significant study of Pelmatohydra in Douglas

Lake, Michigan, Miller (1936, p. 157) associates his sea

sonal data with data published by Welch and Eggleton (papers cited by Miller), and agrees with the conclusion of Welch and Loomis, namely, that, during the summer, the whole epi- liranion and most of the thermocllne are inhabitable for hydras but that the hypollmnlon is usually unfavorable, especially with regard to oxygen content. He mentions that even during the summer and winter stagnation periods the hydrogen-ion concentration Is never so high as to be unfavor able for hydras. Free carbon dioxide up to 5 p.p.m. appeared to have no unfavorable effects at low temperatures.

In western Lake Erie thermal stratification is a rare phenomenon; a thermocllne is temporary. Seldom in the known history of the lake has its hydra population been subjected to adverse chemical conditions. The nearest approach to such conditions occurred during the period of the present

study at the end of the late summer calm in 1953. Britt

(1955a.) reports the catastrophic effects of the oxygen defi cit resulting from stratification on the Hexagenia popula tion. On September 5, at a station between Rattlesnake and

Green islands, he measured the lowest dissolved oxygen con tent ever recorded for the region. It was 0.70 p.p.m.

- 31 - This is close to the 0.1|3 p.p.m. that appear to be lethal to

hydras. In the littoral zone, however, where the hydra popu

lation was concentrated, the oxygen content was well above

the danger point: in Fishery Bay on September 2, dissolved

oxygen measured 3*92 p.p.m. at the surface and l±.3 Q p.p.m.

at the bottom. The decline in the hydra population here is

attributed to the mounting temperatures (Fig. 5>) rather than to low oxygen in the prolonged period. A record high of 29° was measured.

DISCUSSION OF HABITATS AND DISTRIBUTION RECORDS

Collection methods varied with the type of habitat.

No quantitative work was attempted except in Fishery Bay where hydras were collected from the rocky bottom at the shore stations by colonization of microscope slides contained

in a rack. This rack, described in detail later, was bridled

to. a large piece of square dolomite rubble (see Pt. Ill, Sect. 1). The hydras settling on rubble and similar surfaces

can be removed without injury along with associated perphytcn

by brushing the submerged surfaces with a nylon bristle handbrush. Samples of gravel and sometimes a piece of rubble

can be grappled with the Petersen dredge, but stones are best obtained in shallow water with a long-handled spade, or by diving and bringing up selected rocks.

The best device for collecting hydras in deep water from all types of bottom was a small, rectangular cage trawl, designed by Dr. Anthony Bodola of the Laboratory staff. - 32 - Flexible rake claws — set at the lower edge of the metal straps supporting the cage — rake the substratum, and some of the hydras thus detached or adhering to mollusc shells and debris are carried into the cage opening above and caught in the No. 20 mesh screen which encloses its frame on five sides. Especially useful for collecting from soft bottoms at a trawling speed of about three statute miles per hour was the 30-foot otter trawl described by Kinney (195ij.)« The short tunnel of the quarter-inch mesh trawl net gathers in shells and debris, to which hydras may be found attached.

Samples of mud, muck, and ooze were occasionally taken by means of the Ekman dredge. The few specimens of hydras ob tained in collections from sediments were found in samples dredged and screened by the method described by Wood (19£3) in his bottom fauna work on the western basin. Plants were grappled with a Pieters plant hook or long- handled rake. Bullrushes were pulled loose and examined for hydras in the field by submerging them in water in a long glass tube equipped with a screw cap. (Such tubes can be secured from music instrument concerns where they are used as containers for violin strings.)

The final operation in the field usually consisted in screening the collection through a standard sieve with open ings of 0.2£ mm. (U. S. Sieve Series, Mesh No. 60). Hydras were segregated in the laboratory by searching the collection, portion by portion, in petri dishes under a widefleld stereo scopic microscope. A pair of jeweler's tweezers was used in - 33 - this manipulation. Sorted hydras were held in petri dishes

half-filled -with strained lake water kept in a constant

temperature cabinet set about 5°C. below lake temperature

during the process of identification. Temperature at the

point of collection was taken with a Negretti and Zambra

reversible thermometer. These temperature readings, type of

gear used, depth and nature of the bottom are mentioned in

the following discussion.

Aggregations on Nets

The largest aggregations of hydras encountered during " the study were observed on the pound nets of the fishery

operated at pelee Island. The species at the Island, how ever, was not the H. oligactls which Clemens (1922) reported at the pound-net fisheries on the Canadian mainland. All

specimens studied from collections made at the pelee Island

nets each season (1951-1951+) were identical with H. littoralls, the predominant species of the Bass Island region.

Thirteen collections were made from the Pelee Island

nets: 195>1; July 11, 11+, 21+, 25; 1952: July 2, 20, 30;

1953: July 20, August 11, 25, and October ll+; 19^1+: June

30, August 6. Each season, hydras were found in abundance

as part of the perphytic community which covers the tarred twine.

The fishery Is operated according to standard proce

dures for pound-net fishing described by Langlois (1951+,

pp. 323-331; see Pig. 1+8 for photograph of Pelee Island

pound net). The five nets are set In early spring. They are - 31+ - spaced along the west shore of pelee island from Pish point due north for a distance of about two miles. The crib, held in place on the rock and hard clay bottom by stone anchors, is set in about 18 feet of water. (Soundings varied from Ij. to 6 meters with changing water levels of the seasons.)

The lead net, about 100 rods long, is set at a right angle to the shore, so that the head (heart and crib) is about one- third of a mile offshore. The mesh is inohes, diagonal stretch.

It is evident that each of the nets affords an extensive web for studying the vertical and horizontal distribution of periphyton which grows on the twine. Examination of complete nets when the crews were pulling them for cleaning on July Z%

1951 (2 3 .5 °) indicated the hydras settled on the leads well into shallow water. Our collections unfortunately were mostly confined to samples which could be taken from the crib, usually the head (the outshore face of the pot, or pound proper). Regular sampling was limited to the most northerly located net. By grasping the top line of the net, reaching inside the crib and overhauling the web by hand from the side of the boat, the deepest level we could reach was about 36 meshes. Measured as the net stretched in the water, the maximum depth at which samples were obtainable, was approxi mately 1 3/h meters. Working up from this level, by counting meshes, it was possible to segregate samples from 1 3/h meters to the 1 meter level, from this level to the meter

- 35 - level, and from this point to the surface. Ten bars extend ing through each zone were scraped by means of a small cylin drical aluminum cup, notched at the lip to fit the twine.

The typical diatom-fungus cast, brown in color and felt-like in texture, which encases the twine of the bar was caught in the cup and washed into a 12$ ml. jar half-filled with lake water. The hydras showed no ill effects from exposure to air during collection. When not overcrowded in the jars and not exposed to temperatures much exceeding those of the lake, the specimens were usually expanded and in good condition for study upon arrival at the laboratory. It was thus possible to make a crude estimate of the density of numbers per bar and the per cent bearing buds.

Interpretation of results of the net studies indicate:

(1) Aggregations of H. llttoralls establish themselves on every net sometime during each summer season. The com plete nets, including stakes, are pulled before freezing weather. Members of the population must therefore (a) either maintain themselves on substrata of the bottom in the vicin ity during the winter and migrate to the twine after the nets are set in the spring, or (b) individuals from other localities are carried to them in the plankton or attached to drifting plants.

Hydras were not taken in horizontal or vertical plankton tows made routinely between nets at each visit to Pelee island. Only once did an exception occur. On July 20, 19£>3,

- 36 - a single specimen (H. littoralls) was taken In a vertical

tow. It is possible that this Individual was dislodged from

the rocky bottom by the metal hoop of the net. (Depth £ o meters, temperature 2i|..8 .) There are no zones of rooted

vegetation along the shore. Drifting plants, Valllsneria

and clumps of Cladophora, frequently tangle in the nets

during the late summer season. But this happens after the

hydra aggregation is established on the twine. Rooted aqua tics have not reached sufficient growth in Lake Erie so that

they are torn loose by storms at the season when the nets

are set. That colonization Is effected by possibility (b) appears doubtful.

No collections were made at Pelee island shore during

the winter, but seasonal collections at South Bass island

show that members of the population survive at near freezing

temperatures. During the fall visit (October II4., 19S>3, sur face 15>.1 °, bottom 15*0°), the nets had been freshly tarred,

and no hydras were found on the twine. Scrapings from the

bottom to top of a free down-line at one of the crib stakes

yielded 37 specimens, all H. littoralls. A third of them

were budding. None had formed gonads.

Winter eggs may aid in maintaining the population, but the problem of how hydras hatching from them start aggrega

tions on the nets is not altered. Newly hatched hydras as well as hydras surviving from the previous season must find a suitable substratum on the bottom. Prom even a remnant of

- 37 - the preceding season's population, rapid increase in the num

bers of hydras by asexual reproduction can build up a popula

tion as the waters warm in the late spring. When the nets contact the bottom habitats of the hydras and a suitable sub

stratum forms on the twine, colonization probably begins.

The mesh provides an excellent web for the upward migra

tion of the hydras. Ewer (19^7), in an elaborate series of

experiments with Hydra vulgaris, has demonstrated that buds

immediately after separation from the parent migrate upwards.

The reaction is a negative geotaxis, and not a response to a

gradient in oxygen concentration. This gravity reaction —

coupled with positive response of hydra to light — may be significant in our species, acting to bring about vertical

distribution on supports and to prevent overcrowding.

Mature individuals of Hydra are known to be positively phototactic (Wilson, 1891; Welch and Loomis, 1921}.; Ewer,

19lj-7). Orientation to light is effected by trial movements.

The animal walks, by inching movements upon the substratum in an upward direction toward a zone of optimal illumination.

This reaction, too, would contribute to the spread of the

budding aggregation, especially in the less turbid water mass surrounding Pelee island (See Berduin, 19£la, 1951b, for com

parative turbidity measurements.) Recolonization of nets, which have been pulled to clean off the "net-moss” during the fishing season, probably also occurs in the same manner.

(2) The aggregation did not extend into the upper

- 38 - one-half meter zones of the nets. Wave action here Is almost constant, and during storms reaches such force that pound- stakes are sometimes pulled loose. Periphyton is sparse near the surface. Wave action is apparently the cause, as young

(19l+5>) concludes in his thorough quantitative study of peri phyton made at Douglas Lake.

Hydras were abundant, as many as 100 to the two-inch bar early in July, in the zone from one meter to one and three-quarter meters. Their comparative scarcity in the adjacent half-meter zone is attributed to lack of foothold in the heavy growth on the twine there rather than directly to wave action. Plankton collections made at the cribs after storms and during rough weather contained no hydras even when the aggregations on nets at the lower zone was heavy.

A slide rack collection, discussed below, indicates that the one-meter level may be optimal for hydras multiplying on artificial supports.

(3) Hydra littoralls will rapidly colonize a perphyton- free natural support set at the bottom in the vicinity of a crib. Clean microscope slides suspended in new racks at one-half meter from the bottom (depth 6 metersJ and at one meter from the surface are also heavily settled with hydras in a ten-day period.

A set-up meeting the conditions Indicated above was made at the net nearest Pish Point on July 20, 195>2. The rig was tied off a back heart-stake with Kordite lines so that the slide racks hung free, slides vertical to bottom. The clean

dolomite, square rubble (20x20x5 cm.) used as the natural

support, was set on the bottom about two meters from the

stake. Ten days later the rig was lifted and the settlement examined.

Bar scrapings from the adjacent crib-wall were also

taken, 5 bars at each level. The cast where the hydras grew was essentially a periphytic community composed of sessile diatoms of the genera Gomphonema, Oymbella and chlorophyceon

Mougeotia, among which had settled members of the planktonic

Asterionella-Cyclotella community. Naviculate diatoms were also numerous and the desmid Cosmarium, among other green algae, lived in the tangle of diatom stalks and fungus mycelia. (Date: July 30, 1952. Choppy sea. Temperatures:

21^*14.° surface, 2lj.,2 ° bottom. Depth: 6 meters. Water sam ples at one-half meter: dissolved oxygen 7.50 p.p.m., equal ing 88 per cent saturation; pH 8,I|..)

Large numbers of hydras were observed on all substrata.

The majority bore from one to three buds, indicating a high rate of reproduction. On the stone, which had but a sparse periphyton colony, they were found on the top and sides. None were on the bottom surface which apparently had rested flat on shifting sand. Actual count of hydras on the stone was (60 per cent with buds). Also found on the stone were

13 large planarians, 2 caddis-fly larvae, and 1|. mayfly larvae

(Btenonema). On the racks, hydras attached to the slides

- 1*0 - were too numerous to count in the field. The total number on the two sides of 18 slides in the rack was estimated by actual count of one slide in each rack as: llj.00 for the rack at bottom level, and 2000 for the rack at the one-meter level

(about 60 per cent budding in each case). Slides at the lower level were quite heavily colonized with Plumatella.

Estimated number of hydras from scrapings at the one meter to one and three-quarter level of the crib was 8 per bar, about 50 per cent budding. At this level, on July 2 o (22.6 ) the numbers were much greater — from 80 to 100 per bar with 76 per cent bearing one to four buds. No such large numbers of hydras of any species were ever taken on slides or other substrata at South Bass island in collections of the same submergence period made at the same season of the year. It may be that that the lower turbidity of the Pelee Island water mass results in a more abundant entomostracan food supply and that hydras growing on supports well off the bottom are in a better position to capture these plankters.

* *»■ «•

Only trap nets and a few gill nets are used by commer cial fishermen at the Bass Islands (see Langlois, 1951+j pp.

303-315 for operation). These are submerged, and are not accessible for routine collections. A complete check of the string of trap nets operated along the shore at South Bass

Island was made on April 30, 1952 (water temperature 11°C.}.

- kl - Hydras were found on the rubble in the vicinity of the nets.

Failure to find hydras on any of the gear is attributed not to the early season, but to unsuitability of the substratum on the twine and brails. Very little periphyton had covered the tar.

During the season of 1951+ $ when water temperatures had reached 11° C. in early May, scrapings were made from the tarred twine of hoop nets and the untarred twine of gill nets submerged in Fishery Bay for comparable periods. No hydras were found on the hoop-net twine, whereas a sample of 50 hydras (H. oligactis) were easily collected from a short length of gill nets. At the beginning of February 1951+ when the water temperature had reached only 1+°C., H. oligactis was collected from the untarred twine of gill nets set under the ice cover of the bay. The density of population was estimated by Moen (1951) as about 7 oligactis per inch on a fill net submerged for 18 hours in an Iowa nursery lake

(temperature at submergence 61+°F. (17.7°C,). This affinity of hydras for string was originally ob served by Trembley (17i^)» He used pieces of string for transferring hydras in culture work. A drawing of his "long-armed polyp” (H. oligactis) attached to a little string is reproduced in his second memoire (pi. !+, Fig. 3). Trembley says (p. 8lj.): "pour avoir des Polypes attaches a de la ficelle, il suffit d'en mettre en quantite' dans une verre garni de Polypes. Il y en aura toujours quelques-uns qui iront se fixer sur cette ficelle." From my own experience

- ^2 - with Trembley*s technique and observations of settlement of

hydras on clean net twine, I can state that hydras attach to

such surfaces as one of the first members of the forming perlphytic community.

* * »

The widespread distribution of hydras along the Canadian shore during the season of 1953 is reported by Mr. Robert G.

Ferguson, of the Ontario Department of Lands and Forests

(personal comraunication dated August 30, 1953):

Pursuant to our conversation at put-in-Bay on Aug. 15 I am setting forth my observations regarding Hydra on pound nets.

On July 1*, 1953 I accompanied the four men that operate the Cove Fisheries at Clear Creek, Ontario, while they fished their nets. The surface temperature this day was Their nets are set off Clear Creek and towards Long Pt.; their furthest net east being a few miles up the pt. from the base. I am sorry to say that I did not record the depths of these nets, but recall them as being 2£ to 1*0 feet.

All these nets (7-10^ were covered with red "slime" towards the bottom. I would estimate that from 20 feet above the bottom to the bottom there was abundant red "slime." I identified this slime as Hydra. The men operating this fishery are: Clarence puddlcombe, Lee Didrick, Lloyd Kenline and Bill Kenllne.

On July 6th, 1953 I accompanied jack ("Buck") Wamsley of port Dover while he fished his pound nets on the east side of Long pt. His nets are 36 to 29 feet deep, and were set from 2 to 6 miles from the tip of Long Point. His nets showed the same condition as to Hydra as did the Cove Fisheries; further these Hydra all appeared to have one or more buds. Wamsley has since gone out of business.

The Crewes at Port Crewe, the Getty’s of Wheatley, and various others have mentioned red slime on their nets, and also that it irritates the skin on their arms - 1*3 - at times. Charles Pi lion of Erleau mentioned that red slime gets on gill nets at times, even on nets newly- set overnight. This latter observation doesn't appear right to me. I hope that I have recorded here all the informa tion you requested, but if I have not drop me a line and I will be most pleased to be of assistance. I have not run into any specimens for you yet, but have put out some feelers.

These records from Mr. Ferguson, quoted with his permis sion, extend the known range of Hydra to a point in the east basin of Lake Erie approximately 120 miles upshore from the place where Clemens (1922, quoted in Introduction) originally reported H. ollgactls. The reports from Wheatley, Port Crewe, and Erleau Indicate that hydras are still settling in large numbers on the nets at the fisheries where Clemens observed them 33 years earlier.

No animals from these nets were obtained for species identification. The reddish or orange color seems to be peculiar to Lake Erie hydras at this time of year due to the reddish oils of certain vernal copepods they eat. The color of the Mslime" on the nets at Pelee island, about 30 miles southward, was always brownish or green from algal growths.

H. littoralls collected there was brown, white, or violet- tinted, never orange. Daphnids were abundant at the time, and these do not produce orange coloration in hydras (see

III: 2-b, ]+-b).

I know of no peripbytic organisms except hydras that will cause the red or orange slime which fishermen find on nets. Such reports may signalize the presence of hydras for

- kk - biologists interested in identifying them* It is noteworthy,

that fishermen handling nets covered with hydra slime com plain of skin irritations. The question of the poisoning of the fishermen, raised by Clemens (1922), is discussed later

(see ill: 6-b).

Red slime on gill nets is also reported by Mr. William

Lamb, owner of the Pelee Island fishery, occurring on gill nets set at Little Chicken island, five miles due west of his fishery.

Mr. Edward Brennan, of South Bass Island, working a string of gill nets at Mouse Island and Scott Point on the

American shore, during the spring of 191*6, observed red slime covering the twine, and reports that all members of the crew suffered from the ’’itch.”

The only information concerning possible widespread distribution of hydras in the other Great Lakes originates from reports of "red slime" on nets and the associated

"itch." Observations of the phenomenon in Lake Michigan, supplied by Dr. Walter Koelz, are given by Welch and Loomis

(1921*, p. 215):

On November 11, 1920, the large gill nets set about seventeen miles from shore, northwest of Michigan City, Indiana, at a depth of 32 fathoms were so covered with red hydra that they were literally slimy with them and this condition prevailed throughout the entire length of the "gang11 of nets which was 15,120 feet.... Dr. Koelz also learned from a fisherman at Michigan City that in regions of Lake Michigan off that city the hydra nuisance on the nets occurs only in the autumn with September as the earliest date. Its range was given as 10 to 30 fathoms but was most abundant in 10 to 15 fathoms. Prom this same source comes the informa tion that immense hydra populations occur in other parts - J+5 - of Lake Michigan and are well known to the fisherman because of a form of "poisoning” of the hands which they attribute to them.

Similar occurrences of hydra aggregations in the north ern waters of Lake Michigan and Lake Huron are indicated by the observations of Mr. Orrin Robertson, of Birmingham,

Michigan. He has kindly reviewed with me his ten years of continuous commercial fishing experience ending 1939. He observed the characteristic "red slime" on the nets at all fisheries where he worked. It occurred in the fall on the gill nets fished in Lake Michigan off Scotts Point, Port Inland, near the Straits of Mackinaw, especially after a heavy blow when "moss" was torn loose from the bottom.

Sometimes gill nets strung under the ice-cover in Lake Huron at the Drummond island reef were also covered with the accu mulation. At Forty Mile Point and farther south in Lake

Huron at Alpena, where trap nets and pound nets were operated in depths of from 8 to 16 fathoms, red patches were noticed in the accumulation on the twine at the season of the mid summer cleaning. Mr. Robertson also reports skin Irrita tions among the fishermen who handled the trap nets or the gill nets after they were dried.

The large aggregations noticed on human artifacts may lead one to postulate sizeable hydra populations In the G-reat

Lakes waters. But they might merely represent "explosions" of a population which is normally quite small.

- - Occurrence in the plankton

The single instance in which I have taken a hydra in a plankton tow is reported above. Horizontal and vertical tows with a large net was routine procedure on trips to Pelee

Island and during transects trips of the western basin of

Lake Erie. Hydras were never noticed by Verduin (personal communication) in his numerous surface collections by the

mobile sampling method (1951a, 195l"b)- In Fishery Bay, at

seasons when the hydras were abundant on substrata, tows were made after storms, but no hydras were taken. In the many

plankton-trap samples taken by Stone institute investigators, no occurrence of hydras is mentioned. This experience agrees with that of Miller (1936, pp.

ll+O-ll+l). in 120 quantative plankton samples taken in Doug las Lake he found only a single hydra in a 1+0 1. collection

from the surface. In 60 qualitative samples, five were taken by means of a tow net pulled for about one-half mile through

the bulrush area where H. oligactis was abundant. Welch and Loomis (192I+, p. 20£), studying the species in the same lake,

found "that the floating population was relatively small,

even during the season when the vegetation was loaded with hydra, except at times of strong wave action." Reighard (l89l+a, p. 13U)» reporting on the first plank ton study made in the Great Lakes, records the occurrence of hydras in Lake St. Clair. They were taken occasionally in a - 1+7 - BIrge net towed at the surface, and Reighard says they were

"evidently wanderers from the ‘bottom." In his complete report on the collections in Lake St. Clair and Lake Erie,

(l89lj.b, p. 17), Reighard emphasizes that the occurrence of hydras in the plankton is accidental. None were found in the Lake Erie collections, and those collected at Anchor Bay,

Lake St. Clair, were taken in tows through vegetation to which they were attached. During my collections at Anchor

Bay in 1953, I took hydras in the tow net only when it was dragged through Potamogeton beds. A series of quantitative surface samples, taken at various points along the American and Canadian shores during August, 1950, contained no hydras.

Eddy (19275 found hydras only once in his Lake Michigan plankton collections. He states (p. 25?): "Hydra ollgactus

Coligactisj pallas was found sparingly (100 per cm.5 in a silk-net collection Oct, 17, 1926 from Michigan City. It occurred In small numbers in the December 1887 and September 1888 collections." (At Chicago: Table l.J Eddy»s report is not accepted as a valid species record. Not even a pro visional identification could have been made from the speci mens, which were preserved In formalin and glycerine.

Hydras then are not actual members of a plankton commu nity. They are merely transients. Apparently the tentacles do not capture prey unless the animal is attached to a sub stratum. I have observed that hydras when floating in

- 48 - plankton-filled cultures do not feed. When detached from a substratum they sink slowly to the bottom. The specific gravity of a hydra is slightly greater than that of water.

Deep-Water Communities

The greatest depth at which we have taken hydras in Lake

Erie is 9 fathoms. No collections were made in deeper waters of the eastern basin. In the western basin 6-fathom depths are the maximum. The deepest sounding datum for Lake Erie,

35 fathoms (6ij. meters), lies 8 miles due east from the tip of Long Point (U. S. Lake Survey Chart No. 3). Such truly profundal regions (Forel, 1901j., p. 258) are all east of longitude 80°30i. Associations of animals living at depths of 30 feet may be regarded as ’’deep-water communities” in shallow Lake Erie.

The location of our deepest collection point is 13 miles north of Vermillion, Ohio. Depth on July 13, 1951, when the hydras were collected, was 55 feet. No temperatures were taken. Ten hydras, along with numerous dead shells, were dredged from the bottom of gray clayish mud by means of the cage-trawl. planarians, oligochaetes, and snails were also found among the living animals. But most remarkable were the 23 specimens of Manayunkia speciosa Leidy (reported by

Pettibone, 1953) found in the washings by Dr. George M. Moore, professor In charge of the Invertebrate zoology class. I observed none of these little polychaetous annelids In the hydras. Nor did I find oligochaetes or any other recogniz able food in them. The color was purplish-gray, like the

- 1*9 - color of the mud, but I could not recognize mud in squashes of the hydras. The hydras would not Ingest entomostracans or oligochaetes in cultures, and became depressed by July 31.

I was unable to determine the species, but can state that these specimens were not H. oligactis or pseudoligactls.

An effort to collect sexual hydras from the same locality was made on October 11, 1951 (water temp. ll|.,7°}'« but no hydras were taken there nor in any of the numerous dredge hauls made in the deep water during the transect. Hydras were collected from a depth of lj.7 feet on June

30, 195>lj-, (water temp. 19°C.) in a short haul made with the otter-trawl over sand bottom in the pelee Passage about 10 miles south of point Pelee, All 28 hydras found were attached to the shells of the 11 clams collected: 8 Lamp- silis siliquoidea, 2 L. ventrlcosa (average size x 3.3 ram.), and 1 empty Leptodea fragilis shell (9.5 x 6 mm.).

Hone were found in washings from the snails and sphaerid clams in the collection, but examination in the field of the segregated mussels disclosed stalked hydras with long ten tacles on every shell where it would normally protrude above the bottom. Gomphonema and other naviculate diatoms pre dominated in the shell microhabit of the hydras. The hydras were of an orange tint, small, and with a column length of about 8 ram. They showed no ill effects from a fairly heavy amoebic infestation (see section on parasites). No traces of ingested animals were found when all the specimens were squashed for nematocyst examination 2Ip hours after collection.

- $0 - Of the 28 specimens: 21+ were H. oligactls, I4. were H. pseudoligactis.

Two hauls of numerous clams and snails taken from the mud bottom in 30 feet of water off Kelleys Island on this

trip yielded no hydras. The bottom debris brought up here

smelled quite foul. Hydras, of course, do not thrive in anoxic habitats (von Brand, 191+6, p. 60). Unlike some spe cies of sponges, turbellarians, and Oorethra larvae, they are not facultative anaerobes (Eggleston, 1931). Pew hydras were ever found in collections from the mud bottom dredged in transects of the western basin, where 78 per cent of the benthos is composed of mussels (see plotted maps, distribution of bottom-dwelling invertebrates, Wood,

1953). Examination of washings from the fine-gauge screen made on trips with Dr. Wood in September and October, 1951 yielded negative results. Hydras were taken only twice — on September 21+, 1951 about two miles E.S.E. of Middle Sister

Island to Maumee Bay. (Depth: 33 feet; Temperature: 1 6 . 6 ° ; Bottom: muc.) The five specimens found were all H. oligactis. one was attached to a snail of the genus Bythinla.

The others may have been dislodged from the numerous snails and clams when the dredgings were hosed.

H. oligactis will apparently survive submergence in mud for a short period. The incident above, at an air tempera ture of 1 6 ° , is an example. Another observation made on

July 9, 1953 is Indicative of this species' capacity to sur vive mudding-over at higher temperatures. Healthy specimens - 51 - were retrieved from a collection of clams trawled from the mud of Sandusky Bay by Dr. Kinney and held for six hours with mud adhering in a bucket containing just enough water to cover them. The collection was exposed to a temperature of 2 £ ° , about two degrees higher than the water temperature.

*■ # •»

Hydras can withstand the tremendous pressures of profundal waters. The single record published on the occurrence of Hydra in Lake Superior (Smith and Verrill, 1871, p. 1+1+8) states that it !lwas brought up in many of the dredgings from

8 to li+.8 fathoms. In 32 fathoms, Neepigon Bay, and in £9 fathoms, off Simmon's Harbor, it was brought up from a soft clayey bottom. In the deep dredgings, it frequently came up near the bottom of the clay in the dredge, and was evidently not caught while the dredge was near the surface." Smith (1871, pp. 373-374) in a preliminary report of this U. S. Lake Survey work states: "The temperature, every where below 30 or 1+0 fathoms, varied very little from 39°, while at surface (at the time of the observations, during August) it varied from £0 to £f>°." He also notes that water brought from 169 fathoms (the deepest point known in the lake) "gave no precipitate with nitrate of silver." He mentions "a species of Hydra" in this report as being found down to 15>9 fathoms.

The Smith and Verrill report (p. 41+8) lists the species as Hydra camea Agassiz. A beautiful Hydra, agreeing with

- £2 - Ayres's description of this species, was very abundant at

the eastern end of St. Ignace, upon rocks along the shore.

Ayres (183>1|.) provides a description which is more com

plete than that given by Agassiz (185>0, p. 35^) • Whether

Smith and Verrill, despite the fact that they were students

of Louis Agassiz, could distinguish their specimens by the

shorter "tentacula" from H. fusca [a H. oligactis] is ques tionable. The habitat description they give suggests that

the species may have been Hydra littoralis, which was not then known. The Lake Superior record of II4.8 fathoms (270.8 meters) may well constitute the world's deep-water record for Hydra.

Forel, in his work on Lake Geneva (190lj., p. 21j3) lists one

specimen of H. rubra (* H. vulgaris)as taken at 110 meters with "la drague a filet" on October 9, 1883; four specimens taken with "la drague metallique" at meters on March 18, l88ij.. in his table of regions and zones of the lake (p.

2£8 ), he specifies a temperature of 0.£°c. at 100 meters in the "Region profonde - zone inferieure" as "limite de la variation thermique annuelles," and 2-3 ° at £0 meters in the upper zone of the profundal region. The Lake Superior depth would be close to the limit (290'meters) of Forel's lower zone.

- 5 3 - Vegetation Zones

The only extensive ecological Investigations of hydras, all of which have been made in this country, deal with

Pelmatohydra (H. oligactis and H. pseudoligactis). With the

exception of Bryden*s population study of H. oligactis in Kirkpatricks Lake, Tennessee (1952), the work has been cen tered at Douglas Lake, Michigan (Welch and Loomis, 192I+;

Miller, 1936; Young, 191+5The normal habitat of these species is rooted aquatic plants. Investigation was focused

in these studies consequently on factors affecting distribu

tion and seasonal fluctuation of the hydra population in the vegetation zones.

In the present investigation a fairly large population

of H. oligactis was found available for study in the vege

tation zone of the bay at South Bass Island (Fig. 2, station 5 )* Mixed in the oligactis population on the Myriopbyllum plants, which grow densely in this habitat, were a few H. pseudoligactis. The associates of these still-water species in the microhabitats of the milfoil leaves are discussed in the section on community interactions. Adjacent to station 5 , growing around near the labora tory dock, is a bed of Vallisneria americana (for photo, see

Langlois, 195^+* Fig* 7 — ’’Tape grass flattening waves in Fishery Bay”). Potamogeton pusillus Is interspersed in the stand. H. oligaotls, and H. pseudoligactis in fewer number^

- 51+ - were found in collections from these plants also. The spe cies were also found in collections from Elodea plants grow ing in the boatwell at Gibraltar Island where large pale greenish-white hydras were observed earlier (Langlois, 195>lj.# p. 110).

The animal populations of these same stands of submerged aquatic plants were studied quantitatively by Krecker (1939) during the summer of 1935 and 1936. He also included data from mixed stands In Squaw Harbor at Put-in-Bay and In East

Harbor at the mainland. Krecker does not tabulate his data by identified stations, but the grouped data provides valu able information concerning the comparative numbers of hydras, annelids, arthropods, insects, mollusks and other groups of the larger animals, on plants of the seven species In mixed stands (E. canadensis, N. flexilis, M. spicatum, V. spiralis, and Potamogeton - compressus, pectlnatus, crispus). Hydras, as well as the other animals In the sam ples, were identified to genus only. The species of Hydra were probably the same as found twenty-five years later —

H. oligactis mixed with a few pseudoligactis.

Hydras are recorded as occurring on all the species of plants except Vallisneria spiralis, which harbored the least numbers of the other animals. Counts from ten linear feet of each plant sample were made under a microscope, and aver age and maximum numbers tabulated. The largest average num ber of hydras (f>0) was found on P. cri spus. Elodea and - 55 - Myriophyllum ranked next with averages of 22 and 20 respec tively. The densest numbers of other animals were also found on P. orispus and Myriophyllum. Midge larvae and annelids made up 59 to 93 per cent of the population, occurring more numerously on the Myri ophyllum, which has finely subdivided leaves. Krecker says (p. 561): "The midges were able to cling to such leaves with their hook-bearing appendages and the annelids could coil about them with ease." He observes that the broad-leafed p. crispus harbored the largest number of semi-sessile animals like Hydra.

In passing, it is interesting to note that early workers on the Protozoa of Lake Erie mention Hydra with its parasitic ciliates taken from plant collections at East Harbor and

Sandusky Bay (Jennings, 1901b, p. 113; Landacre, 1908, pp. ). Field notes of 1916 by Dr. Stephen R. Williams on file at the Laboratory record Hydra fusca as "very abundant on weeds, floating material about dock. Season had been very cold and rainy to the opening of the laboratory. The para site Trichodina pedicuius very common also. Hydra decreased in number after first warm days. Few by July ij.th." Ho one mentions finding hydras attached to the filaments of the algal plant, Cladophora, which grows in heavy wavy masses on the rocks at the shoreline of Bass Islands from

May to late July. The green streamers are torn loose by storms and carried to the open waters. I found only a few specimens of H. oligactis and H. littoralis in many collec tions of Cladophora clumps, and these in May and June.

- 56 - The occurrence of hydras in Lake St. Clair is mentioned

by Reighard in his pioneer liranological investigation of the

Great Lakes (l89lj.bJ. He states (p. 17): "Both Hydra viridis

and Hydra grisea are reported commonly attached to the vege

tation and were not infrequently taken in the tow net, where their presence was no doubt accidental." A facsimile of a

record sheet (p. 10) for a collection made near New Balti

more specifies H. grisea as "many," H. viridis as "scarce."

The tow was taken with a Birge net attached to a small wire

dredge at 11 feet on a weedy bottom. (Date: September 1, o o 1893; water temperature 68 F., 20 C.)

Since the fall of 195>3» when I started studies of hy

dras in Lake St. Clair, I have found but one species, namely, Hydra oligactis. These hydras were identical with the

' 2* ollgactls studied in Lake Erie. They were fairly abundant

in fall, spring and summer collections on the stands of Potamogeton crispus and Scirpus validus which dominate the

vegetation zone in Anchor Bay from the Salt Creek area up to

New Baltimore. No hydras were found in collections from the bottom, which is covered with a matting of Chara.

Reighard’s record of the green hydra »H. viridis Lin

naeus, 1767 z Chlorohydra viridissima (Pallas, 1766)’ is noteworthy. It is the only published record of this hydra

for the Great Lakes. Yet there can be no doubt that the

species was correctly determined, for the single species of

the green hydra is easily distinguished from other species.

- £7 - The validity of the record for Hydra grisea Linnaeus,

1767, must be regarded as questionable, however. Reighard and his associates probably followed the practice then preva lent among zoologists of designating all hydras which were neither green nor brown as H. grisea. As pointed out above, it is doubtful whether this species, now known as Hydra vul garis Pallas, 1766, occurs in this country.

Possibly the Hydra species encountered in the early

Lake St. Clair study was Hydra americana Hyman, 1929. This species is believed by Hyman (1929, pp. 2ij.8, 2^1} to have been previously identified as H. vulgaris. Thus far I have not found H. americana in Lake St. Clair, but my collections have been limited to the area between the mouth of Salt

Creek and New Baltimore.

This area of Anchor Bay, abounding in stands of pond- weeds and bulrushes, affords suitable habitats for the Still water forms of Hydra. The aquatic meadows, which develop in the shallow, comparatively quiet waters by mid-July, harbor an abundant supply of oligachaetes and chironomid larvae and support what appears to be a fairly large population of H. oligactis. Further field work will probably disclose members of H. pseudoligactis and H. americana mixed in the oligactis population. It is predicted, however, that popu lations of H. littoralis do not establish themselves in Lake

St. Clair, in contrast to Lake Erie, Lake St. Clair has no well-defined terrace and lacks the rock-strewn shores where this moving-water species thrives. (See Reighard, l89l|.b,

- 58 - pp. 11-16 , for analysis of shore development and effect of morphemetrie features on Lake St. Clair biota.)

Results of seasonal field work in progress at a station in Anchor Bay adjacent to the mouth of Salt Creek will be treated in a separate report. This station is close to the location of the Station I of the early reconnaissance (see sketch map, Reighard, l89l4-b, facing p. 60). The thorough work of Reighard and his associates done in 1893 provides an excellent base-line for examining the biological changes which have taken place in half a century. Current observa tions suggest that a full-scale ecological investigation of Lake St. Clair would yield valuable knowledge.

To what extent hydras occur in the Detroit River is not known. But at times they constitute a nuisance in waterworks operations at the City of Detroit. For instance, in the fall of the main filters from the settling tanks became clogged with a salmon-colored jelly. Samples brought to

Wayne university were found to be composed of living hydras.

They had been pumped into the tanks from the intake at the head of Belle Isle (personal communication, Dr. Charles W.

Creaser). Perhaps, in this instance, hydras were components of the plankton.

- 5 9 - Lake Erie island Ponds

The most common species found in the permanent ponds of the Bass Islands was H. oligactis. Fischer’s pond at Middle

Bass Island, where thrifty beds of Anacharis canadensis grow, appears to provide the best of the pond habitats. Budding oligactis were always present on the leaves of this water weed during summer visits to the pond. Few hydras were ever found, however, on the leaves of the homwort, Ceratophyllum demersum, which is interspersed In the Anacharis growth.

A few green hydras (Chlorobydra vlridissima) were found on the Elodea plants during the summer of 195^. Haunck's Pond, on Middle Bass island, is quite stagnant, and It is noteworthy that specimens occasionally taken there were living close to the surface film on the undersides of the thalli and dangling roots of Lemna minor. Scotland

(193l|-), in her study of the Lemna association, mentions hydras among the facultative residents of the duckweed com munity, attributing their presence to the supply of Crus tacea and insect larvae which swim among the maze or roots.

The better oxygen supply of the water at the surface where duckweeds float is also a factor in maintaining the hydras in this association. Ewer's experiments (191+7J demonstrate that the response evoked In adult Hydra by increased carbon dioxide serves to bring them up to the surface when there is a shortage of oxygen lower down. In the still waters of

- 6 0 - ponds, hydras are sometimes seen suspended from the surface film. Here they establish a niche in the surface-film com munity by feeding on some of its members and on the true plankton.

The single permanent pond on South Bass Island, known as Terwillegar's pond (Pig. 2), is connected with the lake by a narrow channel. Reversing currents caused by seiche oscillations produce a rhythmical inflow and outflow of waters between the pond and the bay (Krecker, 1928). In addition, during blows from the northeast, water from the lake piles up into the pond, raising its level as much as three feet, and then resurges into the bay when the wind reverses or subsides. (See Langlois, 195l|., Pigs. 1 and 2, for photographs of Fishery Bay and Terwillegar's Pond at high and low levels resulting from wind action, March 22-23, 19S2.)

True pond conditions exist in the shallow waters border ing the channel where patches of Myri ophyllum, Potamogeton, and Vallisneria take root in the mucky bottom. Stands of

Sagittaria latifolla grow along the shore and In the tempo rary pool which forms near Its northern margin. The bottom near the shore is covered with dead leaves and branches, mainly those from the surrounding white oaks.

The communication of the pond with the lake and the varying substrata in the pond waters afforded an opportunity to make some casual observations on habitat selection of hydra species existing in the lake. Accordingly, collections

- 61 - of plants or bottom debris from the ponds were usually made

on dates when collections at station 5 were made. It was found that all types of plants In the pond supported small

aggregations of H. oligactis during seasons when this species

were abundant on the Myri ophyllum plants at station 5 nearby in Fishery Bay (Fig. 2). Specimens of H. pseudoligactis and

H. americana were sometimes found on vegetation in the pond

at those periods when they were appearing in company with H. oligactis in the bay. On the other hand, H. littoralls,

the most widespread species in the bay, was not taken in

collections of plants from the pond. Yet that individuals

of this species gained ingress to the pond was evident from the presence of specimens on branches and dead leaves caught

on the inflow side of a net set at the inlet. Occasionally a specimen was also taken from the gravel bottom on dead leaves or branches well inside the pond channel. This obser

vation lends support to the hypothesis that H. littoralls requires moving water as a condition of existence.

Dead tree leaves appear to be the main means of trans

port between the H. oligactis population of the pond and

that of the bay. A slide rack was kept suspended from the

bridge over the inlet at a depth of about one meter from the

channel bottom during the 1951-1952 season, but settlements of hydras did not appear on the slides. Nor were any hydras

captured in plankton tows made from the bridge. It was ob

served, however, that a few leaves drifting along the bottom may carry a dense propagating hydra aggregation. For example, - 62 - on May 17* 195?lf, five leaves picked at random from among

those caught in the bottom of a net set at the bridge yielded

50 H. oligactis, 60 per cent of which bore buds. Leaves carried by the current may also serve to transport embryonic

hydras. Thecated eggs of H. oligactis and H. pseudoligactis

were found adhering to leaves picked from the net on Decem

ber 11, April 8, and May 18, 190?k-

The temporary pool adjacent to the pond also harbors

H. oligactis. On June 19, 19£2, eight stalks of Sagittaria

pulled from the pool supported budding hydras, having

blackish hypostomes.

«• -s* «•

The uncharted swamp located on Pelee Island at the

beginning of Pish Point was the only place where Hydra came a was found. The green hydra, Chlorohydra vlridissima, also lives in company with H. carnea in this habitat. Individuals

of both species, collected on October Ilf, 19J>3, became sexual and definitive determinations could be made.

Members of the two species form part of the Lemna asso

ciation discussed above. They were not present in samples of debris scooped from the mucky bottom. The swamp is very shallow— about three feet in depth — and most of its surface during summer and autumn is covered with Lemna minor inter spersed with Spirodela polyrhiza. Prom observations in cul ture, it appears that both species of hydras may occupy the same raicrohabitat, living attached to the fronds and roots

- 63 - of a single duckweed plant. Budding H. carnea were collected from the duckweeds when the pond was revisited August 6, 19Sb* tout no green hydras were found. The water temperature was 21.2°. The absence of H. oligactis, generally the commonest pond form, suggests that the water in the shallow Pelee habi tat may have been too foul for this species even at the sur face. Specimens of H. carnea and C. viridissima thrived in a battery jar culture at room temperature for three and a half months without change of water. The green hydras sur vived the other species in the fouled water, but when

H. oligactis specimens were placed in it, they succumbed within two days. Wilson (1891, p. lj.19) mentions that "H. viridis [= (3. viridissimal is far more hardy than H. fusea [ = H. oligactis], being able to live for many days or weeks in foul water that would quickly prove fatal to the latter species. This power of endurance may be due to the liberation of oxygen through the assimilative action of the chlorophyll." As mentioned in the introduction, neither H. carnea nor

C. viridissima have been taken in Lake Erie. The only record of the occurrence of the green hydra in the Great Lakes or its waterways is Reighard’s old record (189l|.to) for Lake St.

Clair. As pointed out above, the green hydra is easily recognized and is cosmopolitan in distribution. How does one account then for its rare occurrence in large lakes?

- 6Lj. - It seems quite likely, for example, that members of the pelee swamp green hydra population are carried into waters of Lake

Erie on duckweeds that cling to the feet of the various water birds which frequent the pond. The margin of the swamp reaches to within about I4.O feet of the lake shore on the west side and to about 15>0 feet on the east side. The nets, set in the lake adjacent to the location of the pond at Fish Point, would afford an excellent substratum upon which an aggregation of these green hydras might occur. They will attach to pieces of the net twine suspended in a culture.

But they are very sensitive to the slightest disturbance; they contract and are easily dislodged from their place of attachment. What probably happens is that the green hydras are transported into the lake, but fail to gain a foothold on any of the substrata in its agitated waters. Hence they do not produce a population by budding, and their transitory occurrence is not notices by biologists.

Wave-swept Shore s

Further Investigation may disclose that H. littoralls, which predominates in the region of the Bass Islands, is the most widespread species of the shore zone In Lake Erie, and possibly in the other Great Lakes. Hyman (1931a, 1938J originally found the species in the Yacht Harbor in Jackson

Park, Chicago, on wave-swept stones where the harbor opens into Lake Michigan. No records of Its discovery in Lake Michigan or the other Great Lakes have been published to

- 65 - date* All records (Hyman, 1931b, 1938; Trowbridge, Bragg and Self, 1936; Bragg and Self, 1937) report the species as occurring in running waters or on wave-swept shores.

As the Lake Erie findings indicate, H. littoralls is not resident in ponds or in the still water of lake vegetation zones. It was not found in deep-water collections. Collec tions made along the shores of South Bass island during the season when the hydras were abundant revealed that like other species It was absent from sand, clay, or smooth pebble beaches. Nor was it present on the bare shelving rock shore. At South Bass Island and the other Lake Erie islands, the raicrohabitat of the species was rubble of miscellaneous sizes. (See Krecker and Lancaster, 1933, for photographs of island beaches in Lake Erie and classification of rubble types.)

These stout, long-tentacled hydras are an integral part of the typical swift-water community of the island shores.

Pieces of dolomite block rubble — strewn by water action from the shoreline to the three-meter zone — furnish the principal supports for the plants and animals which make up a complex association of sessisle, sedentary, and clinging forms. Characteristically, H. littoralls Is the only coelenterate species having permanent residence in the community.

Seasonal abundance of the swift-water hydra, Its interactions with various associates of the microhabitats as observed in the Fishery Bay study area, are discussed In subsequent sec tions. - 66 - PART III

COMMUNITY INTERACTIONS WITH SPECIAL REFERENCE TO HYDRA LITTORALIS

Our knowledge of the ecological niches which hydras

occupy in the communities of major habitats is very limited.

Only in the case of the "common hydra" (H. oligactis) do we

have some information concerning "the status of the organism

in its community," i.e., the "niche," as Charles Elton (1927)

originally used the term. This is not surprising in view of the fact that only three full-scale studies — all dealing

with H. oligactis and carried out in this country — have been

published on the ecology of hydras, insofar as I have been able to ascertain (Welch and Loomis, 192l|.; Miller, 1936; Bryden, 19^2).

The first two of these investigations were made at Douglas Lake, Michigan in South Fish-Tail Bay and the adjoin

ing Big Shoal area; the most recent was done at Kirkpatricks

Lake, a shallow, privately owned lake located in Tennessee. All are essentially population studies dealing with the verti

cal, horizontal, and seasonal distribution of H. oligactis.

Standard limnological procedures are applied in the measure ment of factors which might affect the observed distribution.

These studies comprise most valuable contributions

toward an understanding of the ecology of hydras. But since

- 67 - they are primarily devoted to problems of organization at the species population level, little attention is devoted to qualitative observation of hydras as members of the biotic communities of the raicrohabitats. The field methods employed precluded adequate observation of living specimens, which must be made at the time of collection when studying hydras in order to ascertain the niche of the species. Consequently, except for a few observations recorded by Miller, these papers contain little information concerning interactions of hydras with other organisms. Questions centering about food relations, so essential to an understanding of the ecological niche, are left unanswered.

Prom the inception of the present study, it was recog nized that conditions in Lake Erie habitats made it imprac ticable for the individual investigator to approach the ecol ogy of hydras at the species population level. It was felt, however, that an effort to discover and describe the niche of the predominant species was a worthwhile objective of re search. Accordingly, from observations made during the period® when it was possible for me to make collections in Fishery Bay, I have endeavored to identify the organisms closely associating with hydras in the communities of the microhabitats, to work out some food relations, to determine the nature and extent of parasite-host relationships, and to note phoretic associations with molluscs and arthropods.

The results of these observations on interactions, though largely qualitative and in many respects fragmentary, - 68 - are summarized at the present time as a contribution toward an understanding of the natural history of hydras. A criti cal appraisal of the meager literature on the subject and some suggestions for further research are made during the course of discussions.

Attention has been focused on the interesting endemic

American hydra, E. littoralis, for two reasons; it appears to be the predominant hydra species in the communities of the wave-swept, rocky shores of the Lake Erie archipelago; its ecology has not been previously studied. The collection method used in Fishery Bay to facilitate rapid examination of thiB species and its associates has also yielded some quantitative seasonal data which may represent a rough measurement of relative abundance, reproductive potential by budding, and the duration of the sexual period.

METHODS USED IN THE STUD3T AREA

Reconnaissance during 1951 at the beginning of the study established that suitable habitats for aggregations of Hydra li ttoralis existed in Fishery Bay of South Bass Island where the research laboratory was located. The horizontal distribu tion of this species, which had become the focus of interest, was ascertained by the summer of 1952. Except for stray agg;regations attached to clam shells or dead leaves dredged from the center of the bay, the members of the local popu lation were confined to rocky shore habitats of Gibraltar Island and the rubble beaches on the South Bass Island sides - 69 - of the bay (see map, Pig. 2).

The methods mentioned In the section on discussion of habitats were used in these intensive collections. Transects of the bay were made during summer and autumn months, through

the ice cover, and during the spring. The lines of transect were three t (1) Peach Point from the angular rubble beach to the open water of the bay with a maximum depth of about six meters at the north point of Gibraltar Island; (2) Laboratory

dock through the weed beds, over the mud bottom at the center of the bay to the opposite rubble shore; (3) Oak Point to the

south tip of Gibraltar Island along the gravel bar, where

the water is so shallow only small boats can cross with safe

ty into the sheltered waters of Put-in-Bay proper.

Seven transects in all were made, one each during the

following months: July, September and November, 19J?1; Janu ary and February (through ice-cover), April and June of 195>2.

Samples were taken at varying space intervals, depending upon weather conditions. At the shore zone pieces of rubble of various sizes and shapes were grappled from the bottom at half meter intervals. Hydras were always found on these

supports except during the period of the Ice-cover. Along Gibraltar Island the rubble Is strewn from the shore line out to a depth of about six meters. These pieces of dolo mite broken from the cliff face contributed the microhabitats for the aggregations of H. littoralis. Occasionally a few

small short-tentacled hydras were found in the same micro habitat with the long-tentaoled unstalked form. These were - 70 - L A K E ER TE

PEACH POINT o 6

GIBRALTAR. ISLAND FISHERY BAY o 5 TERWILLl GAR’S POND

O A K POINT

SOUTH B A S S ISLAND POT-IN-BAY S C A E £ AV F £ * T

TOO ZOO BOO

Fig. 2. Map shoving location of collecting stations in Fishery Bay. (Shoreline from pantograph tracing of U.S. Lake Survey Field Sheet No. 1-1891, sheet 5 of 15, dated 1949-50.) later Identified as H. americana (see I: 3-c). The restriction of the oligactis-pseudoligactis popula

tion to the beds of water plants in the comparatively stiller

waters of the bay along the South Bass island shore has

already been discussed. It will be recalled that H. lit-

toralis is not a plant dweller. Vertical distribution does

not enter into the ecological picture of this species there fore.

It seemed desirable to have a rough estimate of the

horizontal seasonal distribution of our species. Collec

tions made with Dr. N. Wilson Britt from his concrete block

stations along the Gibraltar island shore in 19^1 indicated

that the mean number of organisms per square meter of rubble bottom might be determined from the samples. (See Britt,

19£5b for description of this new method; depths and loca

tion of stations are given in Table III.) But the operation

was not feasible because it required two men in the field;

the labor of separating the hydras from the mass of peri-

phytic and clinging organisms obtained in these large samples

also made collections at frequent intervals impractical.

A measure of the bottom fauna could also be obtained by

submerging rubble in a square decimeter frame sieve of sturdy

construction and sampling the pieces or rubble periodically

with a two centimeter hollow-square scraper such as Young

(19lj.5>) used in his quantitative study of periphyton. This method is not practicable, however, in rough water over half

- 72 - a meter deep. Consequently, a slide-rack and stone-anchor rig was devised to meet the requirements of conditions in the study area.

In general, the rack for holding the microscope slides followed the design originated by Bisonnette (1930). This type of rack with improvements was first used in a hydra study by Miller (1936). Specifications are available in Welch»s manual (19i|.8, pp. 260- 2 6 2 ) . Racks built according to these specifications have been employed as the best method of measuring the mass productivity rate of periphytic orga nisms by suspending a series in a deep lake at different depths ( Newcombe, 195>0).

To fit the slide rack for withstanding the wear-and- tear of the rock-bottom situation, the strips of cypress composing the frame were securely fastened with brass screws.

The strength of the rectangle was increased by insetting the end pieces. The hardware screen of the lid was faced with galvanized metal at the locking edge. To protect the screen on the bottom and to give the rack proper counterbalance, a heavy lead seine weight was sawed lengthwise and screwed to the lower side of the end pieces.

Slots on the inside faces of the long pieces were cut to accommodate a pair of slides instead of a single slide.

The exposed surfaces of a pair can thus be turned up in a petri dish containing water. A single slide must be propped with capillary rods to prevent crushing of the delicate organisms. Also the slide must be turned after one side is - 73 - examined. Organisms on both sides also make counting more difficult.

When loaded, the rack contained a set of 19 pairs, each two slides spaced about 12 mm. apart. It weighed about two pounds.

To hold the rack in place at the bottom stations, a piece of square rubble, about 20 x 20 x ^ cm., and weighing around 10 pounds, was selected for an anchor. "Kordite" (approximate strength 300 pounds) was used for all lines in the rigging; piece of one meter attached as sling to rack by side-piece eyelets; cross-ties to cradle anchor; and line attached to light cedar marking float. The rack was bridled to the anchor with a bowline bent through rack sling and anchor cradle with the free end of the float line. (This knot has the advantage of being as rapidly untied as it Is tied, and does not shear the "Kordite11 like permanently attached metal clips.)

When the rig Is lowered into the water, the rack is so bridled and weighted that it floats free from its anchor.

It comes to rest on the bottom in a horizontal or tilted position, depending on the size of the rubble present, but it Is counterbalanced by the seine leads so that it remains lid-side up. The slide rack and stone anchor lie in juxta position in an area roughly one-half meter square. Some Idea of the rig In operating position may be gained from the photograph (Pig. 3).

- 7k - Fig. 3. Photograph showing slide-rack and stone-anchor collecting rig exposed on bottom at Oak Point bar, station 4, during seiche of December 10, 1953.

Fig. 4-. Photograph of station 1 location between the two large dolomite rocks (at left) split off from Gibraltar Island cliff face.

- 72 - Trial collections made with slide-rack and stone-anchor

rigs during June and July of 195>1 indicated that it provided

the most suitable collecting method for the purposes of the

study. It was found that a sample of the hydras and asso

ciated periphytic organisms could be obtained from the

rubble-bottom habitats in an immersion period as short as

a week. Hydras from the rubble on which the rack and anchor

rested settled on their surfaces and colonized them. On the stone the hydras were attached to the sides in clusters,

seldom on the top surface, and never on the bottom surface

unless the square block had been tilted. On the rack, they appeared on every kind of material except the brass hinges

of the lid. Some also attached to the "Kordite" and migrated

up the float line unless it was slimy with blue-green algae. It appeared that the hydras would attach and grow on

freshly cleaned surfaces as well as those on which periphyton was already growing, unless this growth was exceedingly

dense. This assumption was later confirmed by experiments made in the hatchery trough. Observations made then also

afforded a basis for interpreting microhabitat selection of the hydras with respect to influences of silting, current,

and tilt angle of the surface. These matters will be dis

cussed later (see III: 2,3). The following procedure was followed in working the rigs:

(1) Float was grasped, and line overhauled into rowboat. Slide rack and stone were immediately immersed in separate - 76 - containers which had been filled with surface water as the station was approached. Bowline in float-line looping rack- sling to anchor-eradle was untied and float with line attached was stowed, pan containing rack was covered with lid to protect hydras from sun. Stone was submerged in large bucket. Collection was immediately worked over in laboratory. Precaution was taken to keep containers cool.

Strained lake water from the tap was used in all operations.

(2) Settlement from stone was removed by brushing sur faces thoroughly. This was done in the bucket of lake water containing it. Stone was stowed under water in separate collecting bucket. Washings from original collection water were strained through a standard sieve. (0.25 mra. opening, TJ. S. Sieve Series, Mesh No. 60.) The sievings were then washed into finger powls or petri dishes with a syringe. Amphipods had been counted and discarded during the sieving operation. Snails were segregated immediately, checked for hydras, identified and discarded. The periphytic residue was held in a constant temperature cabinet set about 5° below lake temperature while the slides were being inspected.

(3) Counts of hydras were made from every pair of slides in the rack except the pair in the middle slot. This was preserved in a screw-cap jar in five per cent formalin for reference later. Tallies are thus recorded on the basis of 18 slides. The hydras were segregated from the rest of the periphyton by searching each pair of slides under a widefield

- 77 - "binocular microscope. This was accomplished by turning each pair of slides free-face down in a separate clean petri dish filled with sufficient water to allow the hydras to extend. The hydra was separated from the glass substratum by grasping it gently near the pedal disc with a pair of jeweler's twee zers and transferring it to the water of a petri dish. A number 3 D Dumont Fils tweezers serves best for this manipu lation. With a little practice the holdfast can be loosened from the substratum without injuring the delicate tissues of the animal. Hydras containing undigested prey or those har boring commensals or parasites were isolated in separate dishes. All sorted hydras were then stored in the constant temperature box. A couple of representative slides was also held for further examination. The rest were dumped into chem ical cleaning solution.

(ij.) Hydras were then sorted from the settlement removed from the stone-anchor. In this operation two jewelers twee zers were needed to disentangle the hydra from the mass of periphyton. The animals were held with a tweezer, inspected, tallied, and segregated into respective petri dishes for further observation on food and parasites. Specimens of the larger microfauna, along with selected snails and insect larvae were preserved for identification. The residual peri phyton and the assorted hydras were held in the constant temperature cabinet.

(f?) The empty rack was then cleaned by brushing and

- 78 - loaded with fresh slides. Slide-rack and stone-anchor rig was then transported to Its station and reset. On the return trip, the rig from the next station was picked up. Work on the collections was completed during the evening as a rule. Identification of the hydras must be made with living material (see I: 2). The specimens containing food or carrying parasites were given first attention. Spot identifications to genus of the microfauna and flora eventu ally became possible. Counts from selected slides were made to estimate the density of certain of the predominant forms and comparisons were made with the associates of the micro habitat from the stone-anchor collections.

The record of findings for each collection was kept on a separate sheet. These were organized for reference by stations in a ring-binder. Laboratory notes were recorded at the time of observation on these work sheets along with all field notes pertaining to the collection such as weather conditions, water temperature, hour of lifting and resetting the rig. Temperatures in the laboratory were also recorded. On this sheet, set up at the start of the collection, the counts of hydras tallied by number of buds and number of ten tacles on clicker counters were also recorded.

It should be mentioned here that by using direct illu mination from a water-cooled light source and powers of from

8.7£ to 37*£ diameters, one can clearly see the condition of the hydras attached to slides under the widefield stereoscopic - 79 - microscope, and recognize ectoparasites and commensals, some

of the larger arthropod prey, and other animals in the micro

habitat. Thus also it was possible to tally the counts of hydras made at the sort just after collection by number of

buds as well as tentacle number. Incidence of abnormalities

such as split tentacles, duplication, and the like, were recorded for future reference. During the course of the

study a total of about £,000 living individuals of the littoralls species were closely inspected on the slides alone. The method of using slides for securing hydras to be

observed at the time of collection has distinct advantages. Miller made some use of his slides in this manner, but his

standard practice was to estimate the total number when a

rack was lifted by counting the number on a single slide placed in a little jar of water. This was done in the field

and the rack with its large settlement again suspended in

the lake. With the small number of hydras colonizing slides during short immersion intervals, counts per slide showed this method to be subject to great error because of the

tendency of buds to settle close to the parent hydra. Miller found that very few hydras were lost from slides

when a rack was lifted through 9 meters of water. Towing a

rack containing slides bearing a known number of hydras 100 meters through the open water produced a 10 per cent loss.

To test the extent to which littoralis might be dislodged,

a rack was pulled rapidly through a nine-meter stretch of a

hatchery tank just before it was to be drained and scrubbed. - 80 - Six specimens were sieved from the effluent water at the drain. Around 300 hydras remained attached to the slides.

Other observations made in the hatchery trough also showed that hydras are not dislodged from glass, wood, or rubble

supports by swiftly running water. As Miller concluded, few? hydras are probably lost when a rack is lifted except when the slides are thick with heavy periphytic accumulations from long submergence in summer.

Since none of the stations in the bay finally chosen for routine collections were located at depths much beyond two meters, there is little likelihood that the significant losses from the slides occurred in lifting the rigs. Wave action apparently does not tear the hydras loose from their microhabitats in any number. This is strongly indicated at any rate by failure to obtain a single hydra in horizontal and vertical plankton tows made in the bay after heavy blows

(see II: 2-b).

SEASONAL ABUNDANCE To arrive at even a crude estimate of the actual size of the H. littoralis population in the area at any given time was obviously not possible. The ’’ecological density” of a hydra population (i.e., the number of individuals of a species per unit area of habitat that can be colonized by the population) can be determined in a pond or a small shal low lake by application of proper methodology. For example,

Bryden was able to determine the size and some other

- 81 - parameters of the H. ollgaotis population in his Kirkpatricks

Lake study (19£2). Under the conditions in Fishery Bay just described, it did seem possible, however, that periodical collections by the slide-rack and stone-anchor method might provide quantitative data which would reflect changes in the size of the population. A measure of density per unit time has special value in ecological studies of sedentary organ isms which may multiply rapidly by asexual reproduction. Time-relative density may be taken as a fairly reliable index of seasonal abundance provided sampling methods and size of samples are adequate (Allee et al., 1949, Ch. 18). Because of the high reproductive rate by budding, changes in the size of a hydra population can occur quite suddenly. The tendency of the offspring to settle close to the sedentary parent results in a highly "clumped” disper sion in the available habitats. Since only a relatively few samples could be taken at one time, these peculiarities inherent in the population were kept in mind in selecting collection stations and intervals between collections.

Five rubble-bottom collecting stations were chosen on the basis of the reconnaissance experience. These are located by numbered circles on the map (Fig. 2).

The three stations along the shore of Gibraltar island and the station at Peach Point were most exposed to violent wave action during storms. The station at the Oak Point bar was the most protected of the rubble-bottom stations. This station also differed in the nature of the bottom; fewer

- 82 - scattered pieces of rubble rested on the gravel bar (see

Pig* 3) • Here also the water was shallower — about one meter In depth. At the shallowest station on the other side of the bar the rig lay in only one-half meter of water. At these two stations the rigs were tied off the shore. At the other stations water depth ranged around two meters, and the rigs were located by floats.

At station 1, usually the roughest water station, three rigs were worked during the 19i?2 collections. These were spaced in a line so that the first was at a depth of six meters, the second at four meters, and the third at two meters. Their locations are designated a, b, c respectively on the map. Subsequently frequent loss of the two rigs lying out in the deeper water occurred with the opening of the bass-fishing season. So collections at station 1 were limited to the rig located in two meters of water between the large dolomite rocks split off from the Gibraltar Island cliff face (see photograph, Pig. 1|.).

These five stations then provided samples which took into account the "dispersion factor" of the littoralis popu lation. The collections from the three stations located on the Gibraltar shore were usually made and worked up on two consecutive days, those from the other two stations the fol lowing day. Each station was handled as a separate operation according to standard practice outlined in the section on methods. Thus the immersion intervals at all the stations

- 83 - were kept fairly unifoim. Variations resulting from stormy weather or other situations beyond the operator’s control

are noted in Tables IV - VII summarizing the collection data.

Since effort was concentrated on obtaining adequate

quantitative samples of the llttoralls population, only

rough qualitative sampling the mixed oligactis-pseudollgactia

population in the bay was attempted, periodic collections were limited to one station near the laboratory dock (num

bered 5 on map). The habitat here consists of a dense stand

of Myrlophyllum ex albescens. This milfoil is one of the

plants, it will be recalled, on which Krecker (1939) found

hydras most numerous In his comparative measurements of ani

mal populations on submerged aquatics in the island vegeta tion zones. The milfoil grows throughout the year. At

station 5, it Is rooted in muck at a depth of about two meters. This part of the bay is the least subjected to

agitation from waves. (See ll:2-d for description of this habitat.)

A rough estimate of seasonal fluctuations in the oligaotis - pseudollgactis population could be made by sampling

the milfoil bed at station 5. Washings from a quart-jar full

of the plants, packed loosely, usually yielded hydras in sufficient quantity to constitute a fair count for proportion

budding. However, the taxonomic difficulties involved made it impractical to separate the two forms In routine field work. Miller encountered the same obstacle In his work with these two similar-appearing stalked forms, and was obliged - a ii . - to group M s otherwise valuable quantitative data. As in the Douglas Lake mixed population, it was found by nematocyst examination of a few samples, that ollgactis outnum bered pseudollgaotis about ten to one. Fortunately, these two forms did not occupy the same habitat as the members of the littoralis population. When heavy seining operations through the vegetation zone of the bay began in 19£ij., Indi viduals from the still-water population were intermixed with the littoralis population early in spring. But taxonomic analysis of the collections showed that the stalked species did not remain permanent residents of the swift-water asso ciation as the season progressed.

A slide-rack and stone-anchor collecting rig was oper ated on the bottom at station 5. T M s was lifted and counts made on approximately the same dates as those shown In the tables for the rubble-bottom collections. In t M s way some idea was gained of the extent to w M c h the hydras might utilize supports on the bottom when the Myriophyllum microhabitats became physiologically unfit for them. Also a slide rack was kept suspended at one meter depth from the laboratory dock near station 5. T M s was examined at periods when the hydras on the water plants appeared to be increasing or declining.

The single samples and the small numbers resulting from them rendered the data unfit for statistical treatment.

Consequently, only the crude data will be referred to in

- 85 - comparing certain differences which seem to exist between the seasonal cycles of the ollgactiB-pseudoligaetls populations and the littoralis population.

The Annual Cycle The seasonal data obtained from the slide-rack collec tions during 19£2-195>^ are presented in Tables IV - VII. In collating the crude data from counts of the total numbers of hydras and the number with buds from station tallies, the mean number of each per station was calculated. Ratios be tween these means at certain immersion intervals and analysis of their trends through periods of critical changes in water temperatures provide a rough picture of the hydra cycle. Graphing of this data proved unfeasible because of the varying immersion intervals used during different years.

Also lack of sufficient quantitative data for the winter, spring, and early summer months made extrapolation unjusti fiable. But by drawing upon qualitative information and grouping the quantitative data available, a projection of an annual cycle which may be typical of H. llttoralis In western Lake Erie can be synthesized.

This cycle is diagramatically represented against the plot of five-day mean temperatures for the period of the study in Figure

The nature of the cycle can best be discussed under the six functional seasons which affect community periodicity in large lakes of the temperate zone. Interval dates and

- 86 - temperature ranges are for the study area. But, tempera tures as well as oxygen, carbon dioxide, and pH are remark ably uniform through the open-water littoral zones of the island region according to measurements made by Krecker and

Lancaster (1933). The study area, as earlier discussion of abiotic factors indicates, forms an integral part of the lake ecosystem. (See Verduin, 19£>6 re unpublished data by

Stansbery showing that even in calm weather the flow through the bay renews the water at least once a day.) In the fol lowing summary of the hydra cycle, the quantitative data available in the tables is reviewed in context with addition al information accrued from field observations. Hibernal Season. Middle of December to end of March: below I4.0j under ice cover at 0.2 °; up till the last week in March at 2°.

Hydras are dormant. The few that can be found on the rubble in the near-freezing water are small and white. They are without buds. Colonization of substrata over long immer sion intervals is so meager that collections are hardly worth the labor. (Tables III, IV, VIJ also no hydras settled on a series of racks at stations 1 and I4. set Feb. 12, 19£2.)

P re vernal Season. The month of April: the water slow ly reaches 10° by the end of the month. Heavy blows roil the lake.

Production of new hydras from winter survivors is very slow. This Is reflected by absence of settlements on slides in racks which have survived break-up of the ice (Table VII). - 87 - Toward the end of the month, when the water Is warming to

10°, a few budding individuals can be secured from the rubble at the bay stations. This is probably also the time when some new clones are originating from hatches of winter eggs (see pt. IV, Sect* 3)« Whatever the source of the small prevernal population may be, the nature of its repro ductive potential soon becomes apparent.

Vernal Season. Beginning of May to middle of June: o temperatures rise from 10 to 20 , mounting quite rapidly in some years, slowly in others. The population increases with a burst of budding. Aggregations are composed of stout, active individuals.

They are now yellow or orange-colored from pigments of digested food. The majority of the animals bear one, two, or three buds — a few as many as four.

Asexual reproductive activity apparently begins as water temperatures approach 10°, During 19S>2, results of this activity were just becoming apparent at the opening of the season (Table III). Slide racks submerged at stations 1 and ij. throughout May yielded sizeable settlements at the end of the month. In samples of 1 0 0 hydras from these col lections, about $0 per cent of the individuals were budding.

Again during the 19S>1|- season, a remarkable increase in the size of the settlements for comparable immersion intervals is seen in collections made at the beginning and end of the season. The proportionately high number of individuals

- 88 - bearing buds at the beginning of the season (I4J4.-67 per cent) contrasts with lowered number (2£ per cent) at the end of the season (Table VII). This may simply mean that the aggregations at the end of the season were largely made up of young individuals which had not yet reached the age of bud production. Collateral evidence concerning the nature of reproductive potentials in the hydras, which will be reviewed later, indicates that although a relatively large percentage of budding individuals is characteristic of a growing population, the rate of growth of the population actually depends upon rate of bud release. In the Lake Erie seasonal cycle, the growth rate of the population is proba bly the most rapid during the vernal season.

Aestival Season. Middle of June to end of August: mean temperatures permanently above 22° throughout July and

August, ranging around 23 or 2lj.° during August. This is the period of maximum abundance. The hydra pulse reaches Its peak toward the end of June. Additions to the population decline during July, fluctuate during August, But the numbers of hydras remain fairly abundant ■until the end of the season.

The eruptive nature of the population Increase before mean temperatures level off above 22° is reflected in colloca tions made at the end of June. Slides submerged for only a week accrue settlements as large as slides submerged for a month prior to the end of the vernal season (Table VII).

— 89 — The ratio of mean number of hydras per week immersion inter val at the beginning of the pulse and the end is about 6 to

1. The population may be reaching its summer maximum under optimum temperature and food conditions. Space-relative abundance at this peak may range from 30 up to 1700 animals per square meter of rubble bottom (Table III). Decreasing reproductive rates after the hydras are sub jected to continuing temperatures above 22 ° may be the cause of the declining trend in time-relative abundance during

July (Table VI). A significant drop in ratios of budding to non-budding individuals is noticeable at the end of July.

The mean number of hydras on slides submerged throughout the month is about the same as those accruing during an interval of a week at the beginning of the season (Table VII). Possibly certain clones in the population become accli matized to the high temperatures and continue to reproduce at a reduced uniform rate during August. The slight fluctua tions in mean numbers per weekly interval (Tables IV, V, VI) is characteristic of the so-called population equilibrium state. A downward trend is noticeable at the end of the month.

Serotinal Season. September to middle of October: tem peratures drop below 2 2 ° and sink to II4.0 as the warm mass of water begins to lose heat to the atmosphere. This is the period of Indian summer in the island Region. The late summer season Is apparently a critical period for the hydra population. When a midsummer calm occurs and - 90 - temperatures mount beyond the usual annual summer mean of 22° toward the end of August, the population goes into a sudden decline furing early September. The surviving hydras are small, white and without buds. Also they show the symptoms of "depression" — a pathological condition discussed in detail later. But even after such a catastrophe the popula tion re-establishes itself by the end of the serotinal sea son (see plot of 195>3 temperatures, Fig. 5> and Table VI). In normal years also some decline in relative abundance just after the close of aestival season occurs (Tables IV and V ). The mean numbers of hydras per square meter of rubble bottom range from 3 to 173 — a distinct contrast to the large numbers at the early summer peak (Table III).

Mean numbers fluctuate as the water cools, and by the end of the season abundance may be relatively high.

Autumnal Season. Middle of October to middle of Decem ber: temperatures drop rapidly to 10°, then decline toward i|i° as the water approaches its maximum density.

The decline in relative abundance as measured at the opening and the end of the season is striking. Decrease in mean numbers per square meter of rubble bottom area as a whole is about 85 per cent (Table III). Abundance at the early autumn peak appears to be as great as that of the early summer peak. The autumnal decline is of about the same order of magnitude as the decline from the aestival maximum to the late summer minimum. Tables IV - VI show

- 91 - that the autumnal decline begins after the water cools below o 10 In November and gains momentum as the season closes. In

December about 2£ per cent of the hydras are still bearing one or two buds. But after the dormant period of winter sets in budding ceases. ■ft "5fr ■M1

The annual cycle of H. 11ttoralis in broad outline Is characterized by two population pulses: one during the ver nal season resulting in the summer maximum, another in the autumnal season after a phase of late summer decline. The autumnal maximum is reached at the time in the life cycle of the hydras when some are entering Into the sexual period.

As a small portion of the population is developing sexually during autumn (see Part IV), the population as a whole is dwindling. Asexual reproduction gradually ceases, and hiber nation begins with the onset of winter. Remnants of the population remain dormant in the near freezing water — the standing crop for next year's hydra cycle. (See diagram matic presentation, Pig. £.)

•a ■» •*

A similar pattern of seasonal abundance was observed in the oilgactls-pseudollgactis complex. However, the stalked species Inhabiting the vegetation zone of the bay evidently maintain a larger standing crop during the winter. Also the growing period starts earlier than In littoralis. - 92 - m a r c m APR/L M A Y JUNE JULY A U S U S 7 SEPT. OCT. NOV. DEC. JAN. FEB. to ?o nc '13 :o ; ir. 211 ?

^ <2. daijr/w aa^ H. //tlorotis H. oJ/gactis I- H. poot/dohiaacits PERIOD OF MAXIMUM SEXUAL P E R IO D S -I9 S Z 2*- ABUNDANCE

2 7- 19 Si • 26^ » 2 5 ~ /SB?* , • a 2 4- ’ fi

» - VERNAL PULSt

2 2 - FROM

7 1- BUDDING HYDRAS

20 - e ♦ •9 - /»- n - /«- is- 14- . I n- iz- //. N E W CLONES fiBOM ! *• iO- WINTER EGGS AUTUMNAL DECLINE - HIBERNATION 9 - 8- 7 — « e + * 6 r • r 5 -

4 - * 1 Fig. 5. Diagram of the annual hydra cycle In j western Lake Erie. (Five-day mean temperaturea °c.,' 3 - 19S1-195!).; legend _19S l * , 1952 * , 1953 o , 1951*. t- ) ; 2 - • n. ' k *. * ' • • a c-c-o -0-0 —O-O- Q-.O—:] 50 fS JO IS MAY JumB vtUiY AUGUST S £ P T . OCT, Q C C . J A N . BBS. 3 Some budding individuals can be found under the ice- cover in February. At the beginning of the prevernal season, a sample of hydras can easily be obtained from a quart of the Myriophyllum, which is starting to flourish at station

By the middle of April, such samples are composed of yellow ish or orange-colored individuals, 5>0 P©x* cent of which bear from one to seven buds. Throughout May budding aggregations become abundant on every type of vegetation in the bay. In contrast with the lagging littoralis, the spring pulse is over by the end of the vernal season. Maximum summer abundance is reached during June and early July. As the aestival season progresses, it becomes

Increasingly difficult to obtain samples of the ollgaotls- pseudoligaotis population. Zoologists working at the labora tory in years past have also observed this disappearance of the hydras during the summer season.

An increase in numbers is noticeable during October.

After the late autumnal decline some members remain reproductively active in the winter waters. -ts-

From the foregoing presentations it is obvious that the hydra populations wax and wane with changes in the annual temperature cycle of the lake. A range of temperatures in spring and in autumn appears to be optimum for asexual repro duction. Growth in the size of the populations of these

- 9k - sedentary animals must result from the addition of new mem bers to the standing crop by release of buds. .

Some measurements of the theoretical reproductive poten

tial of the better known species, to be reviewed in the next section, provide a basis for interpreting the fluctuating production rate of our species in natural populations.

Sexual reproduction is almost completely suppressed in the hydras, and plays a minor role in maintaining the species

(see Part IV). That the capacity of some individuals to withstand extremes in temperature and other adverse condi tions accounts for the survival of the local populations from year to year has already been suggested in our presen tation of the annual cycle. The manner in which unfavorable temperatures, lack of food, predation, parasitism, and other factors of environmental resistance may operate to suppress the high biotic potential of hydra populations will be exam ined against the background of community interactions at work within the changing microhabitats of the species.

Reproductive Potential under Culture Conditions

Unlike sexual reproduction—* at most only a transitory episode during the hydra’s existence — asexual reproduction is a continuing epigenetic phenomenon throughout the life span of the animal. Regulative processes inherent in the individual express themselves in a pattern of bud arrangement on the column and outgrowth of tentacles on the buds which is remarkably constant for the species (Hyman, 1928; Rulon

- 95 - and Child, 1937; Ewer, 19^7; Brian, 19^1). Nutrition and temperature are the most important exter nal factors affecting blastogenesis. Numerous observations of mass hydra cultures, beginning with those of Trerabley

< 171+J+), have established that extremes of temperature as well

as withdrawal of food under otherwise favorable conditions will suppress bud formation.

The few clone-culture studies of hydras that have been made indicate that the rate at which buds grow and separate from the parent animal depends upon temperature conditions.

Maintaining such conditions with the facilities available in most laboratories is very difficult. As all workers with hydras know, considerable vigilance is required to keep even single-clone mass cultures of such easily cultivated species as H. oligactis or C. viridissima flourishing for long periods. Unless precautions are taken to use suitable water, avoid exposure to high temperatures or bright sunlight, remove decaying food animals and prevent accumulation of metabolites from overcrowding, the hydras will eventually go into a state known as depression. Symptoms, etiology, and progress of this pathological condition are fully treated by Hyman (1928).

Isolation clone culturing requires procedures which in volve tedious manipulations and detailed record keeping. The few zoologists who have undertaken such work usually use microcrustacea for food and conditioned water for the culture

- 96 - medium. To start a series of isolation clones, a healthy

individual is selected from a single-clone mass culture which has been acclimated to laboratory conditions. (See

Hyman 1928, 1930, 19l|l for methods of cultivating food supply

and maintaining mass cultures.) Each bud detached from the ’’stem” animal is isolated. These produce buds, which in turn,

are isolated. Thus the histories of all the individuals in

the whole series can be followed. Small finger bowls, crys

tallizing dishes, or Stender dishes, which provide a maximum surface area per standard volume of water, are used. Each animal, of course, is reared separately in the same type of container and all the cultures are kept under similar condi tions. Manipulations are made at 2i|-hour intervals. At each transfer, the hydras are fed by introducing an over supply of the food animal into the culture dish. The hydras feed to capacity with a few hours, killing more animals than they ingest. Then the dead animals must be removed, and the water replaced with filtered culture water. To prevent bac terial growth, culture dishes need to be replaced with clean ones at weekly intervals.

The theoretical reproductive rate (’’biotic potential” ) of a hydra species can be determined only by isolation clone culturing methods, in practice, the technical problem to be solved is finding out what constitutes optimum conditions of temperature and nutrition for the local species one wants to study. Experienced workers with hydras find that different species of hydras react differently to laboratory conditions. - 97 - Some species, H. oligactls for example, are hardy and feed voraciously. Thus, if properly maintained, a single clone will multiply rapidly. Other species, particularly those from swift-water habitats, will not multiply in the labora tory, and can be maintained only with difficulty for short periods. (Personal communication from Dr, Charles E. Hadley; see also Hadley and Forrest, 1952; Hyman, 1931b, 1938.)

During the present investigation, I found that H. littoralis from Lake Erie habitats would multiply only in flow ing water. Repeated efforts to obtain single-clone cultures in standing water failed under all conditions. Bubbling air through the container did not help. Individuals kept in finger bowls, watch glasses, or other small containers would capture and ingest enchytraeids, chironomid larvae, and microcrustaceans; but none would produce buds regularly even under the most carefully controlled conditions of culture medium, temperature, and light. Efforts to culture clones by the new method of Loomis (1953) also failed. Cultivation of budding individuals taken directly from the habitat by holding them captive in screen-covered funnels suspended in the lake was tried in late spring and early sum mer when the species was multiplying rapidly in the bay.

This method of cultivating hydras under "natural conditions'* was originated by Bryden (1952) to measure the reproductive potential of H. oligactis in Kirkpatricks Lake. It did not work successfully for H. littoralis in Lake Erie, although - 98 - some microcrustacea entered the screening and were eaten by the hydra. Apparently the container deprived the specimens of the needed water movement — the 11 current demand” which seems to be a physiological necessity for maintaining metab olism in the local population. The other local species could be cultivated with the good facilities available in the laboratory, but the labor of clone culturing them in isola tion cultures was not undertaken. Unfortunately the theo retical reproductive rate of H. littoralis, the species upon which the study was focused, could not be measured because of culturing difficulties.

Isolation clone culture work has been done with the cosmopolitan H. oligaotis and C. viridissima, the most easily cultivated of the hydras. The little information on repro ductive rates of these common species available in the liter ature discloses certain aspects of hydra biology of interest to the ecologist. Most of this literature is fairly recent, and since none of it has been reviewed, it seems worthwhile to examine it rather thoroughly.

The most extensive clone-culture work has been done by

Turner (1950). His comprehensive paper on the reproductive potential of a single clone of Hydra oligactls is especially valuable to ecologists because it contains a detailed record of observations made on a long series of isolation cultures and a complete statement of conditions under which the hydras were reared. Turner's findings and calculations on reproduc tive rates as stated in his summary follow: - 99 - (1) New buds are formed every five hours when reproduction is at its height and buds may be detached in 17 hours after their first appearance. (2) periods of maximal reproduction alternate with periods of depression during which specimens become in active and in many instances degenerate at the apical ends. Some specimens die during the periods of depres sion but most of them regenerate the lost parts and resume reproduction by budding.

(3) The average daily rate of bud production in l8ij. specimens, including those which eventually died during depression, was l.llj.. (1|) There is no general decline, during a 75? day period, in reproductive vigor and rate in single speci mens or in succeeding generations. During the 75? day period 18 generations were produced.

(5) The total potential production by a single animal for a 75 day period can be obtained by using the average total number of doubling (budding) for the period, 75 x l.lij., as an exponent of 2,

Theoretically, then, a single hydra under favorable environmental conditions could produce an immense population

- on the order of about x 102 ^ in two and one-half months.

To any field biologist who has encountered the tremen dous hydra aggregations which suddenly appear in natural habitats this phenomenal rate of increase does not seem im probable. (See Pt. XI, Sect. 2-a.)

Turner advances the hypothesis that declines in natural populations of hydras can best be explained by physiological depression which follows rapid reproduction. He suggests that some substance necessary for normal metabolism is ex hausted or that some toxic substance is formed which lowers the rate of metabolism so that the animals go into a state of senescence and perish because they can no longer adjust to

- 100 - changing environmental conditions.

The ecological significance which Turner attaches to his laboratory findings must be carefully weighed. It is evident from the author's clearly stated observations that the clone specimens were undergoing the physiological de pression peculiar to hydras during the 3 to 12 day periods when budding ceased. Symptoms and progress of the patho logical condition were similar to those previously described by Hyman (1928): incapacity to capture and ingest food ani mals; contraction of column and tentacles, which are normally extended to full length; gradual resorption of tentacles and column; finally, complete disintegration of the whole organ ism. Animals recovering from the depression before the con dition becomes advanced resume feeding and regenerate lost tentacles. Recuperating specimens show loss of morphogenetic control, marked by anomalies such as duplications at apical and basal ends, supernumerary tentacles, and forking of ten tacles. This cycle of depression and recovery is now recog nized by all workers with hydras as distinct from inanition changes. When food is withdrawn, the polyp ceases to bud, loses its color, and gradually becomes smaller. Unless starvation is very advanced, the animal can capture and in gest prey, and will again produce buds rapidly under favor able conditions.

The depression states observed by Turner were apparently not due to faulty culture technique. A healthy specimen

- 101 - from a single-clone mass culture of a male strain was selec ted to start the isolation cultures. The segregated progeny were kept in 12£ ml. culture jars in dim light. All dead microcrustacea were removed six hours after feeding, and the water replaced with fresh filtered conditioned water until the next daily feeding. This water varied in pH but was always slightly normal. The cultures were not maintained under constant temperature conditions, however, temperatures varied from 22 to 28°. That such temperature conditions are not favorable for prolonged periods of cultivation and may have been the cause of depression in Turner’s cultures is indicated by the results of other investigators, whose papers are cited in the following discussion of evidence.

Prom reproductive rate studies of H. oligactis made tinder field conditions there is good evidence that the spe cies produces buds most rapidly in lake waters which are between 22 and 28°, the temperatures under which Turner's hydras were reared. These comparatively high temperatures would be lethal for some species of Hydra, but oligactis and pseudollgactis can apparently maintain a high production rate under them if not subjected to sudden variation. Both

Miller (1936) and Bryden (19^2) obtained the maximum rate of production in the species when the lake water was at its warmest. Miller, using "seeded" slides in racks suspended near the one meter level, estimated that the population doubled every two days during July and August when daily

- 102 - o maximum-minimum temperatures in Douglas Lake were 20-27 • Bryden, culturing individuals in screen-covered funnels sus

pended in Kirkpatricks Lake, found the species produced mean numbers of buds at a rate of 3 to Ij. per week at water tem

peratures as high as 29°. He was able to calculate the rela

tive daily percentage increase in the size of the population in Kirkpatricks Lake from transect data. At the period of maximum increase from June through October, the rate of net

increase was 6.1 per cent per day. He concluded that high temperatures contributed to the large size of the population in late summer. Miller also concluded that summer was the best period for rapid asexual reproduction in Douglas Lake providing satisfactory substrata were available. Neither Miller nor Bryden mention observing depressed animals during

the season of maximum budding activity; but their field pro cedures did not include close examination of individuals.

Results of growth and reproduction studies of H. oligac tis, H. attenuate, and £. viridissima are summarized by Professor Brien in his presidential address before the Soci- ete Zoologizue de Prance (1951)• Brien cultured these spe- o cies at constant temperatures of from 18-20 in 200 ml. of conditioned water, feeding each animal with three or four daphnids daily and employing careful isolation clone cultur ing technique. He gives no calculations of theoretical reproductive fcates, but his conclusions regarding depression and senescence are in direct disagreement with those of - 103 - Turner. He says (p. 279): \ La premiere constatation a laquelle nous conduit cette methods d'elevage est celle de 1 'immortal!te de chaque Hydre. Nouse avons dresse par exemple le graphique d'une souche d'Hydra fusca [s oligactis], nee le 31 decembre 191+8# Inscrivant en abscissa le nombre de jours, en ordonnee de bourgeons erais. Depuis plus de deux ans et demi le graphique se poursuit san marquer la moindre depression physiologique. L'animal est aujourd'hui d»un aspect aussi normal, il est aussi prolifique que ses descendants lea plus recants. Cette Hydre a emis a ce jours (23 Mai 1951) 1+76 bourgeons. Il n'y a pas de raison pour qu'elle ne continue a vivre et la prolifSrer au m6me rythme dans les conditions constantes d'elevage defInies plus haute. Tout individu en culture nous en donne la confirmation. L 1 evidence des faits nous conduit done il conclure que 1 'Hydre est un Metozoaire qui maintient Indefiniment son raetabo- lisme, la perennite de son individualite morphologique et physiologique, dans le renouvellement constant de ses elements constituents.

A similar long life duration and productivity for oli gactis is reported by Bryden, who raised individuals for several years in balanced 250 ml. aquaria. One specimen lived for 50 months, producing 385 buds. Bryden does not include the food or the temperature conditions, which are of obvious importance in studying any developmental process, but he states definitely (p. 66): ’’Depression was never ob served, and the hydras that disappeared seemed to disinte grate suddenly within one day. In each case in which a hydra ’’died," the aquarium showed signs of becoming unbalanced and turbid."

Lashley (1915), apparently the first investigator in this country to employ isolation clone culturing in labora tory studies of hydra, does not consider the theory of

"Altersschw&che" advocated by the German experimentalists to

- 101+ - be sound. He criticizes the work of Hase (1909 — cited by Lashley, pp. 18 5 - 1 8 6 ) , who studied life duration in a large

series of isolated polyps, and graphed the distribution of

ages at death. The mean age for H. oligactis as 5 5 . 2 days,

for H. vulgaris 94*8 days. Lashley points out that the

relatively early ages at which the majority of specimens

died were due to two periods of unfavorable conditions in his cultures. Hase did not remove killed or egested daph-

nids from the containers or change the culture water. Even

so some of the animals lived about three months. Unfortu nately Hase*s data has been widely accepted in ecology texts mainly due to its inclusion, by pearl and Miner (193£) in their life tables for lower organisms.

Lashley himself kept 20 asexual generations of C!. viri-

dissima under observation for four months. He states (p.

186): "At no time have I observed an epidemic of depression in the individual cultures belonging to the same clone; less

than 1 per cent of the polyps have shown depression and always the closest relatives of these remained normal, a

condition which is not at all in harmony with theories of

clonal senescense.... The reproductive rate, the first

character modified by a reduction of the vitality of the polyps, was as high at the end of the experiment as at the beginning." Lashley kept his cultures on a table "where they were exposed to uniform conditions of light and tem perature." Again the temperature as measured is not stated

- 105 - in the otherwise complete description of culture methods

(pp. 176-1 7 6 ).

A recent report by Loomis (1953) indicates that rapid exponential increase can be obtained with no outbreaks of depression by cultivating hydras under controlled conditions of temperature, medium, and food. As the medium, Loomis uses a balanced salt solution made by filling a gallon jug with water from a laboratory demineralizer and adding to it

10 ml. each of two stock solutions: 133.0 g. NaCl, 2 6 . 6 g.

CaClg* demineralized water to 1 literj 3.8 g. NaHC0 3 , de mineralized water to 1 liter. For food, brine shrimp eggs are hatched on a 14.8-hour schedule by dusting a quarter tea spoonful of the eggs on the surface of 5>00 ml. of 3.5 g*/l» NaCl solution. (Corning 3-q.uart Pyrex utility dishes are employed for containers.) Newly hatched larvae are separated by phototactic migration, collected in a net, washed in the hydra solution, and added to the hydra cultures. Solution is changed 2I4. hours after feeding. Cultures are maintained at a constant temperature of 20°.

From the foregoing reviews, it is apparent that mainte nance of an optimal constant temperature range is the most important single factor in the successful cultivation of hydras. Brien has found that this temperature is 18 to 20° for the species he studied (H. oligactis, H. attenuata, and

C. viridissima). Loomis presents a four-day graph showing rapid logarithmic increase of C. viridissima and H. littomHn - 106 - (provisional identification) cultured at 20°. By employing

the new culture method, the author states that he has ob

tained thousands of hydras daily and that depression has been entirely avoided. During the present investigation, it was found that none of the species from Lake Brie could be cul tivated at temperatures above 22;° without incurring outbreaks of depression. The high temperatures (22-28°) to which Turner»s cul tures were subjected coupled with too rich feed probably

account for the recurrent episodes of depression he observed

in his clone. Hyman (1928, pp. 67-69) emphasizes that tem- o peratures over 20 and prolonged rich feeding are the main factors in causing rapid onset of depression in cultures.

Using the direct susceptibility method of Child (1915, Ch. 3) in experiments with clones of H. oligactis and another spe cies (now known to be H. americana Hyman, 1929), she estab lished that depressed specimens were in a low metabolic state. Both Hyman and Turner conclude that depression occurs in natural populations, and may represent a condition of senescence (Child, 1915) during which the lowered rate of physiological processes renders the animals less able to adjust to adverse environmental changes. Hyman suggests that depression in a state of nature probably occurs in con sequence of too high temperature. Turner, on the other hand, maintains (p. 297) "that great populations are reduced in part by extrinsic unfavorable factors in the environment but

- 107 - that an important contributing factor is the intrinsic depression which inevitably follows any initial burst of reproductive acceleration.” Although the papers of both

Hyman and of Welch and Loomis are cited, the author fails to recognize that his position is not tenable since no clones were cultured under lower temperatures.

Evidence to be presented in the next section indicates, nevertheless, that Turner’s results provide a valuable clue to the manner in which depression may operate to limit hydra populations under summer environmental conditions.

Survival under Adverse Conditions The findings of Welch and Loomis (192l{.), Miller (1936), and Bryden (19£2) all point to temperature as the limiting factor in the seasonal abundance of H. oligactis. In these investigations, as in the present study, evidence that level ing off and subsequent decline in numbers could be attributed to lack of food, to predation, or to heavy parasitic infesta tions was negative. Emigration and immigration do not, of course, contribute to seasonal fluctuations in sedentary, asexually reproducing populations like hydras. Additions to the population can be calculated in small private lakes by determining the natality rate; but, as Bryden found, even a rough determination of the mortality rate in a hydra popula tion is not possible with present field methods.

Ecologists have recognized the capacity of hydra popu lations to tolerate a wide range of changing chemical and

- 108 - physical conditions in the general habitat since the pioneer work of Welch and Loomis (see 11:1}. Miller and later Young

(19^5) made us aware of the importance of associated sessile organisms in rendering substrata available in the habitats unsuitable for hydras. Antibiosis is without doubt an im-- portant factor in determining distribution of hydras in the microhabitats. It may well be the end cause of mortality. In any event, this little understood type of antagonistic action within the microcommunity is intensified by summer temperatures (see section on crowding). In seeking an explanation of changes in observed hydra abundance, according ly, the naturalist»s attention Is directed — perhaps somewhat naively — toward observation of the adverse effects of high temperatures.

Effects of prolonged Heat. Experiments on the effect of temperature performed by Welch and Loomis (192l|.) at Doug las Lake, though not conclusive, indicated that H. oligactis died off in a balanced aquarium when subjected to a tempera ture of 25>° for several days. When exposed to an abrupt increase In temperature (16 to 28° in a period of 2l\. hours), the animals contracted strongly and depression set In. When temperature was controlled at around 22° by keeping the aqua rium in a water bath, the animals showed no abnormal symptoms.

The authors concluded (p. 231|.) 11 that temperature is a very influential, if not the determining factor, In the disappear ance of the hydra population from the surface waters.1' They

- 109 - considered the experiments confirmed the results of field observations extending over seven seasons in which the large hydra population of early summer began to decline when the daily maximum temperature rose to 22°, and faded out almost completely by late summer. Miller, following up on unanswered questions in a year- round investigation under Welch's direction, for some reason did not carry out any experiments on high temperature effects. He observed high budding rates in. some slide-rack populations during summer, but concluded that the over-all decline in the Douglas Lake oligactis-pseudoligactis population during late summer was due to the influence of periphytic accumula tions on the plant substrata. He found no evidence that the hydras migrated to the bottom.

Bryden concluded from his field findings that high tem peratures in Kirkpatricks Lake, Tennessee, where there was no minimum below 29 degrees for a three-week period, did not cause any reduction in the oligactis population. He found that competition for supports was not keen and that hydras always occurred on the bottom, especially on leaves, up to a depth of l.f> meters. The mean annual standing crop was

106 hydras per square meter of bottom.

In the present investigation, results of laboratory experiments and collateral field observations made during the season of 19!?3 yielded evidence of the adverse effects of high summer temperatures on the hydra species from western

- 110 - Lake Erie.

By utilizing facilities of the Ohio State Pish Hatchery located adjacent to the laboratory it was possible to cul ture freshly collected specimens from the bay at temperatures closely approximating those of the habitat# Observations were made on single-clone mass cultures of the local species from the first of July to the end of August, 19^3# No attempt was made to estimate reproductive rates. The objec tive was to find out the effects of gradually Increasing tem peratures on the animals under culture conditions and to com pare these results with field data obtained during the same period.

It will be noted In the following resume that the work was carried out under slightly more refined conditions than those employed by Welch and Loomis.

This was possible because the flowing water pumped directly from the lake through the hatchery troughs provided excellent water-bath facilities. Conditions in the hatchery room were such that containers almost submerged in them could be kept at temperatures within a degree of those ob taining in the lake. The increases in daily maximum tempera tures to which the hydras were subjected over the two-month period (22-26°) is shown on the five-day mean graph (Pig. 5). The hatchery is so constructed that the troughs are shielded from sunlight. Thus the experimental animals were never sub jected to the trauma of bright sunlight or rapid step-up In - Ill - temperature as were those of Welch and Loomis. The earlier experiments were also vitiated somewhat, as the authors recognized, by their short duration— 2ij.-hour man, twice

repeated. ... Six small aquaria (12 x 8 j 6| inches), the same size

specified by Welch and Loomis, were fitted up according to

standard practice: sloping sand bottom to facilitate siphon ing of dead daphnids, Elodea sprigs planted at the back.

These were filled with filtered lake water and covered with glass plates to reduce evaporation. Algal growth was not a problem since light in the hatchery was uniformly mild, in

these balanced aquaria, clones of oligactis, pseudoligactis, and americana were started with budding parasite-free speci mens transferred from the bay habitats. Species determina tions were made from the first individuals produced in each culture by analysis of nematocysts.

Daphnia magna was used as food since experience had shown that the local hydras could quickly capture and Ingest the smaller specimens of this rapidly reproducing cladoceran; clones can be easily cultured in Banta's manure-soil medium

(1921). An abundant supply of washed specimens was intro duced into the aquaria every other day, the remains removed by siphoning on the succeeding day. About half the water in the aquaria was siphoned from the bottom each week and replaced with fresh filtered lake water.

The aquaria cultures were divided into duplicate series,

- 112 - one for each of the three species. One series was kept at lake temperature on a water table set up in the hatchery trough so that the aquaria were bathed to within a couple of inches of the top rims by the flowing water. The other series was used as a control. These three aquaria were kept in a limestone side-cellar of the laboratory where tempera tures could be held between 18 and 21 degrees under fluor escent lamps which furnished light for the plants.

Changes in the condition of the experimental and con trol animals were observed with the aid of a widefield lOx hand lens. Samples of the aquaria populations were inspected more closely under the stereoscopic microscope for bud counts, parasitic infestation, and symptoms of pathology. This sam pling, done when the cultures were flourishing, served to eliminate the hazard of overcrowding.

Results obtained under the culture conditions just described appear significant when related to collateral field findings.

(1) The control cultures were successfully maintained over the two-month period of the experiment. The healthy offspring of the parent animals remained attached to aquarium sides or plant leaves in the fully extended attitudes of the species. Samples of 100 specimens pipetted at random from the glass on July 15 and August 20 showed no significant difference in the ratio of budding to non-budding individ uals. Approximately $0 per cent were budding, the majority with one bud. - 113 - (2) The experimental animals multiplied rapidly from

July 1 up to the middle of the month. Thereafter progres sively severe physiological depression occurred in all the o clones. As temperatures mounted above 22+ at the close of

the month, the individuals which had recovered ceased to bud. The americana clone became extinct by the first week in

August, the pseudoligactis clone the following week. Five

individuals in the oligactis clone survived until the end

of the month. At this time daily maximum temperatures had reached 28°. oxygen saturation and pH in both lake water and aquaria water were well within the hydra toleration range.

(3) The lethal effect of high temperatures on hydras in the bay habitats was also evident from field results obtained during the period. At the beginning of July, a slide rack

suspended at a depth of one meter from the laboratory dock adjacent to station 5 supported an oligactis-pseudoligactis aggregation of about 1500 healthy individuals. Approximately

2+0 per cent were budding. On July 18, inspection of every other slide in the rack disclosed onset of depression in the aggregation. By the middle of August, the slide-raek population had dwindled to approximately 3 0 0 ; about 12 per cent of these individuals were in various stages of depression. At the end of August the slides were removed from the rack.

\ Only 20 hydras remained: 18 oligactis and 2 pseudoligactis. They were without buds. Column and tentacles were contracted,

- 111+ - None responded to hand feeding in watch glass cultures at 20°,

Knobbing of tentacles and other signs of advancing depression appeared. Squashes showed that the pathology was not caused by amoebic infestation. Similar evidence of endemic de pression was seen in the few specimens which could be col lected from the plants at station 5 or from the slide-rack and stone-anchor rig set on the bottom there. After the middle of July, as temperatures mounted during the prolonged calm, the oligactis-pseudoligactis population declined to such an extent it became increasingly difficult to obtain specimens. In the case of H. americana none could be found after the beginning of August.

(4) Analysis of the 1953 field data (Table VI) indicates that H. littoralis survives under high temperature conditions longer than the other species Inhabiting the bay. Episodes of depression apparently occurred in the population, however, as it was subjected to abnormally high temperatures. Death from physiological depression was probably an important fac tor in causing the eventual decline of the abundant summer population. This was suggested, at least, by the severity of pathology seen in collections at the end of the season compared with its absence at the beginning. The majority of the 2£5 specimens examined July 11 bore from one to four buds, and none were In a state of depression. Among the de clining numbers of hydras found on the slides in subsequent

July collections the majority were in various stages of

- 1 1 5 - depression. Relatively few depressed individuals were seen among the large numbers of hydras colonizing the slides during the weekly immersion intervals of the first part of

August. Recovery from depression was indicated by the high percentage of budding individuals; also by the occurrence of a number of abnormal individuals. Ten of the 6I4.9 specimens exhibited anomalies: three cases of duplication at the oral end, seven cases of forked tentacles. The marked decline during the last half of August was accompanied by onset of depression: 10 per cent of the 181 specimens in the August

collection were in a state of advanced depression. At the end of the dead calm only a few individuals of the population apparently survived. Intensive collecting the first three days in September from the rubble habitats along the Gibral tar Island shore, where littoralis had been abundant earlier in the season, yielded only 2£ specimens. These, like the

16 obtained in the slide-rack collection of September 3, were without buds, colorless, and in a state of incipient physiological depression.

At this time, oxygen in p.p.m. was low; 3.92 at the surface, I4..38 at the bottom (3 meters). The low saturation value is close to the limit of tolerance for hydra species; but as subsequent discussion will make clear, the catastroph ic decline was caused primarily by lethal action of high tem peratures on the population.

Such a conclusion is supported by supplementary observa tions of three single-clone mass cultures of H. littoralis - 116 - maintained in hatchery jars at lake temperatures during the

period of field observations. The water flowing up through

the jars contained an abundant supply of copepods pumped In

from the lake, and the single individuals multiplied rapidly.

By the third week In July number per jar was estimated at from 200 to 300, Samples of 100 scraped from each jar dis

closed that although about 35 per cent were budding; around 15 per cent were in a state of depression. None of the individuals examined were parasitized. Numbers declined

rapidly in the cultures the following week. This initial

decline was followed by a pulse of budding, presumably due to recovery from depression among the surviving individuals

In the clones. During the last two weeks of August, however,

depression again set in. By the end of the period (tempera

ture 28°), the clones had dwindled to from 25 to $0 per jar.

These survivors, like those observed in the habitat, were

without buds, colorless, and did not capture the copepods which were still abundant in the water supply.

Survival of a sufficient number of clones to produce a

sizeable autumnal littoralis population is evidenced in the collection of October 12 (Table VI). During the five and

one-half weeks after September if, when the calm was ended by

a storm, temperatures declined rapidly to 16° (Pig. 5] * The 388 hydras which had colonized the slides during this period were healthy in appearance. The characteristic orange or brown coloration of the animals when well nourished was again present. Twenty-three per cent bore one or two buds, a sign - 117 - of fairly high reproductive activity. None were depressed.

Again, as after recovery from the episode of depression earlier in the summer, examples of abnormalities were found:

two cases with forked tentacles, one with supernumerary

tentacles, and one with functional double hypostome sur rounded by six normal tentacles.

Structural anomalies such as those mentioned above are

commonly seen in cultures after the hydras are subjected to unfavorable conditions (Hyman, 1928; Turner, 195>0; Chang,

Hsieh, and Liu, 195>2). In nature, however, their occurrence is extremely rare. Hyman (pp. 72-73) estimated the ratio in annual collections of H. americana at about one per one thousand. (See Figs. 2-20 for illustrations of the 19 ab normal individuals occurring among twenty thousand speci mens collected in fall.) An occasional specimen with forked tentacles was taken in Lake Erie collections throughout the year; but preliminary analysis of the data indicates that multiple basal or apical structures occur with greater fre quency during midsummer and early autumn.

Maximum Survival Temperature. The highest temperature at which survival is possible for most of the hydra species probably lies well above the maximum daily temperatures to which the animals would be exposed in the habitats. This was recognized by Mast (1903) in discussion of results ob tained in an experiment with a hydra from the Huron River at

Ann Arbor, Michigan, which he designated as Hydra vulgar!s.

- 118 - The "ultramaximum temperature" for this species (probably

H. americana) was about 3l+°C. When five specimens in a beaker at 22° were warmed to 3^° by gradually increasing

the temperature of the water bath over the period of £6 min utes, the animals became contracted at 28°, released their o foothold at 31 , and died during the 23 minutes required to raise the temperature from 31 to 31+°. Some species no doubt have a somewhat higher survival temperature. For instance, Chang, Hsieh, and Liu (1952) reported that they have successfully cultured a previously undescribed species collected from ponds and streams in the vicinity of Peking at temperatures above 30 degrees. In fact, they state the species buds most rapidly at the ex ceedingly high temperature of 33°.

Outside of Mast’s early experiments on kill tempera tures, I could find no other determinations of maximum sur vival temperatures for hydras in the literature. A knowledge of survival temperatures might aid some in predicting the geographical ranges of the known hydra species. For example, reports on temperature tolerance of C. viridissima and

H. oligactis lead to the conclusion that populations of these cosmopolitan species can become adapted to a wide range of temperatures. Carrying out the extensive experi mentation necessary to determine acclimatization to tempera ture extremes was not possible during this preliminary Inves tigation of hydras in the Great Lakes. But from the field - 119 - findings it is evident that in the western basin of Lake

Erie — the warmest area in any of the Great Lakes — tempera tures never reach levels lethal to whole hydra populations.

(See recent paper by Verber, 19££, for temperature records.)

Minimum Survival Temperature. At least some members of the hydra populations in western Lake Erie withstand the extremely low temperatures of the winter months (Pig. £).

This was conclusively demonstrated by field results, which have been summarized In delineating the annual cycle. Here attention is called to the fact that survivors of the autum nal populations can maintain their metabolism at 0.2°C. Organisms in the habitats of the island region may be exposed to such a constant low temperature for a period of from one to two months, depending upon the duration of the ice sheet.

Comparative study of collections taken under the ice- cover indicates that H. oligactls and H. pseudoligactis have a greater cold hardiness than the other local species. The few specimens of H. littoralis and H. americana which could be found under the ice were in an advanced state of hiberna tion; they were all smaller than normal, colorless, and without buds. Specimens of oligactls and pseudoligactis were not only obtainable in greater number, but they showed evidence of a comparatively high rate of metabolic activity. This could be seen, for example, in the condition of speci mens collected on February 3, 19S>i|. at station A quart of

Myrlophyllum grappled from under the ice yielded 27 oligactls

- 120 - and 6 pseudoligactis. One or two buds were b o m by about a

third of the specimens. One of the oligactis specimens

bearing two buds was a mature male; motile sperm were seen

in the testes (see section on sexual reproduction). Several

of the hydras had ingested Chironomus larvae or copepods.

The orange coloration of the gastrodermis is characteristic

of the local species when feeding on such prey (see section on food relations). All the animals in the collection had been exposed to a temperature within 0,2° of freezing for

about a month.

The capacity of oligactis-pseudoligactis populations to maintain themselves at extremely low temperatures under ice-

covers is especially well shown by the findings of Killer (1936). He reports an abundant population on plants near the

ice in Lake Washington, Minnesota. The majority were budding with one or two buds throughout the three and one-half month period of the ice-cover (pp. llj.8-llj.9, Table 11). Large nub- bers of budding hydras were also observed on a slide rack

suspended at one meter beneath the ice of Douglas Lake over

a three-month period. The temperature was close to 1°C. (p. l£l, Table 12).

There is good evidence that hydra populations in the

Great Lakes area can survive at near freezing temperatures.

That any of the hydra species can withstand actual freezing is questionable. Miller, in his review of the literature on the subject, states that "Trembley (17l*lj.) and Sch&ffer (1751*

- 121 - — cited p. 193) claimed that hydras can withstand freezing.”

Miller reports that he cooled 21 hydras in a vial of water

from 20° to 0° in two hours, and subsequently froze them for

one hour. He states (pp. I6I4.-I6 5 ): ”The temperature was then gradually increased to 2ij.°C. in 3.5 hrs. Some of the

hydras were still alive at the end of the experiment, others

were disintegrating." Mast (1903) observed that the species of hydra he used as his experiments, when kept at 0° for

twelve hours without being frozen, recovered as soon as the

temperature was raised to 20°C. He states (p. 177): "if the temperature of water is gradually lowered, Hydras may be frozen in a partially expanded condition.... Frozen Hydras slpwly thawed in a temperature of 22p do not recover. Their ultraminimum temperature is therefore below 0°, — death being probably due to the physical changes in freezing or thawing."

No freezing experiments were performed with Lake Erie hydras. Some aggregations close to the shoreline are no doubt trapped in the ice when the lake freezes over. As

Mast observed, hydras are unable to migrate from lethal temperatures. Like other sedentary forms, these creatures survive unfavorable environmental conditions or perish.

Effects of Cold. Some evidence of low temperature effects on the hydra species has been gained from studies of natural populations. The whole subject of the action of temperature as a factor in gonad formation is discussed in

- 122 - the section on sexual reproduction. Here the evidence relating to the effects of the direct action of declining temperatures on asexually reproducing hydras will he exam ined.

One fact which emerges from year-round ecological inves tigations of hydras in lakes of the temperate zone is that population densities decrease with the onset of winter. This decline in numbers is apparently a direct effect of lowering in the rates of biological reactions at temperatures below 10°C. After the water cools to the critical temperature of

J4.0, production of new individuals by budding probably ceases. Some species may bear buds, such as the examples in oligactls and pseudoligactis cited above, at much lower temperatures.

But as Boecker (1918, pp. i4.9 O-l4.9ii first pointed out, buds are retained by the parent for long periods at winter tem peratures. According to McConnell’s study of mitosis in H. oligactls (1933ai, cell division takes place in hydras under all conditions. Although the rate of mitosis at vari ous temperatures has not been determined in the hydra species!, it is now generally known that mitotic rates and growth rates decrease in poikllothermous animals as temperatures decrease. Bryden (1952) found that H. oligactls when subjected to the temperature of around 7° prevailing in Kirkpatricks Lake during winter ceased budding and eventually underwent resorp tion, which was seen in cellular breakdown of survivors in early spring. Miller (1936) concluded from his study of

- 123 - H. oligactls at Douglas Lake that no buds are probably re

leased in winter at temperatures of below I4.0. No evidence

of resorption was seen by Miller in the winter hydras, how

ever. Likewise, I have never observed cases of resorption

or depression in late fall, winter, and spring collections ollga.ct;i.s or any of the other species from Lake Erie.

Like Bryden and Miller, I found that budding specimens were

fewer after the lake dropped to the critical temperature of 7°, and that only remnants of the autumnal populations re mained throughout the winter. H. llttoralls and H. americana became so scarce during all winter seasons specimens were hard to find; the few that could be found at temperatures . o below 4 were without buds. Cessation of bud production is without doubt a contrib uting factor to the winter decline. But how does one account

for the mortality which must occur in the fairly large popu lations present in autumn? Deaths from predation or para

sitism are known to be few. Starvation is apparently not

the cause; some food animals are available in the habitats

of the three lakes throughout the year. Regardless of food availability, it appears that nutritional demands are slight

at temperatures below 1].°, Some experiments, to be discussed now, show that hydras collected in the fall suffer no ill effects except reduction in size when kept at winter tempera

tures for long periods without food. Attention is called here to the fact that low tempera tures do not cause depression in natural populations. - 121]. - Year-round field observations indicate that though episodes of depression may occur during periods of prolonged heat, cycles of depression and recovery are not seen in autumnal or winter populations. States of depression may be induced in laboratory cultures by extremes of temperature. (See papers of Frischholz, Goetsch, Grosz, Rehm which are cited and reviewed by Hyman, 1928.) But in nature, as Hyman has suggested, depression occurs probably in consequence of too high temperature.

Turner in his paper of 19S>0 (reviewed above) failed to familiarize himself with the work of Miller (1936), Conse quently, he advanced the untenable hypothesis that reproduc tive pulses in H. oligactls could occur in nature at any season of the year; he believed the primary cause of sudden reductions in populations was intrinsic physiological depres sion accompanied by senescence after rapid reproduction. Bryden1s subsequent work did not validate this assumption; the results of the current investigation certainly do not support Turner*s position.

Miller and Bryden are inclined to give some weight to Greeley’s early experiments on low temperature effects on an unidentified species of Hydra. It is obvious, from his paper

(1903), that the experimental animals when exposed to a tem perature of l|. to 6 °C. for seven days exhibited the typical symptoms now recognized as depression. As a matter of fact, the four figures contained in the paper are good illustrations

- 125 - of hydras in stages of progressive depression. Both Miller and Bryden fail to note that McGill (1908) repeated Greeley*s experiments, and obtained contradictory results. McGill, using H. oligactis and £. viridissima, concluded (p. 8£)

"that reduction of temperature for the length of time men tioned by Greeley does not cause Hydra to be resolved into undifferentiated protoplasm. When this does take place it is due to unfavorable conditions and is a degeneration effect and not a temperature effect."

McGill's experiments demonstrated quite conclusively that the hydras collected in summer reacted differently to low temperatures than those collected in autumn. Specimens taken from a pond at high summer temperatures became some what depressed when subjected to a temperature of Ij. to 6° for six or seven days. Specimens taken from a habitat at

8 to 12° could be kept at temperature of 2° for as long as two weeks without ill effects. McGill states (p. 83):

"Hydra that are budding show no absorption of the bud such as described by Greely 'Greeley'. As soon as such Hydra are placed at room temperature the buds, as well as the parent body, become actively contractile." I have confirmed McGill's results, using specimens from fall collections of the Lake Erie hydras.

Sudden onset of cold in northern climates may cause death in hydra populations. H. canadensis is killed off by serious frosts occurring at the type locality near Edmonton,

— 126 — Alberta. Rowan (1930), the author of this species, reporta that when the early fall is very cold, specimens can not be found in the lakes of the vicinity after the beginning of

October. H. carnea, associating with canadensis in the same habitats, is even more susceptible to frost kill. According to Rowan's findings, survival of the populations from year to year depends upon winter eggs produced before the sudden temperature drops. Unfortunately, Rowan's paper, like most taxonomic papers, contains no record of temperatures.

In Lake Erie, H. littoralis appears to be the least cold hardy of the local species. But just how cold may act to cause death by disrupting physiological integration in this species— or any of the other hydras — remains to be determined.

Starvation. The small, colorless, budless H. littoralis specimens found in late winter contrasted with the robust normal-sized, orange-colored, active specimens taken in col lections at the beginning of winter. This observation sug gested that the hibernating animals were in a state approach ing total inanition. Results obtained in the laboratory con firmed this inference.

Changes in the behavior and general appearance of the animals, which are summarized below, were observed by placing freshly collected specimens in a small finger bowl half filled with filtered lake water from the habitat and contain ing a few filaments of Cladophora. These cultures were kept

- 127 - close to the temperatures specified below in compartments of

a refrigerator. The copepods (Cyclops blcuspldatus} killed

during the feeding experiments were removed and culture water was changed; water was left unchanged during starvation periods. It should be noted here that the copepods contained

a reddish oil globule.

A. Ten specimens collected February 2, at 0.2°. Specimens were colorless, budless, thin and small — about

6 mm. whereas normal column length is around 12 ram. (1) Specimens fed every other day at 2° for two weeks: majority rejected food; remained extended; no signs of de pression.

(2) Same specimens fed every other day for two weeks at a temperature of ij. to 6°: ingested Cyclops; after two days orange tints appeared in gastrodermis; became more active after first week, but no noticeable increase in size except thickening of column; buds did not form.

(3) Food withdrawn and temperature lowered to 2° for three weeks: orange coloration faded to white during second week; none attached to substratum at end of second week; columns thinner but fully extended; tentacles, as usual in standing-water cultures of this species, not fully extended.

B. Ten specimens without buds and ten specimens with one bud collected December 7» 1953 at habitat temperature of

specimens large (columns 10 to 12 mm. and stout); orange-colored; budding and budless individuals isolated in

- 128 - separate finger bowls, and kept at to 6° without feeding

up until February 12.

(1) Coloration lost by end of first week in majority of

specimens; by end of second week all were white.

(2) Buds detached in six specimens by end of third week;

in remaining four by end of fourth week; these ten buds were

isolated in some of the culture water and kept under the

same conditions as the parents; none developed depression,

but all underwent gradual reduction in size, dwindling to

about 0.5 mm. by the end of the sixth week.

(3) The adults gradually became thinner and smaller during the two-month period of starvation; column lengths

decreased to about half the original size; columns remained

uncontracted, however, and no signs of depression were ob served.

(4) Feeding was started on February 12. Fourteen of the

twenty specimens recovered from the state of inanition, be coming again orange-colored and active within a week. The

other six remained white, and after two weeks were squashed

for nematocyst examination. Some increase in size was noticed in the surviving specimens, but none had produced buds up to March 1. At this time all the specimens were

squashed for nematocyst examination. Nematocyst structure in specimens which recovered from inanition was the same as in those which did not survive. Orange pigments and oil globules were of course absent from the gastrodermis of the

- 129 - starved individuals, which had not been able to ingest the

copepods.

A similar capacity to survive inanition at low tempera ture was observed in the other species of Hydra from Lake

Erie. Pinger-bowl cultures of oligactls, pseudoligactis,

and americana were kept in the refrigerator at ij. to 6 ° with

out food or change of water for a three-month period approxi mating the duration of winter. Again, the specimens— ten without buds and ten with buds — were selected from an early

December collection (temperature about 5>°) for their large

size and healthy appearance. The histories of these cultures

as recorded at weekly inspections, followed the general pat

tern of the H. littoralis cultures: retention of buds for

three or four weeks; rapid fading of color and progressive

shrinkage in size; higher mortality at the end of 13 weeks among newly detached individuals than among the full-grown animals (ij. or 5? out of 10 in the isolated bud cultures com?- pared with 1 or 2 per 10 of the adults). As one would expect, at higher temperatures, hydras can not maintain metabolism without food for such a long period.

This is indicated by some observations made on cultures sub merged in the pool of one of the caves in South Bass Island, owned by Hr. A. Kindt. (This cave is designated "Kindt»s cave I" by Verber and Stansbery (19^3) in their report on the Island caves.) The water, which is filtered into the pool from Lake Erie through rock layers, remains close to a

- 130 - constant temperature of 11°. The pool is clear and free of animal or plant life excepting bacteria. The water was tested and found suitable for hydra cultures, although total alkalinity is 180 p.p.m., about twice that of Lake Erie water

(Verber and Stansbery, 195>3» p. 360). A series of 10 well- fed specimens from cultures of the various species was set up in 100 ml. bottles covered with bolting cloth; these were submerged in the pool at a depth of 3£ cm. on February 11;.,

19^2. When the cave was again visited on March 13, all the animals were living; they were able to ingest D. magna after a month of starvation. When the cultures were removed from the cave on May 1, 195>2, only about half the animals remained.

These were in a state of total inanition and could not feed.

A few observations of inanition effects in relation to temperature are reported in the contemporary literature.

Bryden (19^2, pp. 5>8-£9) studied I4.O specimens of H. oligactls kept at a constant temperature of for 11 weeks: cessa tion of feeding and bud resorption occurred at the end of the two months; of the 33 survivors only 10 resumed feeding and produced buds at room temperature. Miller (1936, pp. 161).-

165>) reports some specimens of oligactls and pseudoligactis (numbers not stated) collected from under the ice survived in the refrigerator at 8 ° for two months without food.

Loomis (19£3) states he is able to hold hydras (C. virldissima and H. littoralis?) in his solution of salts for several weeks at room temperature and in the refrigerator for several

- 131 - months; stored specimens will bud in approximately I4.8 hours at room temperature after feeding with brine shrimp larvae.

According to Mashtaler (1937), ten hydras ("H. fusea”) sur vived for a month in culture water where plate counts showed the bacteria density to be between one and two million per cc. (temperature not stated); he observed that such prolonged starvation results in decrease in size from the usual column length of 15 mm. to 0.5 mm. A species of hydra under study by Hadley and Forrest (personal communication), when kept without change of water at about 7 ° for several weeks, shows no visible changes except the fading of color and decrease in size which accompany inanition.

Regulation of form apparently remains under control of the oral center during inanition. Starvation does not cause cessation of mitosis (McConnell, 1933a). Depression, however, is accompanied by disturbances of regulative processes. As the condition advances, mitotic activity ceases and buds are resorbed; shortening and loss of tentacles occur; and eventu ally cellular breakdown brings about disintegration of the whole organism. States of inanition can thus be clearly dis tinguished from states of depression. Hyman (1928, p. 67) makes this especially clear in calling attention to the erro neous interpretations of Kepner and Jester (1927), who con cluded that the shortened tentacles and disintegration they observed in some green hydras collected in summer was due to starvation. According to Hyman's criteria, the condition

- 132 - described by Bryden (p. £9) in his starving experimental

animals (buds being resorbed and tentacles short or lacking) was due to depression rather than inanition. Likewise, Tann-

reuther (1909b) probably did not recognize the depression

condition when he concluded that starvation causes bud re sorption.

Inanition up to a certain degree raises the metabolic rate of Hydra. Experiments by Hyman (1928, pp. 75-76) indi cate that the rate of H. oligactis specimens starved at room temperatures 6 to 11 days is greater than that of the fed controls; the rate diminishes with longer starvation between

15 and 18 days; small specimens have a higher metabolic rate than large ones starved the same length of time. Data on metabolic rates of the hydras under inanition conditions at low temperature are not available. Nevertheless, the prin ciple of indirect susceptibility advanced by Child (1915)t according to which animals in a low metabolic state are less able to adjust to slightly adverse conditions than animals in a more active state, may be operative in reducing natural populations of hydras.

Thresholds for Budding. Temperature ranges in which hydras from diverse localities can maintain metabolism most effectively remain to be determined. Present ecological information suggests a range of between if and 214° for the

North American species. In spring populations of Lake Erie hydras, results of food ingestion, rapid digestion, assirailar- tion and cell multiplication are seen in the large size and - 133 - budding condition of the hydras by the time temperatures o reach 10 •

The threshold temperature for bud production in oli- o gactis and pseudoligactis is probably around 5 > for littor alis and americana around 10°.

like Miller (1936, pp. lij.6—II4.9 ), I found no increase in numbers of hydras on slides suspended in racks under the ice- cover at a depth of one meter. Budding individuals seen in winter oligactls-pseudollgactls aggregations apparently retain the buds until the prevemal season. No increase on slides was noticeable at station £ until the beginning of

April. About a third of the l£0 hydras counted on 18 slides

April 6, 19%k bore from one to three buds. A week later the number estimated on the slides was 3 0 0 , about half budding, a few with as many as five buds (temperature about 5>°).

These specimens were large, stout and active. Many were gorged with chironomids, and fully developed buds were seen ingesting reddish colored copepods. Both buds and parents were orange-colored down to the stalks. To test the effect of increasing temperature on bud detachment, two lots of £0 well-fed specimens, each bearing a single bud in the three-tentacle stage, were sorted from the April 13 collection into large crystallizing dishes con taining £00 ml. of filtered lake water. One dish was placed in the constant temperature cabinet at 12°, the other in a bath of flowing lake water at % to 6°. After four days,

- 13k - approximately half of the "buds had detached In the lot held at tne higher temperature whereas only a tenth of the buds had detached in the lot kept at the lake temperature. Effects of rapid production at temperatures around 12° were seen in the abundant aggregations which colonized all types of substrata during May. Samples of 5>0 hydras could easily be obtained from a quart of loosely packed Myrio- phyllum, from Potomageton plants, or from a few dead tree leaves. Numerous budding aggregations settled on gill nets in operation adjacent to station 5 within k 8 hours. Slides were likewise rapidly colonized; the number by the middle of

May was about l£00, about $0 per cent budding. Several Indi viduals bearing six and seven buds were observed in a spot- check of the slides on May 13.

Distribution of bud counts from samples of the mixed ollgactis-pseudoligaCtis population at station 5 during the 195k prevernal and vernal seasons is given in Table VIII.

The random samples of specimens on which the counts are based were taken from collections off the Myrlophyllum plants. Ratios of budding to non-budding individuals and distribution by number of buds were almost identical In simi lar samples taken from other natural substrata such as poto mageton and dead leaves, or from artificial substrata such as nets or microscope slides. As at other seasons oligactls far outnumbered pseudoligactis in the samples; the estimated ratio, based on nematocyst examination of 15 individuals in

- 135 - each sample, was about 10:1. No significant difference in number of buds per individual, bud pattern, or size was found between the two identical-appearing species.

It will be noted that approximately 60 per cent of the hydras bore from one to five buds in the samples taken from the middle of April to the middle of May. The great increase in density of population during this period leads to the conclusion that buds were being formed and released at a relatively high rate after the threshold temperature of £° was passed. Actually the rate of production was probably at its peak from the middle of May up to the end of June at water temperatures in the increasing range of 12 to 22°.

This was indicated, not only by the dense population in June, but also by the greater incidence of four-tentacled individ uals among the non-budding hydras. In the June 19 sample,

5 of the £0 specimens had only four tentacles; in the June

27 sample, there were 9 out of £0, in all the previous sam ples, there were only 6 four-tentacled specimens. All the rest of the individuals had five and six tentacles, the majority (from 38 to Lj.3) in the samples with six. This occurrence Is significant since it is well-known that buds are detached during rapid production before the full comple ment of six tentacles has developed (Hyman, 1930, pp. 32ij.- 32£>J Bryden, 19i>2, pp. 62-63).

Column length of fully-developed buds ready to detach measured between 10 and 12 mm., the stalk being about 1+ mm.

- 136 - Specimens bearing buds measured from 15> to 20 mm., including a distinct stalk of about £ mm. Apparently column above the stalk increases in size before the first bud is formed and grows but little afterwards. Bryden's growth curve derived from measurements of l£ oligactls specimens cultured at room temperature shows a mean length of 12 mm. at time of detach ment from parent (5 days) and 17 mm. at time of bud formation (11 days).

It is worth noting here that flourishing spring oligaotis populations such as those seen in Lake Erie and Lake Washington (Hiller, 1936, p. 1I+9) may be absent in some lakes.

Bryden (p. 6i|.) found the population at its lowest level in Kirkpatricks Lake during spring with no specimens budding in

May. Bryden*s data, based on study of separate specimens held captive in separate funnels suspended in the lake from

June to January, indicate that there is an inverse relation ship between rate of production and number of buds per indi vidual, increase of budding being most rapid at temperatures above 15 °• The higher threshold temperature for budding in H. lit toralis results in an exceedingly sparse population of bud less specimens until temperatures reach 10° at the beginning of May. A high rate of production Is reflected in May and June collections (Tables III and VII). Under optimal condi tions for growth and budding, buds may be detached as rapidly in littoralis as in oligactis. This was indicated by the

- 137 - high incidence of individuals without the full complement of five or six tentacles. At the vernal population peak of

195>l4- as many as 30 per cent of the non-budding specimens in some collections possessed only three and four tentacles. Among the budding specimens, four-tentacled specimens them selves bearing one or two buds were not uncommon. Prelim inary analysis of the seasonal data shows a direct relation ship between increase in incidence of bud production in three- and four-tentacled specimens and increase in tempera ture from 10 up to 2l±°. Maximum number of buds per individ ual in fully-grown littoralis specimens is less than in oligactis or pseudoligactis adults; specimens bearing as many as four buds were rare.

The threshold temperature for budding in H. americana is probably close to that of littoralis; a few specimens of this little hydra could be found after water temperatures reached 10°* but the species was never abundant in any col lections. Few budding individuals were ever taken, and these bore at most only two buds.

Exposure to Air. Like other inhabitants of shoal areas in Lake Erie, H. littoralis may suffer the adverse effects of desiccation due to fluctuations in the water level caused by seiches. The time required to complete seiche oscilla tions varies from a few minutes to several hours. in summer months, the water level at the shoreline is usually not lowered more than 3 or ij. inches for a period of about ij.0

- 137* ^ minutes (Krecker, 1931). In early winter, however, blows from the southwest sometimes lower the water in the bay as much as £ feet for several hours (Britt, 195UK Extent of exposure of the bottom during this kind of seiche is shown in the photograph (Pig. 3) taken at Oak Point bar, December

10, 1953 by Dr. N. Wilson Britt.

Observations made during the seiche cited above lead to the conclusion that the hydras living on the rubble of the bar can survive exposure to cold air for several hours.

Pieces of small rubble were collected from the shoal area, which was laid bare its entire length to the bar at Gibraltar

Island for five hours. The pieces were placed in large jars of water at collection. They had been exposed to an air temperature of 3° lor about four hours. Normal specimens of littoralis, also a few oligactls, were seen attached to rubble or-the glass of the jars when the collections were examined in the laboratory. Similar results were obtained from exposed rubble collected at Peach point and Oak Point during seiches of December I4., 1951 and November 26, 1952 (air and water about 7°).

Capacity of littoralis to survive desiccation for sever al minutes at room temperature was established by a chance observation made on May 6 , 1952. About 30 fire bricks sup porting large numbers of the species which had multiplied in the hatchery fry tanks were accidentally removed and stacked In the coal room. The hydras were out of water (temperature

- 138 - 12°) in air at about 20°; yet when the bricks were placed in

buckets of water after approximately ten minutes, numerous

specimens were still living.

It was also observed that hydras on slides survived ex posure to air at temperatures of about 20° for as long as l£

minutes. Mashtaler (1937* English summary) states: "Hydra

is to some extent capable of surviving after desiccation, but

this is possible only after brief desiccation and must be

preceded by considerable dissociation of the animal’s body.

Anabiosis in Hydra is, accordingly, accompanied by regenera

tion." There is no clear statement of survival time in the

Ukrainian text, though mention is made of desiccating speci

mens for one to one and a half hours at 16°.

Actually, when the habitat substratum is exposed to air,

hydras are somewhat protected from desiccation by associating

periphytes, such as algae, which retain moisture. Reactions to Silting. Hydras can extricate themselves

from sediments which at times may completely cover them in

Lake Erie habitats. Emergence of littoralis, oligactls, and,

pseudoligactis into clear water above layers of sediments was

observed during May, 1952 by subjecting a series of healthy

finger-bowl cultures to samples of sediments collected from

the bay. Forty specimens, adhering to the bottom of the finger bowls, were covered with 1 cm. of muck, mud, sand,

and silt; the bowls were filled with lake water siphoned at a trickle to prevent roiling. The set-ups were left undis

turbed in the constant temperature cabinet at 15° for ?)| - 139 - hours. After this lapse of time, the majority of the speci

mens in each series had freed themselves from the sediments

and could be seen attached to the sides of the container in

the clear water above the sediments. There was no significant difference between the species

in the numbers extricating themselves from the sediments.

However, the number of animals escaping during the time inter val varied somewhat according to type of sediment: muck,

21-2£; mud, 22-29; sand, 30-^0; silt, 36-ij.O.

The mud and muck, collected at station 5> was not ana lyzed for organic content; however, the pH range at the end

of the experiment was 7.0-7.lj.» which is well within the tol

eration limit. Sand, taken from station Ij., was washed through a U.S. Sieve Series so that most of the particles were about

1 mm. in diameter. The silt was obtained from a settlement

on a plate of glass held horizontal in the hatchery tank; ocular micrometer measurements of samples gave a range of

0 ,0$ to 0.001 mm., with only a few of the 0 ,0$ mm. particles

sand; particles smaller than 0.002 mm. were probably clay

from suspensoids. (Weeks, 191+il-* showed the upper 2 cm. of

sediments at a station near South Bass island contained: 39.1 per cent sand, 0.0^-0.002 run. in size; 27.8 per cent silt, 0.0^-0.002 ram.; 32.9 per cent clay less than 0.002 mm. particle size.)

These experiments were repeated in October, 19$2 with confirmatory results. Mention is made by Mashtaler (1937)

- 11^0 .- of H. oligactis ridding itself of sand or silt; he reports that 3 out of 10 hydras covered with a centimeter of sand managed to escape into the water after eight hours. Capacity of oligactis to withstand mudding-over for periods of sever- , o .o al hours at 16 and 25 has been observed during our dredging operations (see section, "Deep-Water Communities"). Examin ations of mud and muck which sometimes loaded the spaces between slides set on the bottom at station 5 disclosed no hydras; they were found only on the exposed surfaces of the slides and rack. Samples taken with an ooze sucker at this station never yielded hydras. Core samples from the muck- bottom slope of Douglas Lake, analyzed for quantitative distribution of microfauna by Moore (1939« contained no hydras.

In most Lake Erie bottom habitats, where silting, sand ing or mudding-over is frequently a hazard to the existence of sedentary and sessile aerobes, the ability of the unpro tected hydra to withdraw itself by contractile motions of column and tentacles may be of some survival value. But unless there Is a substratum available upon which the pedal disc can fasten, the animal sinks back into the mobile layer of sediments at the bottom interface and perishes. On sand and other granular bottoms in shoal areas these naked organ isms would be quickly ground to death by molar action. Thus

In bottom habitats, hydras are found living only on some suitable support — protruding rocks, mollusc shells, dead

- 11*1 - leaves, or wood (see section on habitats). The buoyancy of water supports the soft-bodied hydra so

that its column and tentacles float above the substratum to

which the pedal disc is attached by adhesive secretions. In

this way the delicate tissues escape abrasion and direct con

tact with silted surfaces. Nevertheless, the radially sym

metrical diploblastic form of the body exposes a maximum of

cell surface per unit volume to the turbid medium. To what extent high turbidity directly affects the

physiology of hydras has not been determined. Results of a

crude experiment, however, suggest that neither littoralis

nor oligactis can tolerate a heavy silt suspension. A double

series of two cultures, each containing about £>0 animals,

were set up in battery jars containing a liter of filtered lake water. The water was kept turbulent in each with aera

tors. About 50 cc. of fine silt, taken from an accumulation settled upon a large glass plate in the hatchery tank, was introduced Into two of the cultures. After three days, the majority of the animals in the silty cultures showed signs of depression, evidenced by contraction. The control animals were in the extended postures characteristic of the long-

tentacled forms under favorable culture conditions. Microscopic examination of 30 of the contracted experi mental animals disclosed fine particles of silt in the

gastnFovascular cavities. Silt was also found in the~"diges-

tive cavities of 20 littoralis specimens which remained at

- 11+2 - tho clean edges of the heavily sedimentated glass plate in the hatchery tank. After prolonged periods of high turbidity following spring and autumn storms evidence of silt in the gastrovascular cavities of specimens collected in the bay was observed during routine squash examinations. Particles were in the size-range of clay. Some of the fine clay particles which are held in sus pension in the alkaline waters of Lake Erie (Langlois, 19$k» p. 3i+8) apparently are sedimentated in the gastrovascular cavities of the hydras. It is also possible that sedimenta tion, occurring on the surfaces of the epidermal cells where the projecting cnidocils may collect suspended particles, interferes with respiratory functions. Much work needs to be done before the physiological effects of prolonged high turbidities on hydras and other lake organisms can be under stood.

In nature, as we shall see in the next section, It is difficult to ascertain whether the absence of hydras from a particular microhabitat is due to silting, to crowding from competitors for space, or to a complex of abiotic and biotic factors which elude analysis.

- li+3 - CHANGING AGGREGATIONS IN THE MICROHABITATS Her© an attempt will b© mad© to briefly characterize the

plant and animal life of the communities in which the hydras

occupy a rather unique niche. These communities establish

themselves on any submerged substratum (as was seen in the

general survey of habitats) where the associated organisms can fasten and form an aggregation. Regardless of the type of substrate or its location, rapidly multiplying sessile

’’Aufwuchs” and ’’Bewuchs” organisms — mainly stalked diatoms and vorticellids — are the pioneers of these essentially periphytic communities. The type of substrate, whether liv

ing or dead, as well as its location in the general habitat

exerts a selective influence upon the ultimate composition

of the biocoenosis (Roll, 1939; Young, 19i|-5j Ruttner, 1953). The substrata in Lake Erie which provide the principal

microhabitats for communities in which Hydra may be regarded as the dominant 11 peri phytic" animal are pieces of limestone

rubble, leaves of plants, and shells of molluscs.

Only a crude qualitative reconstruction of these communities as they exist in the biotopes where the hydras were

studied can be provided at this time. It is apparent from Langlois* comprehensive review of the ecological investiga

tions undertaken in western Lake Erie (195^) that much work

remains to be done with the microflora and microfauna of the

little known communities of the'lAufwuchs.1’

- 11* - The generic composition of the larger invertebrate raeta- zoan fauna associated with the microbiota on stones and plants in the South Bass Island habitats appears to have changed little since its summer distribution was quantita tively determined by Krecker and Lancaster (1933) and Krecker (1939). These pioneer studies along with the many excellent life-history studies carried out at the Franz Theodore Stone

Laboratory (summarized by Langlois, 1951+) enabled us to arrive at species identifications of some of the dominant members in the communities under study. Determinations of insect larvae were verified by Dr. N. Wilson Britt; species of mites were identified by Dr. Robert R. Crowell. Diatom identification to genera were verified by Dr. Jacob Verduin; it Is hoped that species determinations can be made even tually from preserved material on hand. Unfortunately the Platyhelminthes have not been investi gated in Lake Erie, and no material was sent to taxonomists in this difficult group. Likewise, since only specialists in residence at the Laboratory were consulted for taxonomic assistance, the periphytic bacteria and fungi remain uniden tified. These groups along with the protozoans and sessile algae appear to form 11 centers of action” (Elton, 191^9) in the communities; their interactions await investigation by teams of microbiologists.

- U+5 - The Swift-Water Community of the Block Rubble The residents of the microhabitats living on the block rubble make up a complex aggregation of plants and animals adapted to existence in swift water. The more permanent residents of this community are sessile and sedentary forms: among the plants diatoms of the order Pennales, the filamen tous chlorophytes Cladophora and Stigeoclonium, some endolithic blue-green algae, and various species of phycomycetes and periphytic bacteriaj among the animals the vorticellid protozoans (principally the colonial Carchesium, the stalked

Vortlcella, and loricate Stentor), colonies of the bryozoans

Plumatella and Fredericella, budding hydras (predominantly

H. littoralls), and the minute hydroid colonies of the fresh water jellyfish, Craspedacusta sowerbii.

These forms, it will be noted, are all capable of form ing aggregations by rapid asexual reproduction and maintain ing attachments to the substratum in the agitated water through specially modified holdfasts. Thus, early in the annual cycle, they rapidly colonize the rubble that covers the bottom in the eulittoral zone. The offspring of these pioneers fasten on the permanent substrate provided by the dolomite rocks and form the core of a stable 11 Aufwuchs” com munity. The earliest aggregations in this microcommunity may be regarded as homotypical or primary associations com posed at first of monosyngenia and then polysyngenia, - lij.6 - according to Deegener’s classification of aggregations

(Allee, 1931).

Year-round observations of ecological succession in the microcomraunity of the "Aufwuchs” made from comparative study

of the settlements on clean slides and fresh pieces of block rubble show the pioneers in the microhabitats of the agitated waters at all seasons are species of the pennate diatoms of Gomphonema, Epithemia, and Cymbella and the peritrichous protozoans Vorticella and Carchesium. Of the metazoans

Hydra littoralis and plumatella repens are the pioneers. Cymbella multiplies in a long stout pectinate tube,

sometimes branched. At the climax of the community in late

summer, these tubes form a microscopic forest so dense the stones appear covered with a brown slippery coating. Closely

appressed to the substratum is the flat Epithemla. Gompho- nema grows on short stout stalk firmly attached to the edges of the rocks. Under the microscope numerous species of

Navicula and Pinnularia can be seen barely moving through the jungle of the gelatinous stalks and tubes, strands of filamentous algae, and mycelia of fungi. Frustules of the phytoplankters Tabellaria, Fraglllaria, Stephanodlscus and

Melosira, and members of the drifting Cyclotella-Asterlonelia community are caught in the tangle. Couched in the branches of the algal forest can be seen the bright green shapes of Cosmarium, pediastrum, and Zygnema.

Clusters of Vorticella can be watched contracting on stout stalks and the colonial Carchesium is often abundant.

- 147 - In early spring, the beautiful Stentor eoeruleus appears in the loricate form, particles of food, probably minute algae

and bacteria, can be seen swept into currents of the peri-

trich vortex. Various free-swimming ciliates and free-living

amoebae originally studied by Jennings (1901b), also some of the sessile rotifers described and figured by Jennings (1901s) especially Mellcerta and Foscularia were noticed in numbers during the summer season. Colonial rotifers of the genus

Conochilus were also seen in the settlements from the plank ton communities. Hemispheric colonies of Spongllla lacustzis appeared in spring, but the sponges never grew large on the rock substratum.

Among the metazoans, the dominant sessile animals of the swift-water "Aufwuchs,” both in size and numbers, are

Hydra littoralis and plumatella repens. To what extent, these two dominants compete with each other for food was not determined, but certain data indicate that the bryozoan colo nies may compete with the hydras for attachment space. During the siammer hydra maximum, the branched growth of the zooecia spread over the surface and edges of the rubble forming a mat covered with diatoms and detritus, and the lophophores of the numerous polypides are extended in active feeding. It was observed that the hydras were never found attached to the zooecium of a bryozoan colony, probably because detritus and silting rendered it unfit. It was also found that col lections at stations 3 and 1*, where the bryozans grew most

- 11*8 - densely, usually yielded fewer hydras compared with the other ruhble-bottom stations. Since conditions, at these stations appeared equally favorable, it seems likely that the hydras, which require at most an area of about one square millimeter for individual foothold, were crowded out of the microhabitat by the creeping branches of the bryozoans.

It can be reported at this time that the other coelenterate periphyte, the colonial hydroid of (J. sowerbil, Is so minute that it can occupy the interstices between the twigs of the bryozoan zooecium. Here, almost hidden by diatom masses, It was first observed in Lake Erie during July of

at stations 3 and 1|. Apparently, the medusoid stage is suppressed, as no one at the Laboratory has encountered schools of the jellyfish which are easily seen with the un aided eye. (See Davis1 recent study of the medusae in Crys tal Lake, Ohio; also Payne’s classical papers on the life history of Craspedacusta, 192ij., 1926.)

By far the most numerous of the larger metazoans in the swift-water community of the block-rubble are midge larvae, caddis-fly larvae, and snails, especially those of the genera

Goniabasis and pbysa. In view of the extensive distribution of these animals, Krecker and Lancaster (1933) characterized the bottom shore fauna up to the depth of six feet in western

Lake Erie as a "midge-caddis-snall association," with the chironomids the dominant animals in point of numbers. Our findings on the distribution and numbers of hydras and other

- U+9 - raicrobiota in the shore zone suggest that the sessile and more permanent part of this community might be termed a "diatom-hydra-bryozoa association.” In any event, the inter actions between the dominant chironomid-trichopteran-gastro- pod fauna and the diatom-hydra-bryozoan microbiota will become evident in our discussion of food-chain relations.

Also hydra's tendency to form heterotypical associations with snails, caddis-fly larvae and nymphs of mayflies will merit some discussion from the viewpoint of population dis persal.

The amoeboid parasites and ciliate commensals living on the hydras, as well as hydra's host-relationshlp with the cladoceran Anchistropus minor, form interesting examples of antagonistic reactions within the community, to be discussed later on.

The large triclad turbellarian. Dugesia tigrina and the minute rhabdocoele flatworms, multipy rapidly by fission as the water warms, and can be seen sliding through the micro jungle of diatom stalks. Their ecology has not yet been studied in Lake Erie. The most numerous representatives of the free-living platyhelminthes are the microscopic specimens of the genus Stenostomum; these rhabdocoeles can be quite easily distinguished from the associated genus Microstomum, whose species appropriate hydra stenoteles; (III: !+-a).

The niche of the minute free-living nematodes, which are often abundant in the microfaunal aggregations during the

- 150 - spring, remains to be determined. The Annelida are only sparsely represented by a few

naid oliglochaetes and the leeches Herpobdella and G-losso-

phonla. The insect larvae, which abound in the community, pre

sent a fascinating study of body features specialized for

maintaining position and feeding in water currents; some of these swift-water animals are figured and discussed at some

length by Welch (195>2), Ruttner (19^2-1953), and Coker (19^|). Larvae of the various species of the genus Chi ronomus live in cases built of silt and ribbed with diatom frustules.

The order Trichoptera is represented by larvae which drag

about their characteristic oases; the commonest caddis larvae

belong to the families Hydroptilldae, Leptoceridae, polyeen-

tropidae, Sericostomidae, Mollanidae, and Sericostomidae.

Often abundant among these swift-water caddis larvae is the net spinner, Hydropsyche, usually found in streams. The

Emphemeroptera are represented primarily by the streamlined

nymphs of the Stenonema complex, Ephoron album, and Ephemera simulans. The "water penny" larvae of psephenus herricki

are glued fast to the surface of the rock; another beetle

larva which is fairly common is that of the long-toed Stenelmis bicarinatus.

More transient members of the community are the clinging

forms of crustacea: ostracods of various species, the gam- marids and Hyalella azteca Gammarua fasciatus, migrating

- 151 - copepods (mainly Cyclops sp.), & few shore-blown members of

the limnetic daphnid communities, the littoral cladoceran

Sida crystallina, and the various species of chydorid cladocerans, which cling to algae in the periphytic communities.

These littoral-zone cladocerans, generally overlooked in western Lake Erie because of concentration on open-water plankton sampling, have been identified to species.

Chydorus sphaerlcus (O.F.M.) thrives in the plant bed during the early spring; Pleuroxus procurvatus Birge and

Pleuroxus denticulatus Birge also make their appearance in the winter waters; Acroperus harpae Baird comes in Hay and

Leydigia quadrangular!s (Leydig) follows in June. The largest of the Chydoridae, Eurycercus lamellatus (O.P.M.), forms dense parthenogenetic swarms in August. Other species, found in sparse numbers during the summer season, are Alona quadrangularls (O.P.M.) and Camptocercus macrurus (O.P.M.) The intimate relationships of these chydorids with the coelenterates raise questions concerning their niches in the changing microcoramunity which will be discussed later.

Larger crustaceans, the isopod Asellus and the crayfish

Orconeotes hide and feed among the exposed rocks of the

Cladophora zone in the shallowest water at the shore.

Hydracarinids, principally of the genera Atractides,

Hygrobates, Megapus, and Libertia, come into contact with the hydras in the rock microhabitats. A specimen of a rare hala- carid mite, found clinging to the column of a H. littoralis,

- 152 - was identified by Dr. Rodger Mitchell, as Soldanelonyx monardi Walter, who established it as a second record for

North America, the only other record for the continent being Walter's from Domalson's Cave, Indiana (personal communication).

Frequent invaders of the shore habitats are the various fishes, whose ecology is discussed by Langlois (19f>lj-> PP« 17^4-—28Ip). occasional visitors are mud-puppies (Necturus maculosus), the water snake (Matrix sipedon insullarum), scavenging turtles (especially G-raptemys geographies), and the wading water birds.

The Myriophyllum-Leaf Community

The metazoan inhabitants of the fine leaves of this densely populated macrophyte which grows in the stiller waters of the South Bass Island shore have already been men tioned in our discussion of vegetation zone habitats. The composition of the microbiota in the "Aufwuchs” does not differ essentially from that of stone, except that filaments of the Oscillator!ales Gleotrlcha, Qscillatoria, and Lyngbya take hold on the living substratum. Also amoeboid forms, especially Amoeba verrucosa, are more common in the heavy settlement of detritus as summer progresses; and, as Young (19l|5) demonstrated in his study of bulrush periphyton, raycelia of the phycomycetes penetrate leaves and stalk as the season wanes. It was observed at station $ that the

- 153 - proximal region of the column of the hydras (H. oligactis and

H. pseudollgaotis)where they had attached to the epidermis of the leaf gradually became overgrown with a brownish diatom- fungus mat during the midsummer calm. The silt collecting in this tangle probably contributes to rendering the changing leaf substratum unsuitable physiologically for the hydras.

Wave action during storms apparently does not dislodge the hydras from their foothold on the leaves (see section on plankton). In a careful quantitative investigation of the effects of stormy weather on periphyton in Douglas Lake,

Young (19l+5>> pp. 8-10) found H. oligactis thriving while other periphytic organisms decreased. Crowding for space, bryozoan colonies which are sparse on the little leaves, is not a factor as it is in some of the rubble microhabitats. One is struck by the similarity of the "Aufwuchs’1 on a

Myriophyllum leaf from western Lake Erie to the community on a leaf of the same genus of plant from the Lunzer Untersee as described and pictured by Ruttner (19!?3* Pig. 1+3). The difference in life forms between the same species of diatoms and vorticelllds growing on the plant in still water and those living on the rock in agitated water are discussed by

Ruttner (pp. l£8-l59).. Rot only are the stalks longer and more branched In the same species on the leaf than on the stone in rough water, but the holdfasts are not as strong.

In this respect it Is interesting to note that the swift- water hydra of the rubble community has a broader pedal disc

- l $ k ~ and stouter column than the oligactis-pseudollgactl s forms attached to the water plants.

The living substrate of the plant leaf, as Ruttner points out, is not nearly as stable as that of the inorganic stone. The periphytic organisms enter into a complex reci procity with the raacrophyte as its tissues grow and decay; metabolites of the plant may become beneficial to the micro organisms early in the season, h a m f u l later on. Similarly the multiplying periphytes may at first contribute metabo lites directly beneficial to the plant, and to certain mem bers of the microbiota associated in the leaf community, but may eventually produce antibiotic effects on each other and on the tissues of the macrophyte. This whole question needs to be investigated, especially the competition of the flora of the "Aufwuchs" for sunlight and the effects of a thick periphytic mat in depriving the plant leaf of its solar energy. In the general habitats of western Lake Erie, the circumstantial evidence indicates that the limiting factor on the production of both phytoplankton and attached microflora is turbidity; not only has allocthonous material held in suspension in the shallow waters reduced the plant habi tats available to periphytic organisms and their associated fish food, but the high turbidity in the key area must limit the flora which forms the foundation for the piant-leaf com munity (Langlois, 1954s PP* 97-104). Availability of the oligochaetous annelids as hydra food

- 155 - may be much greater in the leaf microhabitat than in the stone microhabitat since species of Nais, Stylarla, and

Chae togas ter are abundant at most seasons of the year. The principal difference in the arthropod part of the community is in the absence of mayflies, which abound on the stones, and the presence of nymphs of the order Anisoptera (Coenagri- onidae) and the Zygoptera (Libullulidae) are present here in the water weeds. The clinging gammarid crustaceans and the chydorid cladocerans (species listed above) are more abundant on the leaves than in the periphyton of the block rubble.

Species of snails, especially those of the genera

Heliosoma, Ancylus, and Amnicola are abundant on the plants at times. On the other hand, the dominant snail of the swift-water community, G-onlobasis livescens (Menke) is absent from the plant microhabitats.

Hydras as Epizoites of Molluscs

Attention has already been called to the role of the pelecypod mollusca and certain of the gastropods in forming suitable microhabitats for hydras and other sessile forms on the muddy or shifting sand bottoms of the lake (see section on deep-water communities). In transects of the bay, espe cially during spring, hauls of clams often yielded numerous hydras. At all seasons of the year, hydras were found attached to the shells of the snail species which browse through the bay microhabitats* The chitinous coverings of

- 156 - the insect larvae also served as places of attachment for hydras. Our notes record a number of examples of H. littor- alis1 ”riding habits" on the cephalothorax of the mayfly nymph Stenonema, on the cases of caddis flies and midges, and even on the first pair of legs of adult almid beetles.

In one instance, three hydras remained attached to the exo skeleton of a Stenonema for ten days. In another Instance, the hydra (H. oligactis) rode the gills of a damsel fly larva for two weeks while both coelenterate and insect fed on daphnids introduced into the finger bowl. The movements of in sect carriers, sometimes quite lively, did not disturb their passengers or dislodge them.

This riding habit of the hydras on self-moving objects

— shells of snails and clams or cases of caddis flies — was mentioned by Steche in the popular German monograph devoted to the hydroids (1911, p. if.). Peculiarly, however, few biol ogists have since noted this relationship, which obviously has biological significance. Somehow Krecker in his inten sive work on the distribution of the shore fauna of western

Lake Erie (1933, and 1939— with Lancaster) did not observe the close physical association between hydras and their mol- luscan and arthropod vectors. He recognized the Importance of caddis-fly worms as agents in the distribution of Spongil- la (1920} and reports on the conditions under which Gonio- basis llvescens occurs in the Island Region (192if.), but does, not record any observations on their association with hydras.

- 157 - Similarly, Goodrich (191+5) in his monograph on the ecology of G. livescens in the Great Lakes region makes no mention of the occurrence of the hydras on the shells of this widely distributed pleurocerid species.

Only a few reports concerning hydra’s easily observed riding habit have been published. Clark and Wilson in their survey of the mussel fauna of the Maumee River (1912, p. 13) record that hydra "was quite abundant, and numerous examples were seen attached to the back of the shells of Ancylus."

Griffin and peters (1939), in their description of H. oregona, report that the new species was found attached to the cases of chironomid larvae, as many as six to a case. Bomer, in his monograph on the St. Moritzer Sees (1917, p. lj-8) states that he found H. vulgaris attached to bryozoans of the sub prof undal zone.

Biologists who have previously made intensive investi gations of hydra ecology (see papers by Welch and Loomis,

Miller, Bruden, Boecker previously reviewed) do not record any observations on the heterotypical associations between hydras and snail aggregations. Yet, from the results of the present investigation, one may hazard the prediction that in any habitat where hydras and snails occur in abundance, some polyps will be found attached to the shells of some of the snails if the living specimens are immediately examined at the time of collection.

By handling collections from the stones and plants in

- 158 - the method employed in this investigation and making exten sive observations of the behavior of hydras and snails in the laboratory, some facts have been established concerning this hitherto ignored relationship. These are summarized with special reference to the dominant Hydra littoralis and

Goniobasis livescens of the swift-water community as follows:

(1) All the hydra species will attach to the species of snails that form part of the microcommunity. Hydras were found and maintained for weeks on every species of snail mentioned above.

(2) Species-specifity does not enter into the relation ship. None of the hydras, however, will attach to the shells covered with a dense algal mat. This was especially notice able in midsummer collections of hundreds of G. livescens from the rubble where hydras were abundant. The old "moss- backs” — as these snails have been aptly termed (Goodrich,

191+5)— bore no polyps, whereas at least one hydra could be seen attached to the shell of the younger snails where the periphytic growth was not dense.

(3) The incidence of H. littoralis on G. livescens in the microhabitat is greatest during the vernal hydra pulse.

At this time up to half of the hydras taken in a stone anchor collection may occupy the microhabitat of the snail shell.

To illustrate in a collection from the station on May 16,

1951+* at the beginning of the hydra pulse, 28 hydras were found on the stone anchor and 20 were observed on the shells

- 159 - of the 6 snails removed from it. The number of hydras per shell ranged from 1-6, with the following distribution per snail: 1, 2, 3, 3, £, 6. Attachment of the hydras varied in position from the aperture at the basal whorl to the api cal whorl, the majority being on the more basal whorls. All but i|. of these 20 were budding. A similar high incidence was observed at the height of the hydra pulse on June 29,

195>2 in a month’s settlement on the concrete block at station

B (Table III). Everyone of the 13 snails in the collection had at least one hydra growing on it; some bore as many as four or five. (J+) Aggregations of snails start to migrate from their hibernation places under stones at the beginning of the hydra pulse. Thus the association of the hydras with the molluscs apparently arises from a density-dependent situation obtaining in the community complex (Elton and Miller, 19^1+)•

The tendency of the snails to migrate from the rough water close to the shore onto the sides of the pieces of rubble, where they maintain themselves in the current was studied by

Krecker (192l(.) on the Gibraltar Island shore. This behavior serves to distribute the attached hydras to a favorable microhabitat where the buds they produce can quickly form aggregations. Likewise, the snails’ habit of migrating up wards in still water may be a factor in the vertical dis tribution of the hydra population during the late summer calm. G. livescens is stimulated to move by the waves

- 160 - oscillating the substratum or by the waves directly striking their shells — apparently because the rocking stimulates their organs of equilibrium according to Krecker’s experi ments. Goodrich (19l+5» P» 29) has observed that these pleurocerids move several feet in the course of a few hours.

There can be little doubt that these snails are one of the important agents in the dispersal of the Lake Erie hydra population into favorable locations in the general habitat.

Prom our observations of hydra’s capacity to withstand desiccation, most of those which are exposed on stranded shells during even prolonged seiches survive.

(6) The hydras are in no way injured by the snails who browse through their aggregations. Similarly, the most deli cate exposed tissues of the snail, its tentacles, are unin jured by contact with the nematocyst-loaded tentacles of the hydra.

# * It is clear from these observed facts that the principal role of G. livescens in the ecology of H. littoralis is that of a vector. The snail shell constitutes a mobile microhabi tat for the periphytic community which grows upon it. As in the community of the rubble stone, of which the pleurocerid is an integral part, the diatoms previously mentioned are the pioneers and dominants among the raicroflora. Of the microfauna, the hydra is the largest of animals, towering above the vorticellids, which are the dominants in point of

- 161 - numbers.

Other protozoans seen in the shell "Aufwuchs" include !► suctorians, and numerous free-living ciliates and amoebae.

The single known species of fresh-water endoproctan (TJrna- tella gracilis) was found on the shells, whereas bryozoans were absent. The filamentous algae have not been identified yet, and the endolithic forms which penetrate the lime sub strate remain to be studied from specimens which are avail able in preserved material from all collections.

Both the snails and the hydra on its shell in turn furnish microhabits suitable for their characteristic para sites and commensals — the hydroid for the ectoparasitic

Hy dr amoeba hydroxena and the chydorld Anchistropus minor, and the ciliate ectocommensals Kerona polyporurn and Tricho- dlna pedioulas, the pleurocid for its trematode endoparasites.

The intricacy of the strange relationships existing in the complex of the microcommunities is well illustrated by an observation of Dr. E. E. Dickerman (personal communica tion). While investigating the life-history of proterometra macrostomum, he witnessed the ingestion by the epizoic hydras of the cercariae of this trematode which swarmed from the

Gonlobasis collected at Gibraltar Island. The hydras gorged themselves with the cercaria, which are about 2 mm. long. In this instance, the epizoic habit of the hydra would appear to have nutritional value, and one might consider its rela tionship to the snail as commensal.

- 162 - In general, the biotic relationships existing between the epizoites of the shell and the living animal which in habits it are best viewed as what Deegener terms a " sym- phoria," i.e., a heterotypical association formed when one or more species of animal settles upon another species of animal without obvious interactions of mutualism or para sitism, a situation which Is common In the growth of hydroids and other forms on shells of molluscs and arthropods in the sea.

In the community complexes of this type as observed in the bay, the epizoic habit may be beneficial to certain mem bers of the shell or chi tin community.. For example, the colonial vorticellid Carchesium grows oh almost every sub stratum in the moving waters of the bay. Britt (1950, p.

95) in his study of the white mayfly Ephoron album noted that 98 per cent of the nymphs collected at the Oak Point habitat in the bay (Fig. 3) were covered with Carchesium colonies. He observed that the nymph provides the protozoam a place where they can obtain their food particles from the currents of water.

It may be that further Investigation of the epizoites on snails and arthropods will indicate that many of the pro tozoan forms are In a transitional stage from commensal!sm to parasitism. Such has proved to be the case in the vorti cellid Qpercularia which attaches to various fresh-water arthropods and cannot live when deprived of the water disturbance resulting from the movements of its host. (See

Caullery, 19f?3» p. 32 for experimental evidence relating to a "inquilism.") The hydras epizoic relationship to arthropods and snails on the other hand, is purely phoretic, according to the criteria for commensalism and parasitism clearly pre sented by Baer (19S>1). The inference made by Mashtaler

(1937) from his observation that only hydra which attached to Limnea stagnails in cultures could escape destruction by the snail represent the same kind of symbiotic relationship existing between the actinians and crabs seems far fetched.

POOD-CHAIN RELATIONS

The interspecific relations in the micr©communities where hydras live are too complex for complete analysis here.

Prom what little we know of the physiological requirements cf the species making up these communities, it appears that the major regulatory forces at work in the integration of the aggregations into true communities are: (1) the common de mand for a substratum as a condition of existence, and (2) the predator-prey reactions of the organisms in the species network. A clue to hydra's niche in the communities just described can be gained by a brief examination of its food- chain relations.

- I6I4. - Predators of Hydra

On the basis of a long series of observations made

during the investigation, it can be stated that only one animal in the community preys upon Hydra. This animal is

the microscopic rhabdocoele flatworm Microstomum.

That species of the genus Microstomum were among the few animals known to prey on hydras was called to the attention

of zoologists by Schulze (1917, p. 113 J. But it was not un til recently that the significance of the relationship be tween the rhabdocoele and the coelenterate has been recog

nized. As a result of the behaviorial and histological

studies of Kepner and his coworkers, we are now aware that the eating of Hydra by Microstomum constitutes one of the

strangest examples of instinctive behavior known to etholo

gists (Tinbergen, 195>1, p. 159). The origin of nematocysts in the epidermis of Micro

stomum was traced by Kepner and Barker to the rhabdocoele1s habit of eating hydras (192l|.). They established that the

tiny rhabdocoele, unlike many larger animals associating with

the coelenterate, was able to protect itself from the hydra*s

toxic nettle cells by a secretion spewed from the pharyngeal tube onto a part of the hydra's epidermis. This secretion

blocks the discharge of the nematocysts and enables Microsto mum to ingest part of the hydra, or sometimes the whole hydra. All the tissues of the hydra are digested in the worm's enteron, including the cnidoblasts which secrete the nemato-

- 165 - cysts. The three non-poisonous types of nematocysts are egested along with other undigestible materials. The undis charged stenoteles, however, are ingested by the gastrodermsl cells. The way in which they are transported through the mesenchyme by the "cnidophages" and are distributed by these peculiar wandering cells in the epidermis, orientated so that they can explode in defense or offense, is completely de scribed in the papers of Kepner and Barker (1924), and Kepner and Nuttycombe (1929). In his beautifully illustrated book, 11 Animals Looking into the Future11 (192£), Kepner is inclined to treat the adaptiveness of the behavior exemplified by Microstomum and

Hydra from an unnecessarily teleological point of view;

Nevertheless, I cannot help but feel as does Lashley (1938); after quoting from Kepner*s graphic account of the phenome non, he comments: "Here, in the length of half a millimeter, are encompassed all the major problems of dynamic psychology." My observations of the remarkable relationship between the nematocyst-bearing Microstomum species and the Hydra species studied in aggregations collected on slides through the seasons confirm the findings of Kepner and his associates.

In addition, the Lake Erie field data provide the basis for the following ecological summary: (1) Species of the rhabdocoele genus Microstomum are found in close association with the hydras in the microcomraunity. As provisionally identified, there are two species:

- 166 - Microstomum line are (Milller) and Microstomum cauda turn Leidy. The latter species occurs most commonly; there are probably other species of Microstomum also in the mixed population. (2) Microstomum multiplies rapidly by the formation of zooids at the beginning of the vernal hydra pulse and declines in numbers when the hydra population is dwindling. During the early summer, in a collection of 119 H. littoralis at station 3, as many as 6£ Microstomum were found together with the more numerous Stenostomum which populated the 18 slides.

The number per slide varied from 0 to 20. All the Micro stomum contained the stenotele's of H. littoralis in their epidermis; the number in a sample of Vp ranged from 3j? to

S>00 per individual. (3) The rhabdocoeles were seen preying on the hydras most frequently during the period of early stammer. This was probably due to the number of new individuals produced from the zooid chain which had not yet acquired a full battery of hydra nematocysts. It was found that Microstomum individuals which were loaded with nematocysts avoided the hydras and fed on diatoms (mainly Cymbella).

(Ij.) Microstomum in its attempt to appropriate the polyp’s stinging cells, is sometimes eaten by a hydra. But in gen eral, as Kepner suggested, tke rhabdocoele, when loaded, avoids the coelenterate with nematocysts. Our findings lead to the conclusion suggested by Kepner (192£, p. 68), namely, that Hydra is eaten by Microstoma primarily for its nemato- - 167 - cysts and not for the food that its body represents. (5) There is no species-specificity between the Micro-

stomum species and the Hydra species. The rhabdocoeles were

observed eating every species of Hydra living in the commu nity. (6) The only type of nematocyst of hydra retained by

Microstomum is the poison-containing stenotele. The other three types of hydra nematocysts are egested. Holotrichous

isorhizas were seen in the spongy mesenchyme, but they are never transported to the epidermis by the cnidophages. Kepner and his coworkers do not make this point clear al

though their figures show only stenoteles in the worm's dpi-

dermis. The stenoteles are distributed quite uniformly throughout the peripheral mesenchyme of the rhabdocoele.

They are orientated so that the apex of the pear-shaped

stenotele projects from its attending mesenchymal nurse cell (the cnidophage) into the ciliated epidermis (see excellent

illustration by Kepner, 192J?, Pig. 16). Thus, these stinging

cells can be triggered off by contact of another animal's body. For example, when a Microstomum was isolated in a watchglass- with a small naid worm, the annelid was quickly paralyzed; 35 stenoteles, which had been discharged from the Microstomum, were found penetrating the oligochaete»s body.

Similar cases of Microstomum paralyzing small oligochaetes were observed during inspection.j of the slides, and in two instances the Microstomum attempted to ingest the immobilized

- 168 - annelid. These observations are in accord with those of.

Kepner and Barker (192lj.), who first demonstrated that the nematocysts appropriated from Hydra by Microstomum functioned

as offensive weapons for the turbellarian. (Kepner, 1925,

Pig. 17, illustrates a ’’loaded11 Microstomum swallowing an

annelid.)

4* # »

The only reports of evidence that Microstomum is asso ciated with Hydra found in the ecological literature are

those originating from investigators at Douglas Lake. Miller

(1936, p. 177) mentions that some flatworms (M. lineare) containing nematocysts occurred frequently on his slides

populated with hydra; but he remarks "that the few hydras

eaten by so small a flatworm population would easily be re placed by the active budding of hydras.” This inference

appears correct in the light of the observations accumulated

during our investigation at Lake Erie. Moor (1939, p. 5>l+8) i lists Microstomum sp. as one of the six genera of Rhabdocoela

occurring in his collections of microscopic benthic fauna;

only three Pelmatohydra were found in the six collections

made on sand and muck bottoms where the nematocyst-loaded

Microstomum specimens were abundant. Another platyhelminth which can prey upon Hydra is the

dendrocoele turbellarian Procotyla fluviatilis, Leidy. This

flatworm was not observed in Lake Erie, but according to

Redfield (1915), it can seize the hydra in its grasping

- 169 - organ, (See Illustration after Redfield in Pennak, 1953,

Pig. 70.)

Coker (195^, P« 216) states: "It is said that hydras

are eaten by large copepods." However, I have not encoun

tered any evidence of this dubious predator-prey in the

source literature on the Hydridae.

In his monograph, Schulze (1917, P» 113) mentions that In addition to Microstomum certain fishes, larvae of Chirono-

mus and Corethra, and snails of the genus Limnea are known

to prey upon hydras. Mashtaler (1937) reports an observation of Limnea stagnalis preying on hydras; he states, however,

that various species of fish he studied in aquaria stocked

with hydras do not eat the coelenterates.

Smallwood (1918, p. 331), however, observed trout fry, a few weeks old, feeding on "red hydras" which had settled

in a hatchery trough from the Lake Clear Intake, Saranac Inn, Hew York. He says: "After trout have eaten freely of red

hydra, their droppings are colored red. . . . Doubtless,

the young fish in the lake and small minnows that secure

their food from the stems of plants eat many of these hydra."

It Is quite likely that small fishes feeding on the

Insect larvae and oligochaetes in the vegetation beds or on the rocks of shoal areas accidentally aat hydras. No exten

sive observations on the ingestion of hydras by fishes in

aquaria were made during this study; the fry of the Erie whitefLsh and the yellow pikeperch, however, did eat hydras

- 170 - which were thick on the sides of the hatchery troughs.

Although it can easily he demonstrated that hydras are able to kill newly hatched fry (III; 6-a), it is difficult to ascertain to what extent this reaction reduces aggregations of larval fishes that invade the communities in search of food.

Feeding Reactions and Availability of Prey

Since the publication of Abraham Trembley's memoires on the fresh-water polyps (171+1+), naturalists have been fas cinated by the feeding reactions of hydras. Even the most blase^ ecologist of today cannot help but feel some amazement when he watches a hydra seize and swallow water fleas, aqua tic worms, or insect larvae much larger than its own body. One can hardly improve In either accuracy or clarity the descriptions of hydra feeding which Trembley presented

In his second memoire (pp. 79-llj-8> pis. 6-7) "De la Nourri- ture des Polypes, de la Manie“re dont ils saisissent & avalent leur Proie. . . ." The author provides complete descriptions and many drawings of the feeding behavior of his "polype de la troisieme espece" (which we now recognize as Hydra oligactis Pallas, 1776); his figures show the hydra capturing and Ingesting "poucerons," "vers” and "mille-pieds." (The drawings from the life show that these were dapbnids deter mined by Swammerdam as poucerons branchys and aquatic anne lids termed "Mille-pied a dard" by Reaumur.)

- 171 - Trembley’s portrayal of his ’’long-armed polyp” as a fisherman with many lines is a delight to all students of the Hydridae*

On pourroit aussi comparer un Polype, a un Pecheur a la ligne, mais c'est un Pecheur qui sert un raeme terns de plusiers lignes. Pendant qu'il est occupe' a retenir une prol avec quelque bras, & a la porter a la bojuche (Pi. VI, Fig 3. min), les^ autre restent souvent e"tendus, & saisissent celle qui se" presentent.

Trembley also describes in great detail the action of the hydra’s mouth in swallowing its prey, and shows in his drawings the remarkable distention of its body wall when the enter on is gorged with a large animal. I have watched many times H. oligactis and H. pseudoligaotis fishing with their long threadlike tentacles for the various species of copepods and cladocerans common in the plankton of Lake Erie, and can confirm Trembley's observations. I have also been able to study numerous specimens of the Lake Erie hydras with the stereoscopic microscope while they were capturing, ingesting, digesting, and egesting entomostracans, oligochaete worms, and arthropod larvae. Yet I can add little to Trembley's classical accounts of the way hydras behave when feeding.

It is not difficult to establish the kind of food hydras eat. During the Lake Erie investigation it was demonstrated by feeding experiments that the local hydras would kill and ingest every kind of copepod and cladoceran which make their appearance In the plankton communities of the open waters.

(See Langlois, 19^4# pp. 122-131 for summary of ecological - 172 - studies of zooplankton species; also papers on Copepoda dis tribution and abundance in western Lake Erie by Andrews,

19l|8, 19^3i and Jahoda, 1949.) Our feeding experiments showed that the local hydras will also eat crustacean species which are not indigenous; the larvae of the brine shrimp

Artemi a, the fairy shrimp Eubranchipus, also the largest of the Daphnidae, D. magna. Hydras are known to prey on mos quito larvae (Iablokov, 1926; Twinn, 1931; Hinman, 1934)•

Ho food selection was observable among the various species of Hydra. They ate most entomostracans, including the largest of the cladocerans, the rapacious Leptodora kindtii. (For summary of findings on the ecology of this polyphemid in Lake Erie by Chandler and by Andrews, see

Langlois, 195>4» PP* 123-124.) Peculiarly, however, none of the hydras killed or ingested the delicate Sida crystalline or any of the chydorid cladocerans, which are listed by spe cies in the preceding section. Likewise, none of the other animals of the microcommunity are harmed by contact with the hydras except Chironomids, oligochaetes, and occasionally a small water mite in the nymphal stage. Among the mayflies only the nymphs of Ephemera simulans were paralyzed and eaten. My notebooks contain only three records of freshly- collected hydras which contained ostracods; usually these heavily-shelled crustaceans are rejected as food by the North

American hydras, as Hyman (1930, 1941) states. Certain of the European species — H. circumcincta, for example— feed

-173- on ostracods according to Schulze (1917* p. $k)• Mashtaler (1937) reports that he observed "H. fusea” at the Odessa

Biological Station feeding on rotifers and paramecia. Feed ing experiments with the Lake Erie rotifers, also with Para mecium, yielded results which contradict Mashtaler*s findings.

I have observed H. littoralis feeding on the Vorticella aggregations which populated the slides; but I have never seen hydras ingest Stentor, which were often abundant in the loricate form.

Hydras, when deprived of food, may feed on detritus.

Wilson (1891) reports from results of his aquaria experiments that H. oligactis migrates to the bottom and feeds on sedi ments containing decaying organic matter, diatoms, and infu sorians. Wilson*s findings were confirmed by Welch and Loomis (I92J4.) j Miller (1936) did not find any evidence that oligactis and pseudoligaotis migrated to the bottom or in gested the muck. H. oligactis in Kirkpatricks Lake, however, will ingest detritus at times when microcrustacean food is scarce, according to Bryden (1952, p. 60). Frischholz (1909, p. 208) after repeating Wilson's experiments concluded that migration to the bottom and feeding on sediments occurred only when the hydras were in advanced physiological depres sion. There is no species-specific difference in the kinds of prey which the Lake Erie hydras can paralyze. There appears to be some difference in the amount of prey which the respec-

- 1 7 k - tive species of hydras can capture, convey to their mouths, and engulf with their peristomes. This difference does not depend so far as is known upon variations in the potency of the poisons in the nematocyst capsules or the number of nematocysts discharged. For example, Hydra amerioana, the smallest of the Lake Erie hydras, is just as capable of para lyzing and killing any food animal, which happens to come in contact with its tentacles, as the robust Hydra littoralis.

The large stenoteles of H. amerlcana penetrate into the sur face of the prey»s body, and the ’’hypnotoxins" given off by the armatured tube appear to be as lethal to larger animals as are those exuded from the stinging cells of H. littoralis. It has already been shown (III: 2-b,c) that americana will feed just as voraciously on copepods of daphnids under proper culture conditions as will littoralis, or oligactis and gseudoligactis. The H. americana can ingest small oligochaetes and chironomids also. However little, if any, par ticles of food could be found in the gastrovascular cavities or the gastrodermal cells of the hundreds of ’’white hydras” which were squashed immediately after they were collected and identified as H. americana.

Examination of the circumstantial evidence leads me to hazard the suggestion that H. americana feeds very little on the other animals closely associated with it in the micro community. It may possibly be feeding on periphytic bacte ria. ZoBell (191+6) mentions that such bacteria are abundant - 17S - In the few lake habitats where the bacterial flora has been studied. Their coloration serves as a fairly good index to the state of nutrition in all species of hydras (Ills 2). It has been observed by me in my collections and by Hyman (1928,

1929) in her numerous collections that H. americana when taken from its habitat is almost invariably white. (It was for this reason that Hyman, 1929, after naming the new spe cies, designated it as the "white hydra.") If this species of Hydra were Ingesting and digesting animal food it would have some coloration due to pigments in its gastrodermal cells. Such pigments appear to be derived from the prey's tissue. As we have shown in our feeding experiments, "white hydras" when fed vernal copepods become orange-red; when fed daphnids, they turn brown.

There is now considerable evidence available that the colors of hydras — except the "green hydra" C. viridissima — are dependent to a large extent upon the food eaten. "Red hydras," regardless of the species, are so colored because of the pigments derived from the reddish oils stored in copepods. According to the study of copepod biology by Marshall and Orr

(19$$), the source of oil stored in "red copepods" may be derived from the pigments of diatoms upon which they are now known to feed almost exclusively. Records from the litera ture support the hypothesis that the reddish-orange coloration seen in the Lake Brie hydras, especially during the vernal

- 176 - season, can be traced to the ingestion of ”red copepods” :

Forel, 190lj.: H. rubra (H. oligaotis) -Cyclops magnlceps

Schulze, 1917: H. oiroumointa-Cyclops sp.

Smallwood, 1918 : Hydra sp.-Cyclops americana Rowan, 1930: H. canadensls-Cyclops sp.

Vogel, 1931: H. circumcincta-Cyolops sp.

Hyman, 1931a: H. carnea-Diaptomus sp.

Schulze (1917), using the chemical techniques available at that time, confirmed Studer»s demonstration (191i|-, p. that the colored pigments seen in red hydras were of the carotin-xanthophyll group. He mentions Trembley* s early observation (17i|l|., p. 127) that the color of hydras could be changed by feeding them red water mites, and discusses re sults of experiments by Leydig (l8£lj., p. 282) on the feeding of hydras with red copepods. He reports his own observations on the color changes induced in H. circumcinota by feeding them cyclops or ostracods containing a reddish oil. He demonstrated that the pigments seen in hydra*s tissues were primarily carotenoid, and that they were actively transported in H. ciroumoinota during gamete formation. (See Schulze,

1917* p. for authors cited.) It has now been established, mainly through the re searches of Fox and Pantin (19l|lj.), 11 that carotenoids and their derivatives are probably the main source of color in coelenterates.” The pigments are "lipochromes" as shown by miorochemical tests. The red, orange, and yellow coloration

- 177 - seen in hydras gastroderraal tissue are probably due to carotenoid pigments. Further research may show that they origi nate from the wide-spread occurrence of carotenoids in the diatoms ingested by the copepods, oligochaetes, and chironomid larvae, which appear to be the principal food avail able to the Lake Erie hydras. It may be possible that hydras utilize the energy stored in "plant" carotenoids by feeding directly on the diatoms in the "Aufwuchs." I have observed

H. littoralis feed in several instances upon the aggregations of diatoms surrounding it on the slides, and have witnessed the egestion of empty frustules after digestion. In the case of the Cymbella, the pectin in the tube may be a source of high-protein energy.

To determine the kind and quantity of food eaten by hydras in nature presents one of the most difficult problems in hydra biology. Actually, as results of previous studies indicate, the abundance of hydras cannot be correlated with the abundance of the entomostracan plankton in a lake (III:

2L) • We have already shown that the annual hydra cycle is shaped largely by climatic factors rather than by availabil ity of an abundant food supply. The feeding reactions of hydras under varying conditions of temperature and nutrition have been analyzed. The conflicting evidence relating to the feeding behavior of sexual hydras Is examined later (part IV).

No exact quantitative data can be presented from results of the findings on what the enterons of hydras contained when

- 178 - collected from the hay stations. It can he stated, however, that qualitatively the food of the hydras in Lake Erie throughout the seasons when they are feeding is composed principally of the following animals in the order named: larvae of Chironomus; oligochaetes, primarily those of the genera Nais and Stylaria; copepods, primarily species of Cyclops and some Diaptomus; and daphnids, usually D. retro- curva.

Analysis of data pertaining to whole animals or their parts found in the hydras indicates that, regardless of the season of the year or the location of the station, chironomid larvae formed the largest percentage of the undigested food which could be recognized in the hydras' enterons. For exam ple at the height of the vernal hydra pulse of a total of 1079 H. littoralis were checked for food in the collec tions of June 17 and 2*? (Table VII). All the specimens were healthy, the majority budding. Yet animal food was found in the enterons of only 19i^# or 18 per cent. (Diatoms, mostly Cymbella, were found in I4.8 hydras which contained no animal remains at all.). Per cent distribution of 11 stomach contents," based on the 19l|. specimens follows: chironomid larvae - 77J oligochaetes - 10; copepods - 9j daphnids -Ij.. Only 7 per cent of the hydras containing Chironomus had any other recog nizable food remains in them; 10 specimensi;were gorged with two larvae, and in several cases buds ready to detach con tained one of the smaller larvae.

- 179 - These data suggest that the hydras utilize Chironomus most extensively as food. Probably this is because the lar vae are the most abundant animals within striking range that can be paralyzed and ingested. It is recognized that the comparatively large larvae (from 5 to 7 ram. in length) may be retained longer in the gastrovascular cavity than the minute entomostracans; this might distort the findings of food ingested somewhat. Pending further research on this difficult problem, it is tentatively concluded that Chirono- mus is the principal prey of the hydras in the Lake Erie habitats.

Hydra1s Niche in the Microcommunity

Until we know much more about the nature of the complex biocenoses where hydras exist, any attempt to diagram food relations would be premature. Prom our examination of the more obvious aspects of antagonistic interactions, It appears that Hydra is unique in the position it occupies within the mi crocomrauni ty.

The most unusual feature of Hydra * s ecological niche is its almost complete freedom from enemies. Despite its small size and sedentary habit, the polyp is left unharmed by the larger members of the community. Even the rapacious Gam ma rus leaves it unmolested. It rides with impunity upon the exoskeletons of the grazing Gonlobasls and the herbivorous

Stenonoma. Peculiarly, Hydra1s only consistent predator Is the delicate turbellarian, Microstomum. whose unusual habit

- 180 - of appropriating the polyp’s nematocysts remains a conundrum of instinctual behavior. Hydra, on the other hand, is the tiger of the micro- jungle. It can kill and eat animals larger than Itself, and may destroy more prey than it ever eats. The chi ti nous armor of Chironomus is no protection against the poisoned darts of its stenoteles. This algae-eating larva, the most available of its food animals, is easily paralyzed by H. littoralis stenoteles. Impaled on the writhing tentacles by discharged holotrichous isorhizas, the larva is transported to the gaping mouth in the peristome, and slowly swallowed alive. Daphnia's carapace is especially vulnerable. A single H. littoralis will gorge itself with several of these filter- feeders; and while the parent is eating, mature buds some times ingest these prey. The terminal spine of a large

Daphnia may protrude through the thin body wall. But the polyp’s remarkable regenerative power heals any wound rapidly or replaces any part of its body which may be lost by acci dent.

The detritus-feeding oligochaetes are quickly overcome by hydras, and rapidly digested so that only their setae are usually found in the gastrovascular cavities.

One of the "key-industry animals’1 of the Lake Erie plankton communities, Cyclops, which feed on the diatoms in the phytoplankton, fall prey to Hydra when currents or diurn al migrations bring it into the "Aufwuchs” community. As

- l8l - Schulze (1917, pp. 3 %b) has shown experimentally in confirming Toppe's observations (1910), the desraonemes of

Hydra are effective in impeding the locomotion of animals with projecting bristles, like the darting Cyclops, whereas the stenoteles are discharged when an animal with a smooth chitinous shell, like Daphnia or Chironomus, comes into con tact with the nematocyst batteries.

That hydras destroy a sufficient number of copepods or daphnids to make them competitors for the food supply of young fishes is doubtful. This question, along with other questions raised by Clemens (1922) regarding hydra’s economic importance in killing fry in Lake Erie or poisoning fisher men, will be examined later (111:6 ).

The extent to which the species of Hydra which sometimes occupy the same microhabitat compete with each other for the available food supply is not known; the problem was too com plex for analysis in this investigation. Preliminary observa* tions of the food eaten by Craspedacusta sowerbii indicate that the hydroid of the jelly fish, although it is a vora cious feeder, ingests nematodes and triclad planarians; these animals are not eaten by the hydras.

Enough has been said to make clear to any biologist who subscribes to Elton’s concept of the niche (1927), that the peculiar function which Hydra fulfills in the community results largely from its cnidarian nature, its role in the ecological complex is gained, not so much as a result of its

- 182 - ranking in the "pyramid of numbers," but rather by its coelenterate distinction— the possession of nematocysts. In the trophic levels of the community, hydras are secondary consumers in a biotope where the producers at the first level are sessile diatoms, principally Cymbella, and the primary consumers are herbivores, principally Chironomus.

An attack on the problem of energy fixation and uti lization by natural communities in western Lake Erie has been made by Verduin (195>6), but measurements of rates of energy flow in "Aufwuchs" communities has not yet been attempted.

Physically the individual hydra is restricted to a substratum where its adherent pedal disc can attach. Here it occupies a living space of about one square millimeter, its column borne erect by the buoyancy of the water and its tentacles dangling or carried out in the current. Any small animal which strikes the batteries of nematocysts in the tentacles and discharges them is paralyzed; exceptions, such as the chydorid cladocerans, either do not discharge the batteries or have developed immunity to the nematocyst toxins.

Such motions as Hydra makes from place to place are ran dom, as the experiments of Mast (1903) and Wagner (190£) demonstrated long ago. Locomotion is accomplished by "loop ing" along the substratum. The animal bends over and fastens to the surface, possibly with the aid of the so-called "small glutinants" (atrichous isorhizas), then releases the pedal

- 183 - disc and reattaches it near the oral end; the tentacles are raised by the column for the next "step.” "Somersaulting,” is still described as the mode of locomotion in some text books despite accurate descriptions of the normal mode of locomotion by Trembley (17l|l|-5 and later by Wagner (1905)• The locomotor range of hydras— even when responding to continued stimuli of light, gravity, or chemicals — is probably restricted to several lengths of its body. Hydras thus remain ”on location” in the ecological scene. They cannot leave their place in search of nourishment or space. Their food must be brought within the action range of their nematocyst batteries. Ho doubt this essay presents an oversimplified version of hydra's functional niche. In the final analysis, any interpretation of a lower organism's behavior, as Jennings emphasizes (1906), is determined by the physiological state of the organism as a whole. Unfortunately, our knowledge of many aspects of hydra physiology is exceedingly meager

(see von Brand, I9I+6 ; Hamish, 19S>1 K Until such time as application of available isotope techniques are applied to study of the hydra's nutritional physiology we shall not know what factors may be at work in determining ”... whether the Hydra shall creep upward to the surface and toward the light . . . reverse this position and undertake a laborious tour of exploration” (Jennings, 1906, p. 231).

- I8I4. - PARASITES AND COMMENSALS

In the microcommunity, the hydras themselves serve as separate microhabitats for protozoan parasites and commen sals. Some— like the "polypenlaus" — have been known since the time of Leeuwenhoek, but much remains to be learned of their ecology. In Lake Erie, species of Hydra also fulfill the role of host to a highly specialized arthropod ectopara*- site, a relationship which was observed in natural popula tions in this country for the first time during the present investigation.

True symbiosis occurs in only one member of the Hydridae

— In the "green hydra." The mutualism existing between the single-celled alga Chlorella and Chlorohydra virldissima is so constant that it has been accepted as the basis for a monospecific genus (see 1:1). Some aspects of the nutri tional relationships existing between the symbiotic green alga and its host are examined in the last part of this monograph (IV:2); the reader Is referred to the excellent review of experimental approaches to the problem of symbiosis in Chlorohydra by Hamish (1951* pp. 261-262).

None of the hydra parasites or commensals encountered in the Lake Erie collections were species-specific. Most of the observations summarized in the following discussions of parasite-host relations were made on Hydra littoralis. Some Idea of the extent of parasitism of this species was gained

- 185 - through the inspection of collections through the seasons; but only selected quantitative data will be presented at

this time. Reference will be made to the taxonomic litera

ture, also to the report made by Miller (1936, pp. 1714--177) on the parasites he observed in his study of the oligactis- pseudoligactis population of Douglas Lake, Michigan. Other students of hydra ecology have paid little attention to parasite-host relations.

Amoebic Infestations

The injurious sarcodinian, generally now recognized as

Hydramoeba hydroxena (Entz, 1912), was encountered early in this investigation. Twenty-seven large amoebae were seen under the stereoscopic microscope protruding like warts on the column, peristome, and tentacles on one of 33 hydras obtained from the leaves of Potomageton puisillus collected at station 5 on July 12, 1952. This hydra was isolated in a watch glass in lake water and held at 17°C. Five hydras without amoebae on them were placed with it, and five speci mens were held in a watch glass as a control. After six days, the infested hydra had lost its tentacles; the amoebae moved along the stump of the column with lobose pseudopodia of the 11 A. verrucosa type," mentioned by Entz (1912, p. 36} as typical of specimens when remaining on the surface of the host. Some amoebae in the "A. limax or A. proteus form11 were seen moving on the glass. No signs of amoebae were seen on

- 186 - the other four hydras in the culture; they appeared in as good condition as the control specimens.

All the hydras were then removed and examined in wet mounts under the high powers of the microscope. Nematocyst study placed all the specimens as H. oligactis. None of the control specimens harbored amoebae. From two to seven amoebae in the "A. limax or A. proteus” forms were found in the gastrovaecular cavities in two of the four specimens which had been kept with the infested specimens, but whether these had infected the specimens or were present before the test, was not ascertainable. The amoebae on the heavily infested specimen measured from 75> to 1J?0 microns in length.

Several oligactis nematocysts, undischarged stenoteles and hoDLotrichous isorhizas, along with the nuclei and various colored pigments from the host’s tissues were seen in the granular endoplasm of each amoeba. The amoebae which were left in the bottom of the watch glass disappeared on the fourth day after the experiment was terminated.

Peculiarly, I did not find amoebic infestations in collections from the study area again until July 30, 19£ij.

(temperature 2Ij..6°). Six of the 3>2 H. littoralis from sta tion 2 were infested. No amoebae were observed on the 189

5 * littoralis taken in collections from the other four rubble-bottom stations at that time (see Table VII). Only three or four amoebae were seen on five of the hydras, but one of the six harbored 20 amoebae. These ranged in length

- 187 - from 120 to 300 microns, the majority being around 200 mi crons. They were creeping about the upper column and ten tacles in the typical "wartformig A. yeruccosa’1 shape described by Entz. To test the infecting potentiality and pathogenicity of these amoebae, the following experiment was made:

The heavily infested hydra was placed in a petri dish half-filled with strained lake water along with 20 other hydras from the collection upon which no amoebae could be seen. Twenty other hydras were used as a check. The dishes were placed in the constant temperature cabinet at 20°C.

Ten days later the dishes were removed and the hydras exam ined. In the experimental dish, 10 of the 20 hydras had become infested with from one to four of the large amoebae; these could be easily seen on the column and tentacles with the stereoscopic microscope under 30 diameters. No amoebae were found in the gastrovascular cavities. The original host animal, recognizable because it possessed only four tentacles, now had fewer amoebae on it. About $0 amoebae, some as long as 300 microns, could be seen moving free on t the bottom of the container in the A. yeruccosa manner at the rate of .025 ram. P©** minute. None of the hydras in the experimental dish or the control dish showed symptoms of depression. All were active and apparently healthy. Amoebae from the infested specimens contained the typical nematocysts of H. littoralis, all in the undischarged state. One large

- 188 - amoebae (200 x 270 microns) contained il stenoteles, 2 holo-

trichous isorhizas, and 1 desmoneme. It had also ingested tissues of the host epidermis; some fragments of cells could be clearly seen.

The heaviest infestation occurring in the Lake Erie collections was observed in 28 hydra brought up in the otter-

trawl haul of clams from a depth of about 8 fathoms in the

Pelee Passage on June 30, 1954 (s©© Ilr 2-c). Nine of the 2lj. H. oligactis and one of the Ij. H. pseudoligactis were in fested with 2 to 12 amoebae each. These specimens were unu

sually small for Bydr amoeba hydroxena, which is one of the largest amoebae known. They were only 20 to 60 microns in length. Though the amoebae contained some nematocysts and hydra cells, the hosts showed no ill effects from the para sites at the time of collection. Similarly, several H. oli gactis parasitized with Hydramoeba in a collection from

Anchor Bay, Lake St. Clair (July 22, 1954* temp. 23°C) were in good condition.

The above observations constitute the first report of the occurrence of Hydr amoeba hydroxena in the Great Lakes; also parasitism of H. littoralis by the amoeba is recorded.

The parasite was not observed on H. americana from Lake Erie.

However, Bryden (195>2) states in his paragraph on parasites in the Kirkpatricks Lake hydra population (p. 62):

Hydr amoeba hydroxena appeared in great numbers during the summer of 1 9 4 ^ on Hydra vulgaris, and the whole population of this hydra species disappeared

- 189 - within 30 days, apparently due to the action of the amoeboid organism. After the hydras disappeared, the parasite was not seen again. Neither this parasite nor any of the other parasites were found at any other time.

As previously pointed out (I:3-c), the taxonomic evi dence indicates that the species Bryden has designated as

H. vulgaris is probably H. americana. In view of Miller*s careful investigation of the effects of extensive parasitism of the oligactis-pseudoligactis population in Douglas Lake by H. hydroxena (pp. 17^-177), it seems questionable that a whole population of any hydra species could be killed off solely as a result of amoebic infestation.

Hydr amoeba is known to be especially pathogenic to hydras in cultures. Lashley (19l£, p. 184) reported shortly after Entz, described the new species that an epidemic of the amoeba broke out in his experimental cultures of (3. viridissima, and destroyed all of them. A similar epidemic of the amoeba finally obliterated my cultures of £. vlridissima and H. carnea from Pelee Island (see Pt. IV, Sect. 2).

Reynolds and Looper (1928) placed Amoeba hydroxena, described by Entz (1912) at Budapest as a non-pathogenic parasite on H. oligactis, in the new genus Hydramoeba.

These authors, and later Threlkeld and Reynolds (1929), proved by a long series of experiments that the amoeba ob tained from pools at the University of Virginia was patho genic to C. vlridissima and H. oligactis, killing the infected hosts In three to thirty days. They found that the

- 190 - amoebae could not live longer than four to ten days when removed from the host.

Whether the American authors were justified in erecting a new genus for this parasite of hydra may be open to ques tion. Entz in his detailed morphological study placed the new species in the genus Amoeba set up by Leidy because it possesses contractile vacuoles; truly parasitic forms do not and were classified by Leidy in the genus Entamoeba (p. 38).

The amoebae from Virginia were smaller than the amoebas from the type locality: they ranged in size from 60 to 190 mi crons, with an average size of 123 microns; the type speci mens measured about 100 microns, large specimens from 25>0 to

380 microns (Entz, p. 36). Such observations as I have been able to make show a considerable variation in the size of the amoebae as well as pathogenic effects on the hydras. In general my findings agree with those of Entz. Unlike Reynolds and Looper, Entz was unable to find morphological or physiological grounds for regarding the new amoeba as a true disease-producing parasite, and traced its origin to closely allied free-living forms (pp. 38—J4.O). Entz characterizes the new species in the following summary paragraph (p. I4.3 ): Amoeba hydroxena ist eine Arabbe, welche an der Kttrperoberflkche und im G-astralraum der Hydra oligactis rftuberische Lebensweise ftthrt; ihr Kttrper 1st von an- schaulicher Gr&sse, mit einer Pellicula bedeckt und mehrkemig, mit grossen, mach dem Limax-typus gebauten K e m e n (Protocaryon), mit 1, seltener 2 bis l\. Pulsellen, verschieden geformrten Lobopodien, entweder nach dem Typus der A. verrucosa oder der A. proteus, oder aber von A. Umax. - 191 - /

The excellent plates in Entz’s paper provide a "basis for further study by contemporary protozoologists of the morphology and life cycle of this interesting parasite of hydra.

Ciliate Commensals

Several protozoans, belonging to the subclass Euciliata, exist on hydras. Their relationship with the host ranges from epizoic comraensalism to the threshold of true endozoic parasitism. The gradation from commensalism to parasitism is gradual, and one must apply the ecological criteria pro posed by Baer (1951) and Caullery (1952) to any interpre tation of evidence pertaining to antagonistic reactions be tween the protozoans and the polyps.

The case for commensalism in the association of the well-known peritrich species Trichodina pediculus Ehrenberg with species of Hydra is a fairly clear one. Pulton points this out in his review of the Trichodinae (1923, pp. 22-23):

T. pediculus so long as it inhabits purely aquatic animals — fresh-water fish, Necturus, the larvae of Triturus, Neritina, or Hydra — retains its specific morphological characters and continues its life as an epizoic commensal. However, when found on animals which have metamorphosed and forsaken their aquatic existence, the Trichodinae, having had to leave the gills are found in the bladder. The new environ ment, one is lead to believe, has in time occasioned structural changes in the animal itself, so that a form resembling Trichodina pediculus may thus have been con verted into Triohodlha urlnicola. (See photomicro graphs of T. pediculus and new“ pecies T. uidhocola, obligatory”parasite in the bladder of Bufo sp., described by Pulton.)

- 1 9 2 - !T. pediculus is fairly common in Lake Erie. It is not restricted to an epizoic existence on the species of Hydra, and investigation may disclose that it is widespread on the gills} of Necturus maculosus. Jennings (1901b, p. 113) lists it as occurring: "On Diaptomus from towings in Put-in-Bay; on Hydra from East Harbor, Lake Erie." Landacre (1900, p. lji|.) found it, along with the hypotrich Kerona polyporum

Ehr., in Sandusky Bay. He designates both of the epizoites as "parasitic on Hydra fusca."

There is conflicting evidence concerning the role of the hypotrich Kerona polyporum Ehr. as a true commensal re stricted to the surface of Hydra. According to Schulze

(1917> pp. 22-23, lli+j and Fig. Kerona is highly injuri ous to H. attenuata, ingesting its unexploded penetrants, glut inants, and some epidermal cells. He maintains that the infusiorian is a dangerous obligatory parasite and that heavy Infections cause death from depression. Uhlenmeyer

(1922) observed that Kerona prefers species of Hydra to

C. viridissima, and that it dies when detached from the hydra if it cannot find another hydra. He considers the epizoite an obligatory parasite, which ingests living cells of the hydras and eventually destroys heavily infested hosts. Rulon and Child (1937) report that their experimental cul tures of H. oligactis were killed off by Kerona; they found that the hydras could be freed from the ciliates by washing in water at 33-3^°C, and started parasite-free cultures from

- 193 - those hydras which survived this procedure.

Hyman (1928) does not believe that the ciliates cause depression in hydras, and says (1914-0 , p. 196): "Kerona

inhabits the surface of Hydra apparently as a harmless ecto- commensal."

Prom my observations of Kerona polyporum studied on

the species of Hydra collected in the bay and on C. vlrldissima collected from the Lake Erie Island ponds (II: 2-e),

I can find no evidence that this ciliate is harmful to the hosts. Kerona occurred sporadically on some hydras at all the stations from the middle of June to the beginning of

December. But at no time were samples of the population found to be heavily infested. Hosts did not show symptoms of depression, and remained healthy when maintained in iso lation cultures at lake temperature for from one to two weeks. The ciliates tended to aggregate on the oral portion of the host’s column, sometimes gliding over tentacles and buds. Occasionally they were seen free-swimming for brief periods until they settled on the surface of another host. Kerona was observed on the surface of H. carnea and

0 . viridlssima in a mixed culture originating from the Pelee

Island swamp. Apparently they produced no ill effects for the culture flourished. A few Kerona were seen gliding over the ovaries and eggs when the hydras developed gonads, but these females showed no signs of depression in isolation cultures (see IV: 2 for details). - 19U - No host preference was observable in the behavior of

Kerona polyporum. It appears to be an obligatory ectocom- mensal restricted to species of the hydras. Like Trichodina pedicuius, It probably feeds on particles of food in the lake water rather than on the tissues of its host. Both ciliates are obligatory tfR aumpari si ten," and probably benefit from the constant undulations of the hydras. My observations confirm the findings of earlier investigators: these epizoites die if deprived of a suitable host.

In the Lake Erie collections Trichodina, unlike Kerona, was seldom encountered. Miller (1936) found the peritrich far outnumbering the hypotrichs in his samples of the oligactis-pseudoligactis populations of Lake Washington, Minne sota and Douglas Lake, Michigan. He states (p. 175):

"However, in no instance, was there any evidence that the hydras were harmed by the association." Sometimes a single hydra serves as host to both species of ciliates, as Clark observed In an early American study (1866): At times the Hydra seems to be strangely knotted, and ungainly in outline, when upon close examination, - we ascertain that it is crowded with a swarm of Kerona, upon several of whose backs, 1, 2:, or 3 Trichodinas are seated, enjoying the pleasure of locomotion without the effort of producing it. This dual association of the epizoites was studied on specimens in a collection of 29 hydras made at station 3 on

August 9, 1 9 5 ^ (temp. 23°C). There were from $ to 10 Keronas and from 1 to 1^. Trichodinas on the 1$ host animals: - 195 - (6 littoralis, 6 americana, 3 oligactls). These hydras — each species in a separate petri dish - were held at l£°C in the constant temperature cabinet; the remaining ll|. ciliate- free hydras were used as controls. The specimens were in spected under the high powers of the stereoscopic microscope for about 1$ minutes each day until August 21, at which time all the hydras were squashed for species determination.

None of the hydras showed signs of pathology. I did not happen to witness the peculiar behavior of the peritrich with the hypotrich described by Clark, and considered bene ficial to Trichodina by Pulton (1923, p. 22). Two other ciliate commensals of hydra, both endozoic, are reported in the literature: Balantldium hydrae (de scribed by Entz, 1912, p. 23 and 1913, p. 38 in H. oligactis; type locality — Budapest) and Paraglaucoma sp., characterized by Hauscbka and Doll (19i|i|.) as a facultative endozoan in habiting the gastrovascular cavity of H. americana collected from a pond on the University of Pennsylvania campus. These ciliates were not seen on any of the hydras collected in Lake

Erie or Lake St. Clair.

It can be reported at this time that none of the para sites or commensals of Hydra have been observed on the colo nial hydroids of Craspedacusta sowerbii, which is found occu pying the microhabitats with the hydras in Lake Erie (III: 3-a).

- 1 9 6 - Host Relationship with the Cladooeran Anchlstropus minor

The remarkable association between the chydorid clado oeran Anchlstropus minor Birge, 1893 and Hydra was first called to the attention of zoologists by Hyman. In a short article (1926), Hyman reported her observations of the cladoceran, which accidentally occurred in a culture of H. oligactis. The minute, globular chydorids remained firmly at tached to hydras by the hooked claws of the first pair of feet. These claws are armed with sharp teeth on their con cave sides. Each claw locks into a tooth-like projecting groove on the anteroventral margin of the carapace when the crustacean grips into the hydra’s epidermis.

These characteristics of the claw and the projection of the carapace are diagnostic for Anchlstropus minor, the only

American species of the genus. Birge (l893> pp. 309-311; pi. xiii, Figs. 2-£) in his description distinguishes the new species from Anchlstropus emarginatus described by B. 0. Sars in 1896, new genus, new species. Hyman’s drawing of the claw

(Fig. 6) is clearer than Birge’s figure of it. Another drawing by Hyman (Fig. 5) shows A. minor attached to a hydra, and illustrates the "tooth" on the carapace into which the first foot fits when the chydorid is in gripping position.

With the aid of the excellent drawing of the female and the descriptive notes in Birge’s key to the Cladocera available

- 197 - in Ward and Whipple (1918, Ch. 22), A. minor can be easily- distinguished from the closely associated Chydorus sphaeri-

cus (0. P. Muller, 1785). The parthenogenetic females are

about 0 . 3 5 mm. in length. The male of this species is as yet unknown.

Previous to Hyman's report, A. minor was known only as

a free-living member of the Chydoridae. The parthenogenetic females had been taken in plankton collections from lakes in the states of Michigan, Wisconsin, Maine, and Louisiana

(Birge, 1893; 1918). Hyman mentions (p. 301) that the An- chistropus specimens occurring in her culture probably came from the botany pond on the University of Chicago campus or the lagoon in Jackson Park, Chicago, where the H. oligactis specimens and other culture materials were collected.

Since cladocerans are usually the chief food of hydras,

Hyman was surprised to find that A. minor reproduced rapidly by parthenogenesis in her culture. The young, two from each female as is characteristic of the Chydoridae, became firmly attached to the epidermis of the hydra as soon as they hatched from the brood pouch of the mother, and grew rapidly.

Hyman did not determine whether Anchlstropus is immune to the poisons of the nematocysts of the hydra or whether the hydra fails to discharge its nematocysts at individuals of this species. She believes, however, that the latter alternative is more probable since hydras usually do not attempt to capture very small animals.

- 198 - Hyman established quite conclusively that the hydras

in the culture were killed off by the Anchistropus which

attacked them. Figures 1-lfc, nFour stages in the destruction

of Hydra oligactis by Anchistropus minor,” show how heavily

the hydras were infested and how they gradually disinte

grated. Hyman established that the hydras were not suffer

ing from the effects of depression. The author states

(p. 301): As soon as all of the Hydra had been destroyed by Anchistropus, as happened in about two weeks after Anchistropus was first noticed in the culture, the latter disappeared, showing that they were unable to feed on any other materials in the culture than Hydra. That the culture was favorable for chydorids is proved by the fact that numerous other chydorids were found in it both during and after the destruction of the Hydra.

These and other observations reviewed above led Hyman to suggest at the conclusion of her paper:

It appears probable that this habit of fastening to and feeding upon Hydra (possibly other animals) is the regular mode of life of the genus Anchistropus, and that the members of this genus are particularly adapted for such an existence. The tooth on the cara pace is, as already mentioned, peculiar to the genus and serves the purpose of locking the first feet into a gripping position. The mode of life of Anchistropus minor may account for its reputed rarity for if it is habitually fastened to Hydra it would naturally be seldom taken by seining the open waters.

In view of the ecologically important association be tween Anchistropus minor and Hydra oligactis which Hyman dis covered, it is surprising that subsequent specialists working with the Cladocera or the Hydridae in this country have paid so little attention to the role of hydras as hosts to the chydorid. So far a search of the literature has disclosed - 199 - only one record, of the occurrence of Anchistropus minor on hydras, in his section on parasites occurring in the oligactis-pseudoligaotis population at Douglas Lake, Michigan,

Miller (1936, p. 175) cites Hyman’s note on the arthropod parasite, and mentions his single field finding:

Anchistropus minor was found only once. On Aug. 15, 193ZTJ six Individuals were found on one, otherwise, apparently healthy "budding hydra. The hydra was from a rack which had been lying on the sandy bottom, for several weeks, In water about 3 m. deep.

I have found no record of the occurrence of Anchistropus minor in the Great Lakes. Although Birge (I89IO studied the

collections of Cladocera from Lake St. Clair and Lake Erie obtained through the limnological reconnaissance conducted by Reighard (l89lj.a, l89l).b), he did not find his new species among the various chydorids in the collections. Peculiarly,

Anchistropus was never reported in the numerous papers based on plankton studies subsequently made in Lake Erie (Langlois,

195U. Early in the present investigation (July 1952), it so happened that Anchistropus minor was recognized on hydras collected at all stations of the study area. It was found subsequently that the arthropod occurred on all species of

Hydra Inhabiting the bay; they were never found attached to any other species of animal. Very rarely was a specimen of

A. minor observed free-swimming in collections of chydorids made In the bay (ill: 3-a).

n - 20 0 - The incidence of parasitism as observed in the collec

tions extended from the beginning of the summer season to

the end of the autumnal season. At no time were any of the hydras inspected upon removal from the habitat heavily in fested with the parasite. Usually one, twb, or three — at

the most six specimens were found clinging to the hydras on

its distal column; sometimes one was found on the column of a bud. The hydras harboring the crustaceans were as healthy in appearance as the specimens which were free of the para

sites. Heavy Infestations of 2£ or 35 Anchistropus such as

Hyman records on disintegrating parent hydra and buds (Pigs.

I-I4., 1926) may occur in cultures. It is doubtful, however,

that infestations in the habitat are ever heavy enough to destroy the host animals.

Considerable qualitative and quantitative data pertain ing to the incidence of parasitism of the hydra population in the study area by Anchistropus minor and the life history of the parasite have been accumulated from the field work

and collateral culture studies carried on until the end of the 195^4- season. Complete analysis of these data and illus trations of the life-cycle stages of the parasite will be presented in a separate monograph, which is in preparation.

At this time, only the salient features of hydra's host relationship with the cladoceran will be discussed.

Figure 6 , which presents the first photographic records of Anchlstropus minor parasitic on Hydra, provides a concep-

- 201 - tion of the appearance of the crustaceans as one sees them

on the host under the microscope. Frequently the host har

bors three Anchistropus: one large individual, measuring

between.3 and.ij. mm. in length, and two small individuals, each about half the length of the large one.

The relative sizes of the three individuals and their

typical position on the host is illustrated in 6-a (magni

fication about i+0 x). The claws of the large individual

can be vaguely seen hooked into the column of the hydra. The two small individuals, attached adjacent to each other

on the column above the large individual, are young Anchis

tropus. According to my notebook record of this case, they represent the second hatch from the large individual; this

critical event in the life-history of the cladoceran was

observed in the culture four days after the hydra (H. oli

gactis) was collected (2:00 P.M., July 31* temperatures: lake, 21}..6°; culture, 2l|.-2^0 ).

As soon as the young emerged from the brood sac, they fastened to the hydra; they remained in the position shown

in the photograph, which was taken about six hours later.

The two eggs from which these young developed were formed

just after the first brood hatched (August 1, around noon).

Thus the total time for development of the young at 2l\.-2$° was about ij.8 hours.

Within two hours after the young hatched, the mother molted. The exuvia became detached from the column just

- 202 - f

Fig* 6. Photographs of the cladoceran Anchistropus minor parasitic on hydras.

- 2 0 3 - before the photograph was taken; the parent is seen on the active hydra from the anterior aspect in the close-up view

(Pig. 6-b, magnification about 70 x). Two new eggs are forming in the brood sac. We do not know what instar the female is in, but it can now be characterized as a "multi para." Exuviae from the young of the second hatch were found in the petri dish at midnight on August 1).; so the two new Anchistropus probably entered the first instar about

21). hours after hatching. The characteristic pair of eggs were seen forming in the brood sacs of these "nullipara” just after the first molt occurred. The two young from the first hatch subsequently reattached themselves to the column of the hydra. Here they remained with their parthenogenetic sisters and the parent Anchistropus until August 7, when the observations were terminated. The hydra in this experiment was not fed in order to keep the isolation culture free of other cladocerans. It showed symptoms of advanced inanition

(see III: 2-c); but microscopic examination of the fresh tissues disclosed no signs of lesions.

The hydra (H. littoralis) and the Anchistropus shown in photographs c and d (Pig. 6) were taken in the same collec tion with parasitized hydra, whose case has been reviewed above. This host with its parasites was cultured under the same conditions and photographed the same evening as the other hydra. The large Anchistropus is seen from the pos terior aspect in Figure 6-c and from side view in Figure 6-d.

- 201 ). - The parthenogenetic eggs filling the brood sac show quite

clearly in these photographs.

The female was disturbed by the bright light during photography, and had climbed from its roost on the distal

portion of the column of the hydra up to the hypostome

(Pig. 6-c). The ventral margins of the carapace are raised

above the surface of the hydra by the walking action of the

first pair of legs; if one looks closely, the outline of the

claw of the left leg can be vaguely seen hooked into the clear epidermis of the hypostome.

The animal ambulates over the body of the hydra with a

slow, awkward gait. It appears to be walking on stilts.

t With each step the claws are alternately pulled from the epi

dermis and then reinserted. And at each extraction, the

terminal teeth of the claw rakes out some of the hydras

epithelial cells and some of its undischarged nematocysts.

In Figure 6-d, the Anchistropus has ambled down from the high hypostome of the active H. littoralis; it is seen

stepping along the batteries of nematocysts on one of the

tentacles. When this specimen was inspected under the high powers of the microscope at the end of the period of obser vation, several discharged stenoteles were seen penetrating

the carapace. It had produced two brrods of young in the culture during the 8-day period since it was collected. The multipara with its four young and some of the exuviae were attached to the host. (See shed exoskeleton of one of the

- 205 - young on hydra's column, Pig. 6-0.) The parasites were ac

tive, but the host was beginning to suffer from inanition because of absence of food in the strained lake water.

The case histories of these Anchistropus individuals

and their hosts are typical of the many cases studied in

isolation cultures. Reference to these and to other cases

in the same collection provides insight into how we can

account for the numbers and sizes of Anchlstropus specimens seen on Hydras when taken from the habitat.

The H. oligactis (Pig. 6-a,b) and the H. littoralis

(Pig. 6-c,d) were two of 9 parasited hydras in a settlement of 16£ hydras which had colonized the slides at station 3 during a week's immersion interval ending July 31, 195^4-.

About a quarter of the hydras in the aggregation (predom inantly H. littoralis) were budding. All were active and healthy in appearance, including 21 which were quite heavily infested with ciliate ectocommensals. None of the hydras parasitized by Anchistropus harbored Keronas or Trichodinas, but one of them was infested with several Hydramoeba.

Of the nine hydras (2 oligactis and 7 littoralis) para sitized by Anchistropus minor, 6 bore only one crustacean: a large gravid individual in cases, a small individual with eggs not yet formed in 2 cases. Three of the hydras harbored more than one Anchistropus: one had 6 (two large and four small individuals); one had 2 (both large individ uals with embryos well developed); and one had 3 (one large - 206 - individual with the two eggs just developing and two small

individuals in which eggs had not yet formed). On this

specimen, adjacent to the large Anchistropus, was an exuvia

of about the same size. The molted exoskeleton was firmly

attached to the epidermis of the hydra (H. littoralis) by

the claws of the first pair of legs.

On all the hydras, except the one infested with six

of the chydorids, the parasites were attached to their hosts

on the distal third of the columns. The heavily infested

specimen bore two of the four small crustaceans on its tentacles.

The above sketch gives a picture which is quite charac

teristic of the extent of parasitism of hydras in collections made during midsummer,: when the incidence of A. minor ap pears to reach its peak.

Prom the field experience and the many cases studied in culture up to 19^1+, it can be inferred that the combina tions of Anchistropus individuals occurring on hydras col lected from the study area represent various age groupings of the parasitic population: a single mother and its two young; a single large female or a pair of large females without their young; more rarely a pair of young without their mothers, or a parthenogenetic aggregation composed of six individuals — two mothers and their four offspring..

Anchistropus minor appears to exist only in small num bers. Unlike some of the other species of the local

- 207 - Chydoridae (see III: 3-a),~it does not form parthenogenetic swarms. The rarity of either young or adult specimens of

A. minor in the plankton collections can probably be attrib uted to the habitual mode of existence on hydra, as Hyman has suggested.

The life-history stages of A. minor have not yet been completely worked out. Prom preliminary data, however, it can be predicted that the large individuals represent primi- para or multipara, the small individuals the newly hatched young or nullipara in the first instar. Unlike the mature individuals, the young do not have an amber-colored carapace and dark-brown claws; upon hatching the exoskeletal struc tures are white, and the characteristic coloration of the species does not appear -until after the first molt. Molting takes place on the host; and the exuvia remains attached to the epidermis of the hydra by the hooked claws of discarded exoskeleton. Sometimes only the exuvia of an Anchistropus is found on a hydra. In such cases, the approximate growth of the individual which had been parasitizing the hydra at the time of the molt can be roughly estimated by the size of the in tact exuvia. The young, when ready to enter the first instar, are about 0.19 mm. in length and 0.17 mm. in height.

The older individuals measure between 0.32 and O.lj.2 mm. in length with the greatest height from the dorsal to the ven tral margins 0.27 to 0.37 mm. respectively, and are propor- - 208 - tionately more globose In shape (see Fig. 6-c).

I have not yet found out how many instars the partheno

genetic female passes through before it dies. Nor have I

recognized the male of the species. A specimen may be pres

ent in the preserved material, which has not yet been stud

ied. The parthenogenetic females appearing toward the end

of the seasonal cycle undoubtedly form winter eggs like

other members of Chydoridae. This probably occurs in late

November in western Lake Erie for the Anchistropus popula

tion disappears by early December and does not reappear until about the middle of June. Thus far I have not found the ephippium of A. minor.

Whether or not Anchistropus minor is completely depen dent on Hydra for shelter and nourishment has not yet been conclusively demonstrated. I have made a number of observa tions of large specimens maintained at about l£°C in finger bowls containing several well-fed H. oligacti s, a little detritus, some sessile diatoms (mainly Cymbella in tubes), filaments of Cladophora, and a few Spirodella plants. The behavior of the Anchistropus specimens under these conditions is summarized in the following description.

The cladoceran seldom leaves its host. When swimming, it moves about in a random path by rapid strokes of the second pair of antennae. As soon as it stops swimming, the rotund body, which is sparsely equipped with structures to keep it afloat, sinks quickly to the bottom. Here it lies

' - 209 - on its side as if exhausted for several seconds, then sud denly starts swimming again. Very seldom does it ever attempt to cling to plants. Its efforts to maintain a foot hold on the filaments of Cladophora, the leaves and dangling roots of Spirodella, or the tubes of Cymbella fail. Unlike the other species of Chydoridae It does not appear to be structurally adapted for clinging to such substrata. When a swimming A. minor encounters a hydra, however, it immediately gains a foothold. The place of anchorage is most commonly the distal portion of the column. Very rarely initial attachment is made toward the basal part of the column.

Sometimes the cladoceran latches onto the hypostome or onto one of the hydra's tentacles. The hydra responds to the sudden penetration of the claws by a series of contractions and extensions in different directions. The ventral margin of the carapace is curved inward.

Contractions of the carapace muscles force the tooth-like projection at the first pair of legs upon the epidermis of the hydra so that a little mound of the tissue is pinched up toward the filtration chamber. The posterior end of the feeding Anchistropus remains elevated; the claw of the post abdomen during its repetitive motions just clears the sur face of the hydra’s epidermis.

Prom time to time the Anchistropus retracts its claws without completely removing them from the hydra's epidermis.

At such times fragments of epithelial tissue torn loose by

- 210 - the teeth of the claw can be seen passing in to the feeding mechanism. I have found numerous undischarged nematocysts in the bristles of the cladoceran’s appendages. But whether

these or the epithelial cells of the host are ingested by the parasite, I do not know. No examinations of the intes

tinal contents of the experimental animals have yet been made. As previously mentioned, the activities of Anchls tropus on hydra does not seriously damage the host. Torn

tissues are apparently repaired rapidly by hydra’s regenera

tive capacities.

To what extent Anchistropus minor may gain nourishment

from detritus and other minute particles it filters out of

the water remains to be determined. I have observed parti

cles from lake water carried into the filtration chamber by

currents set up by the feeding mechanism of the animal while

it is fastened to its host. Apparently A. minor cannot multiply or survive when

deprived of hydras. Molting and hatching takes place on the hydra. After the first instar, the young become more active

and leave the hydra on which they were hatched. Unless they

can reattach to the same or another hydra, they are found

dead on the surface film of the water after about 2.]\ hours. More mature individuals do not survive for more than a few

days in cultures from which the hydras have been removed.

I tested the suitability of the conditions in the finger-bowl cultures mentioned above by introducing a few specimens of

- 211 - Chydorus aphaericus into the bowls after the Anchiatropus specimens died. This species, which resembles A. minor in many of its characteristics, multiplied parthenogenetically, and was easily maintained for about a month in the cultures where the more specialized chydorids had perished.

Toward the close of my experiments with Anchistropus minor, I made a search of the literature on the genus Anchis- tropus, and found that Borg (1935>) had published a monograph on the life history, behavior, and distribution of the Euro pean species, Anchistropus emarginatus Sars, 1862. Borg*s extensive monographic treatment of the genus Anchi stropus presents strong evidence toward the conclusion that the two species belonging to this genus of the Chydoridae probably evolved from some form similar to Chydorus sphaerious, and that they have become specialized for a mode of existence as an obligatory ectoparasite on species of Hydra.

THE HYDRA NUISANCE

Certain problems pertaining to the economic importance of hydras were stressed by Clemens (1922) in his paper re porting the tremendous numtoBrs of hydras occurring on fisheiy nets in Lake Erie (see quotation from paper cited in Intro duction). Clemens lists four points (pp. for further investigation:

- 2 1 2 - (1) The amount of interference and injury caused to the nets by these great growths.

(2) The question of the poisoning of the fishermen.

(3) Do these Hydra destroy young fish to any appre ciable extent in open water? (The well-known report of experiments by Beardsley (1902), which conclusively proved that hydras kill trout fry, is cited.)

(4) To what extent do these immense numbers of Hydra reduce the entomostracan food supply of fish and of mature fish such as ciscoes? Certain aspects of these problems have been explored during the present study. Obvious technical obstacles pre cluded obtaining any conclusive answer to the questions posed by Clemens. Examination of the circumstantial evidence, however, leads me to doubt that the activities of hydra aggre gations in Lake Erie or any of the other Great Lakes are of much economic importance.

Let us examine the question of hydras as competitors for the entomostracan food supply of commercially valuable fishes, particularly the species of the whitefish Coregonus. The early researches of Reighard in Lake St. Clair and Lake

Erie (1894a) and of Hankinson in Lake Superior (1914) indi cated that young whitefishes feed almost exclusively on the microcrustacean plankton. Subsequent investigations by

Ewers (1933) showed that the diet of the Erie whitefish

Coregonus clupeaformis latus Koelz up to age of about six weeks is composed almost entirely of Copepoda. Life-history and population studies of other commercially valuable Lake - 213 - Erie species by Doan, Daiber, Kinney, and several other biol ogists, whose work is reviewed by Langlois (1951j-)» have established that the entomostracan food supply is an impor tant factor in the growth of post-larval fishes. The ciscoe or lake herring (Leucicthys sp.) feeds almost exclusively on

Entomostraca throughout its life cycle. It was this food habit which led Clemens to suggest that a large population of hydras in Lake Erie would constitute a serious threat to its food supply. Since Clemen’s time, the ciscoes have de clined greatly in abundance in Lake Erie. A review of the evidence pertaining to causes for this decline by Langlois

(195L}., p. 289-29^) indicates that the ciscoe failure has been largely due to unsuitability of conditions of existence, principally turbidity effects, on the newly-hatched young rather than to man’s fishing activities or competition of other animals for the food supply.

There can be no question that the enormous aggregations of hydras which still are seen on the nets in the fisheries of Lake Erie and in other Great Lakes fisheries consume great numbers of entomostracan s. As we have seen, however, these growths of hydra on artefacts do not represent the abundance of hydras as the species are distributed in the habitats available on rocks and plants along the shore of the lake. In such habitats, the food of hydras appears to be restricted to the daphnids and copepods which are carried into the habitat. The principal food of the hydras is

- 21i(. - Chironomus larva© rather than the plankton Crustacea.

Regarding Clemen's first point listed above, it is now

generally recognized by fishery experts that the main source

of damage to the net twine is due to action of bacteria or

fungi. Masses of hydras growing on the nets along with the diatom-fungus mats which encase the twine do constitute a

nuisance to the commercial fisherman. But, it is the algal

growths — called nnet moss” by the Great Lakes fishermen — which are the principal source of interference in working

the nets.

How let us briefly examine evidence relating to Clemen's

other two questions: the destruction of young fish by hydrasj

and the injury to the skin of fishermen which may be caused by toxic effects due to large numbers of hydras which have settled on the nets.

Destruction of Fish Fry

The ability of species of Hydra to kill fishes in their

larval stages has been known since the early observations

of Trembley (17MJ-)* In his monograph (pp. 213-217) Trembley describes how hydras in an aquarium attacked and killed young roaches which were about 8 mm. long. He portrays a

specimen being engulfed by a hydra (pi. vii, Fig. 3).

Gudger (1927) in his article on hydras as enemies of young

fishes reconstructs Trembley*s rather illegible figure of

the fish being swallowed tail-first by the hydra. He also

- 215 - mentions observations from the early literature reporting the manner in which hydras kill young trout 30-ij.O mm. long.

H© cites cases where hydras were observed to kill the tad poles of frogs.

Outbreaks of hydras sometimes occur in hatcheries, and can destroy the fish being cultivated to a serious degree.

The extent of the damage which hydras can do in trout hatch eries is shown by the observations of Beardsley (1902) and

Smallwood (1918).

The only evidence indicating that hydras can kill young fishes to an appreciable extent in open water is presented in a paper by Moen (1951)* The author studied the decline in abundance of the yellow pikeperch (Stizostedion vitreum vitreum, commonly known as the walleye) in an Iowa nursery lake (160 acres, maximum depth 6 feet) in which a large popu lation of H. oligactis established itself on the water plants. Moen demonstrated that the hydras were able to kill the walleye fry. He put a small amount of vegetation to which about 2000 hydras were attached into a hatchery jar filled with water from the lake. He added 30 fry, 8 mm. In total length to thia jar. He reports the results ( pp. £03-

50lj.) : ’’Within 30 minutes every fish was either dead or com pletely paralyzed. . . . Two fish were partially ingested by individual hydra one hour after the fry were introduced.”

The circumstantial evidence presented by Moen points to the conclusion that the large number of H. oligactis which

- 216 - multiplied in the lake during May and June of 19l|-6 almost completely killed off the young fishes.

As Langlois (1954> P» 112) has mentioned, I established by experiments conducted in the Ohio State Pish Hatchery during the spi*ing of 1952 that the Lake Erie hydras were able to kill newly-hatched whitefish and yellow pickerel.

A photographic record of hydras attacking the fry of the Erie whitefish Coregonus clupeaformis latus Koelz is presented in Figure 7. The fish shown in this action shot taken on April 15, 1952 measured 12.£ mm. from the snout to end of the caudal fin; its width at the eyes was 1.8 mm.

Its age was about 15 days, since the eggs under cultivation had started to hatch on March 28. The hydras shown in the picture were provisionally identified as H. littoralis.

Those in the foreground have already stung the fish when it struck their writhing tentacles with its sides and tail.

The fish has been paralyzed; one of the hydras is attempting to swallow the fish but its head is toolarge. This fish died a few minutes after the picture was taken. Numerous discharged stenoteles were found penetrating the tissues of its skin.

An outbreak of H. littoralis was discovered on May 8 in the hatchery jars which had been set up to incubate pickerel spawn received on April 28, 1952. inspection of the jars disclosed that hydras had attached to the surface of a few of the eggs in each jar of the battery. The hydras multi-

- 217 - Fig. 7. Photograph of hydras attacking fry of the Erie vhitefish Coregonus 'olupeaformia latus

- 2 1 8 - plied rapidly in the batteries of jars. The water pumped from the bay intake contained an abundance of vernal reddish copepods, and soon budding orange-colored H. littoralis covered the sides of the fry tanks. (See Langlois, 19 pp. 386-395 for description of artificial propagation pro cedure in this hatchery plant.)

To test the effects of H. littoralis on the eggs and newly hatched fry of Stizostedion vitreum vitreum (Mitchell)

— known in Lake Erie as pickerel— the following experiment was set up oh May 9. A series of four 150 ml. crystallizing dishes half filled with strained water from the hatchery trough were set up in 3 crystallizing dishes, and placed on a water table so that the temperature was held at about 12°C. (the temperature of the water flowing through the jars). One hundred eggs were placed in each of the dishes. Into two of the dishes

100 hydras.were introduced; the other two were left free of hydras as controls. During the next two days one of the experimental dishes and one of the control dishes were re moved about every four hours for a 15-minute examination under the microscope (temperature in hatchery ro om— 15°C.). The other experimental dish and control dish were left undis- o turbed on the water table at 12 C.

At the end of Lj.8 hours, a tally of the number of the eggs which had hatched in all the dishes was made. The per cent hatch was not statistically significant in. any of the

- 219 - dishes. In the dishes containing hydras, however, all the

fry which had hatched were dead or dying. In the control

dishes, none of the newly hatched fry were dead.

Examination of the five specimens of the fry which had

died in the experimental dishes showed that the tissues of

the skin had been penetrated by discharged stenoteles. Very

few holotrichous isorhizas were seen. The armatured tube of

the stenotele could be especially well seen in the tip of

the caudal finj the tip is' about 1.5 ram. square in these

larvae. The number of stenoteles counted in this area

ranged from 9 to 71+• The conclusion that the newly hatched fry were killed

by the toxins from the stenotele nematocysts of the hydras was substantiated by the periodical observations. Some of

the hydras attached themselves to pickerel eggs, and remained

attached to the surface by their pedal discs. When the

embryo hatched from such an egg, it was stung to death by

the hydra on the surface of the egg or one of the adjacent hydras which had attached to the glass substratum. The place of attack was usually the active tail of the fish. A paralyzed pickerel is seen lying on its side in the photograph (Pig. 8). This specimen had just emerged from

the egg. A few minutes after the photograph was taken the

adjacent hydra attempted to swallow the dying fish head first, and managed to engulf most of it up to the yolk sac.

- 2 2 0 - Fig. 8. Photograph of hydras attached to the eggs of the yellow pikeperch Stlzostedion vitreum vitreum and stinging newly hatched fry.

2 2 1 - Hydras in this and in experiments with fry of other species were frequently observed with peristomes everted and tentacles clinging to the paralyzed or killed fishes.

In only one instance, however, have I seen hydras ingest whole fishes. This case was observed in June of 19^i+ in a smali balanced aquarium in which the young of the carp

(Cyprinus carpio. L.) were being reared at a temperature of

20°C. The larvae, about 30 in number, were being fed Piaph- nia pulex. Ten budding H. oligactis, seen hanging from the

Myriophyllum plants on June 23, were brown from feeding on the daphnids. The next day $ dead carp were seen on the leaves. These were removed and found to be covered with stenoteles. Two of the hydras on the leaves were blackish in color. A carp fry collided with one of the tentacles of a brown-colored hydra, was captured, and swallowed whole.

Two hours later the hydra had turned blackish in color to its budding zone; the three buds it bore were also black in color. The carp had been egested. Both the predator and its prey were immediately subjected to microscopic examina tion. Numerous melanin pigments were found in the gastroderraal cells of the hydra. The egested fry showed signs of the hydra»s digestive activities. The skin was macerated and the melanophores disintegrating. The thorns of numerous stenoteles could be seen in head and tail region. By the next day the remaining hydras had turned black and several additional fry were found dead in the aquarium. Postmortem

- 222 - examination disclosed that all the larvae were penetrated with stenoteles* These young carp were about 5 nnu total length and 1 mm. at the widest part of the trunk* They had hatched from artifloally fertilized eggs of Cyprlnus oarplo*

These observations lead me to believe that the "black" ollgaotls I found in Terwllligar1s pond (lll;2-e)

during the early summer season had been feeding on the newly hatched fry of the carp, which spawn in the pond.

During the 195^ season, experiments conducted in the hatchery with freshly collected specimens of ollgaotia and pseudollgaotls produced results which demonstrated that

these species of Hydra could kill the larvae hatched from the artificially fertilized eggs of the whitefish, the

pickerel, and the pike (Esox luclus L.)•

Observations on the effects of the hydras on the eggs of the whitefish made at that time Indicate the embryos are not injured by the action of the hydra's pedal disc on the

surface of the egg* Langlois (193&) attributed disappear ance of the eggs in the nest of the small-mouth black bass

to rupturing of the egg membranes by the suction from hydra's pedal disc; he observed numerous brown hydras on pebbles in

the nest* We did obtain fertilized eggs of the bass to test

this supposition experimentally.

In the experiment with H. littoralis and S. v. vitreum de scribed above, mortality of the embryos was not due to the ef fects of the. hydras which attached to the eggs. As can be seen

- 2 2 3 - in the photomicrograph (Pig. 8) the eggs are covered with

a heavy growth. This growth was mainly made up of a colo nial protozoan (Carchesium), and species of fungi which are known to cause heavy mortality of pickerel spawn.

Prom what we now know of hydra’s ability to destroy fry tinder experimental conditions, one might well predict

that no species of fresh-water fish in the larval stage is

immune to nematocyst toxins. (See Schulze, 1917, pp. 36-I|l|.

for action of Congestin, the stinging substance, and Thallar-

sin, the paralyzing substance, contained in nematocyst cap sules.) In nature the larvae of certain species may fall

victim to hydras which inhabit their spawning grounds. For

example, S. vitreum vitreum spawns on gravel and rocks in the shoals (Escbmeyer, 1952) and the young may be attacked by hydras at the time of hatching. In open water, however,

the schooling behavior of young fishes would protect most

of the individuals in the aggregation from destruction by hydra.

Injury of Fi shermen»s Skin The reports that Great Lakes fisheimen who handle nets

covered with aggregations suffer from a skin irritation known as the "itch" have been reviewed in an earlier section

(II: 2-a). Also the Irritation of the face, eyes, and hands from "tar dust” experienced by the men who hand the dry nets upon which hydras have settled Is reportedby'(Clemen’s, 1922).

- 22k ~ To find out "whether these Injuries to the skin are caused by poisons from hydra nematocysts or whether the skin is ir

ritated by substances from the diatom-fungus mat that en

cases the twine would require extensive patch-testing. Schwartz and Tabershaw (19*4-5) report that the fre

quency with which dermatitis occurs among fishermen makes

it the chief occupational hazard of the fish industry.

They list the stings from anemones* Jellyfish* sea nettles

and the Portuguese man-of-war as one of the causes of skin lesions among commercial fishermen. Bonnevie (19*4-8) estab

lished that an allergic contact eczema known as "Fisherman*s Dogger Bank Itch" was due to Alcvonidlum hlrsutum (the "sea

chervil"), and demonstrated that this type of eczema suffer

ed by the North Sea fishermen was due to an animal allergen.

The fishermen became eczematously sensitized in the course

of working the nets at Dogger Bank; consequently the dis

abilities resulting from the "itch" were classified under

the Danish Workmen's Occupational Disease Act.

Generally the species of the Cnldarla belonging to

Hydrozoa can be handled without injurious results. De Oreo (19*4-6) established, however, that the colonial hydrold,

Haleclum sp. caused acute urticaria in 25 out of 52 men who

come in contact with the colonies growing on a swimming raft.

The methodological difficulties Involved in establish

ing the cause of the "itch" sometimes experienced by Lake - 225 - Erie fishermen placed investigation of the possibilities of hydras injuring human beings beyond the scope of this inves tigation.

- 226 - PART IV

SEXUAL REPRODUCTION

The transformation of fresh-water polyps from the asex ual to the sexual mode of reproduction has excited the curi osity of biologists since Trembley's time (17I4J4.) • Many investigators have attempted to discover the stimuli which

induce the interstitial cells in the epidermis of a hydra

to differentiate into testes or ovaries. The literature on

the subject is replete with conflicting evidence and provoca

tive speculation. Results of experiments are not conclusive. After two hundred years, the problem of sexuality, like other puzzles in the biology of hydras, remains to be solved.

Prom observations made in"the field, certain findings on environmental factors which may influence gonad formation in the various species of the Hydridae can be contributed by taxonomists and ecologists toward sounder attacks on the problem in the laboratory. But determining what induces

sexuality in the species is not the primary concern of one

studying interspecific relations. To ascertain when the sexual period of each species in the community occurs is of first importance. Information concerning the overlapping sexual periods seems especially necessary for any approach to the question of speciation in such closely associated forms as the hydras. The season of the sexual phase may be impor tant in maintenance of the species in some localities. The

- 227 - dispersal of eggs may be a factor in geographical distribu tion. Failure of females to appear may contribute to eco

logical Isolation. Some aspects of these questions are dis

cussed following the presentation of findings on the sexual

periods of the Lake Erie hydras.

SEXUAL PERIODS, GONAD DEVELOPMENT, AND SEX RATIOS

Throughout the duration of the sexual period special

care was exercised in handling collections to avoid subject ing the gonadal individuals to sudden temperature changes.

Standard practice was followed except that a higher magnifi

cation (37.5 x) was employed while searching the material. All specimens showing any trace of gonadal development were

segregated — males, females, possible hermaphrodites, also

deposited eggs — each into separate watch glasses. These were placed in covered petri dishes and held at lake tempera ture in a constant temperature cabinet.

As soon as the routine tabulations and notations for the station collection were completed, the gonadal specimens were

examined under the highest magnification (75 xj' of the stereo

scopic microscope to observe the gonads more closely and to determine the sexual maturity of each individual. In this way, such significant features in males as the presence of

nipples on testes and the motility of spermatozoa in the testes, could be clearly seen; in some cases, the eruption

of the sperm was observed. In females such features of

- 228 - Importance were noted as the condition of the ovary: whether just forming as swelling on epidermis, more advanced with ovum in scalloped phase, completely mature with egg rounded and ready to "be extruded; the mature egg, held in the epi dermal cup after extrusion; the fertilized egg as indicated by the presence of the embryonic theca; the number of eggs. (See drawings from life in Hyman's treatise, 19^0, Fig. 132.) Thecated eggs were mounted intact on the transected column and examined for taxonomic characters under the compound microscope. The amputated tentacles and distal column of such females were squashed for nematocyst study. Other sex ual specimens were simply squashed in toto after observation so that species diagnosis could be completed by nematocyst examination.

Results of observations made during the sexual periods are summarized for each species. Reference should be made to section on taxonomy, especially the ”comparative notes*’ for citations of Hyman's figures depicting the sexual organs and eggs of the species.

Sexuality in H.

The duration of the sexual period for our swift-water hydra was established by Intensive weekly collections during

195>2 (Table IX and Fig. $). It extended from the middle of

October to the end of November. The temperature of the water on October 13, when the first specimens with gonads were found, was 13.£°C; on December 28, when the last gonadal - 229 - specimens were collected, the temperature had dropped to 0.5°. None of the individuals collected thereafter showed any trace of sexual organs. Of the 696 hydras collected and examined during the

sexual period, 12 per cent were in the sexual state. The

per cent ratio of males to females was 61:39 — about 3 to 2.

It will be noted, however, that only males appeared at first, and that the males outnumbered the females at least 2: to 1

until the latter part of the period.

The incidence of sexual individuals showed an increase during the period, jusging by the following significant per

centages: 2 per cent on October 13; 5 P®r cent on October 27; 20 per cent on November 11. A climax of sexuality appears

to be reached after the organisms have been subjected to a

temperature declining from 13 to 6 degrees over a four-week

interval. Sexual activity may persist in some individuals at extremely low temperatures: on December 28, at the end

of the sexual period when the lake was about to freeze over, a female bearing two thecated eggs and a male with motile

sperm were observed in an aggregation of 20 hydras which had

colonized the stone at station 1-a. The temperature there was 0.5°. Testes and ovaries, during this interval, showed signs

of increasing maturity, in males collected at the beginning of the period, testis development had just started In the f o m

of low ridges covering the distal two-thirds of the column.

- 230 - Individual testes were separating distally, but no nipples had formed. One out of the seven males in the collection of October 27 had well-developed testes with nipples. Motile sperm could be seen in the periphery of these gonads. Two random samples of 100 made at this time from a large settle ment of the species in the hatchery tanks each contained three malesj these were immature. Of the 37 males collected during November, only four were immature, i.e., nipples had not yet appeared on the separate low testes. The remainder all possessed some testes with nipples, the most mature males exhibiting large high testes each with a prominent nipple. These fully mature males bore from seven to nine testes arranged in a tight spiral along the column. Active sperm could easily be seen in the area adjacent to the nipples. Eruption of sperm from the tip of the nipple was observed in several cases.

This gonad development in males is characteristic of this species. Observations of the animals as they mature in

Lake Erie confirm the findings of Hyman (1938, p. 7), who redescribed testis development in H. littoralis, Hyman, 1931, from individuals appearing in a culture of asexual specimens obtained from Pompton Lake, New Jersey. Mention of speim motility or eruption is not made, however. Limited observa tions of Lake Erie specimens indicate that sperm become motile just prior to nipple formation, and that the most distally located testes are the oldest: as the male grows old, - 231 - the nipples of the most distal testes are resorbed and no motile sperm appear in such gonads.

Not a single case of hermaphroditism was encountered. \ No symptoms of protandry were observed in any of the speci mens. None of the individuals maturing as males developed ovaries; individuals destined to become females showed no signs of even abortive testis development. The species is strictly dioecious, as described by Hyman.

The distinctive feature of sexual development in females of this species is the relatively small number of eggs pro duced. Each female apparently produces only two or three eggs, usually one at a time. None of the specimens taken from Lake Erie possessed more than three ova or ovaries.

In no instance did an individual bear more than two completely mature and extruded eggs. Remnants of epidermal cups on such individuals, which would indicate the sites of eggs which had been deposited, were not present. Though eggs formed from ovaries of females held in watch glasses at lake temperature, in no case did an individual produce more than three eggs.

These observations agree with those Hyman (1938, pp. 7-

8) made on females appearing in a culture of the specimens from Pompton Lake, New Jersey. There are no reports of the occurrence of females in the habitats of this locality or other localities where H. littoralis has been found. Hyman

(1938, Fig. 7 and p. 8) describes the thecated eggs, but records no observations on ovary development.

- 232 - In the Lake Erie specimens it was observed that the ovaries originate as a slight swelling encircling the mid region of the column. Formation of the ovum can be recog nized in the scalloped phase seen in other species of hydras and erroneously designated as the "amoeboid stage" by some authors because the protrusions of the differentiating ovum look like pseudopodia. As the egg rounds up into a sphere, the developing ovary takes the form of a hemispherical hump.

The completely mature egg is extruded from the ovary by rup ture of the ovarian epidermis, which shrinks into a calyx like cup that holds the spherical white egg now ready for fertilization.

While the first ovum is maturing to the extrusion phase, a second egg makes its appearance in the scalloped form In the ovary on the opposite side of the column. In rare in stances, two ova mature simultaneously. In such Individuals, the extruded eggs are opposite in position. A third ovum sometimes starts forming in the intermediate zone adjacent to the extruded eggs before they are deposited.

Under laboratory conditions, it takes about three days for an egg to ripen from the scalloped phase to the point where it bursts through the epidermis of the ovary. Unless it is fertilized shortly after extrusion, the egg disinte grates. A fertilized egg secretes the embryonic theca characteristic of the species. This shell enclosing the embryo is brownish yellow; It is about 0.10 mm. thick, so - 233 - that the thecated egg has an outside diameter of around 0.£0 mra. The thecated embryo falls from its cup to the bottom.

It does not adhere to substrata.

Some idea of increasing incidence of egg production in females may be gained from closer examination of the field data summarized in Table IX. Of the seven examined prior to

November 11, no eggs were evident in the ovarian enlargements of five; two in the collection of November ij. showed egg forma tion, one with ovum in the scalloped phase, the other with ovum fully formed in the ovary. Of the 21 females from col lections made at the peak of the sexual period (November 11 and 1 8 ), the Individuals were distributed as follows:

Ovary forming with no ovum evident ...... 2 Ovum in scalloped phase In young ovary . . . 10

Ovum mature in fully formed o v a r y ...... 3

Naked egg extruded In ovarian cup ..... 1+ Thecated egg ready for deposition ...... 2

The tendency of this species to produce but a single egg at a time was observable in this group of females: only three had two eggs forming simultaneously; one of the mature females bore two eggs.

Thecated embryos were not found until November 18: two individuals each bore single thecated eggs; three embryos were found free In the washings from stones. Thecated eggs were not seen again until the end of the period when a single female in the collection of December 28 bore two thecated

- 23k - ©ggsj two embryos were also found In the collection. Subse quent searchings for eggs during the winter period proved futile. Of course, deposited eggs may have been overlooked because of their minute size and brownish color which blends with the masses of diatoms removed from the stones. Never theless, the rarity with which thecated eggs on females appear and the rapid disintegration of extruded eggs, if not fertilized, leads one to suspect that the number of embryos produced during a sexual period is even fewer in H. littoralis than in other species of Hydra.

Supplementary collection data from other seasons tends to confirm the duration of the sexual period and the relative incidence of maturing males and females as established for the species by the 1952 collections.

During 1951 no sexual individuals were found until the middle of October when the water was approaching the critical temperature of II4.0• For example, on October 2, when the lake had cooled to around 17°, gonadal development was absent in

123 specimens examined in a collection made from the rubble and gravel at station lj.. Collections from this habitat and from other habitats along the shore of Gibraltar Island made during the rapid temperature decline of the preceding month contained no sexual specimens.

The large sample of hydras obtained on October 15, when the concrete blocks were first lifted at Britt»s Gibraltar Island shore stations In the bay (Table III), contained eigit - 235 - gonadal individuals — 1.3 per cent of the £8l collected. One of the eight was a female with ovary in the scalloped phase; the seven males were immature, only one with testes developed on the nipples. When the blocks were again lifted on December 5 (tem perature I4..60), females predominated: 5 were found at sta tion B, 3 at C, and 1 at D — about 11 per cent of the 79 individuals collected from the four stations; on the same date, 15 gonadal hydras (thirteen females in various stages of sexual development and two mature males 5 were found in a collection of 59 hydras from four blocks set October 15 at station E, depth 6 meters. ♦ This reversal in ratio of males to females at the begin ning and end of the period was also noticed in 1952, when progressive changes in occurrence and gonad development were closely followed. The relatively large number of gonadal individuals (17 per cent) suggests a high degree of sexual activity; but none of the females except two of the thirteen at station E, where the two males were found, bore a thecated egg; and the occurrence of only these two males among the total of 22 females leads one to suspect a high mortality in the eggs produced by the females.

The lake froze over on December 23, and the floats mark ing the concrete blocks were hidden in or beneath the ice.

The few hydras obtained by collections made through the ice- cover showed no trace of gonads. The ice went out of the bay

- 236 - on March 17, and the blocks could again be located. In the samples obtained from them (March 27 to September 10, 1952 —

Tables IV and V), none of the hydras showed signs of gonads.

The season of 1953, it will be recalled, was character ized by abnormally high temperatures in late summer and early fall (Pig. 5 and Table VI). Collections were suspended on

September 3 when the temperature had risen to 27.6° and the large numbers of hydras colonizing the slides earlier in the season had dwindled almost to the vanishing point. No col lections were made until October 12, when the lake had cooled to 16°. During the intervals, sizeable aggregations of

H. littoralis had reestablished themselves on the slides, but none of the individuals were in the sexual state. No trace of gonads were observed in 37 individuals of the species col lected from the pound nets at Pelee Island two days later, temperature 15.0°. on December 7 when the racks were again lifted, 113 H. littoralis had recolonized the slides, and 13 of them were in advanced sexual development. All of the nine females bore an extruded egg; one had two ready for fertili zation and a third maturing in the scalloped phase. The four males were mature with motile sperm in their testes. The temperature was 5 .i+°.

This relatively high incidence of gonadal individuals (about 12 per cent) compares with a 20 per cent incidence at the peak of sexual period in 1952, November 11, when the temperature was almost the same. In the delayed season of

- 237 - 1953, onset of the sexual period probably did not occur until

the beginning of November, and may not have reached its peak

until the beginning of December: five-day mean temperatures

during October remained close to 16° until the end of the month, when they declined sharply, leveling off around 8° during November, and remaining slightly above 5° from the end

of November to the middle of December — a decided contrast with the season of 1952. Ice covered the bay when the rigs were again lifted 58

days later — February 2, 195^* temperature 0.2°. No hydras had settled on slides or stone, and the lone Individuals

found on pieces of rubble at the stations were without buds

or gonads. Experiences were the same with the two collec tions made at the beginning and middle of April (Table VII:

summary of 195^ seasonal data). Not a single sexual individ

ual was present among the budding aggregations during May, at temperatures of around 11°, or in June, when the water had warned to 20°. Nor were sexual hydras found in the miscel laneous collections made during the spring of previous seasons reviewed above.

Only one sexual period annually appears in Hydra littor alis, as observed in Lake Erie. The possibility of a second

sexual period occurring in the spring was suggested by the report of males of this species being collected in March near Norman, Oklahoma (Trowbridge, Bragg, and Self, 1936). This finding, coupled with observations of gonad induction in

- 238 - cultures, led the author of the species to postulate that

” ... there must be two sexual periods annually In nature.”

(Hyman, 1938, p. 7«) The evidence upon which this conclusion was based will be examined later in discussing the influence

of external factors on gonad formation.

To summarize, in Lake Erie waters, the sexual period of Hydra littoralis extends from the middle of October to the

end of December. The onset of sexual development begins when the water has dropped to a temperature of 1^° and terminates around 0,i|.0 , when the freeze-over is approaching. A climax

of sexual activity appears to be reached about four weeks

after the onset of the period during which the organisms have been exposed to a sharp temperature decline of about 8°.

During this interval, the incidence of sexual individuals in

creases from two per cent to twenty per cent, and evidences

of progressive sexual maturity are seen in both males and

females. Only males appear at first, but as the period pro gresses the ratio of males to females is reversed. At the end of the period, incidence of males is so low that mortal

ity of the two or three eggs produced per female may be high.

I cannot give a detailed account of the sexual phase in

the life histories of the other hydras in the community. As previously mentioned, the occurrence of H. americana was sporadic; adequate quantitative data was not obtained for

H. oligaotlffi and H. pseudoligactis associating in the water- weed habitat. Onset of the sexual period can be reported - 239 - from field experience, however, and certain observations on sexuality compared with the findings of others who have studied these species. Unfortunately, some workers fail to record water temperatures in their reports, mentioning only the month of the year during which sexual specimens were ob served.

Heimaphrodltism in H. amerlcana

This short-tentacled little hydra, in contrast with the other hydras in the community, exhibited two outstanding traits in its life history: it became sexually mature the earliest in the season; both monoecious and dioecious indi viduals developed.

In the series of 19$2 weekly collections (Tables IV and

V), maturing males and females suddenly made their appearance on September 23 when the lake was still around 21°. Two females (one with ovary and bud, the other with two ovaries) and one male with well-developed nipples were present in the aggregation of 29 hydras colonizing slides and stone at sta tion 1—c. Pour others without gonads were found, the remain der being H. littoralis in the asexual state. At the same station the following week (temperature 18°), two more fe males were found among the 18 hydras (£ H. amerlcana) which had recolonized the slides; at station 6 , of the £ H. amerl cana among 18 hydras removed from the stone, one was a female bearing an extruded egg and a bud. Sexual specimens were

- 214.0 - encountered only once again during the season — on October

27 (temperature 10°) at station 6 where two of 12 H. americana in the collection were in advanced sexual development. Both of these specimens were hermaphrodites with ripe testes and extruded eggs: each specimen bore two eggs in their proxiraally located epidermal cups; one egg had been ferti lized, exhibiting the characteristic spherical embryonic theca with long unbranched spines. The nippled conical testes, three and four in number, were distal; motile sperm were observed in the testes of both specimens.

Of the 63 H. littoralis making up the rest of aggrega tion at station 6, four individuals were sexual: 3 immature males, and one female with ovary; this species had just entered its sexual period during the preceding two weeks. Sexuality had not yet occurred in either H. oligactls or H. pseudoligactis associating in the habitat at station £.

The series of samples from the concrete blocks taken during 19^1-1952 (Table III) yielded no sexual specimens of H. americana. Nor did any appear in cultures started from these sources. A clone, subcultured from a mass culture of hydras collected at Oak Point (station Ij.) on October 2, 19f?l, however, produced sexual individuals, all males.

The experience of 195?3 again indicates that the sexual period of this species begins earlier than that of the other species. Among 5>0 which appeared In the collection of Novem ber 12 (Table VI), 23 were sexual: one was an hermaphrodite;

- 2J+1 - the others, immature males (two with a bud), turned hermaph roditic in the laboratory. One hermaphrodite, bearing two testes without nipples and two eggs in the scalloped phase, and one immature male with buds were also found among 19 specimens on a rack near station 6 . At the lake temperature of 16°, onset of the sexual period in H. littoralis or the other species had not yet occurred whereas sexual development in H. americana had probably been in progress since the last week of September when the lake had cooled to 20°. In the

December collection, as in 1951 and 1952, neither sexual nor asexual individuals of the species were found. Thecated em bryos of H. ameri cana were not seen in the debris of any of the collections throughout the period of the study. These field observations show: (1) that sexuality in

H. ameri cana occurs much earlier in the autumn than in the other Lake Erie hydras; (2) that the sexual period begins when the water is approaching a temperature of 20°; (3 ) that either males, females or hermaphrodites may develop; (I).) that in mature hermaphrodites, eggs and testes are both ripe simultaneously; (5 ) that some males and females produce buds while still bearing gonads.

The findings also indicate that the period of sexual activity may be brief, occurring late in September or early

In October, and reaching Its height as the temperature ap proaches 10°. The sexual phase in this species probably ends during November when the other species are attaining full

- 2^2 - sexual maturity. Early sexual maturity was also observed in

the Chicago habitats of the species by Hyman (1929); she

reports males and females as occurring during the months of September and October (p. 2£0). In other latitudes, sexual maturity may occur during the months of March and April.

Bragg and Self (1937) in their collection of this species

from the South Canadian River flood plain near Norman, Okla homa, report finding mature gonadal individuals in the spring

those collected on April 16, 1936 were all females; those

collected March 20, 1937 were all males. Because of its taxonomic significance, the occurrence

of hermaphroditism in the species as observed in Lake Erie

merits some discussion. It will be noted that in the records

just cited the gonadal individuals reported in natural popu

lations were dioecious. Hyman (1929, p. 250) states: "The

sexes are commonly separate but hermaphroditic specimens have occurred occasionally in laboratory cultures." She includes

a drawing done from a fixed preparation in the species de

scription (plate 29, Pig. 6 ), stating (p. 251): "This Indi vidual was one of four hermaphrodites which appeared simul

taneously In a laboratory culture, in which up to that time

only males had occurred.11 other hermaphroditic Individuals were seen in laboratory cultures subsequently. In a later paper (1931a, p. 2lj.)> Hyman mentions receiving several her maphroditic specimens from Salt Lake City, Utah, and says: "Hermaphroditism would seem to be much more common than I had - 2J+3 - supposed since writing the description of this species...

To find out something about hermaphroditic development

in individuals of the species, I followed quite closely the history of the 22 immature males taken in the collection of

November 12, 1953.

At the time of collection, each of these males was iso

lated in a numbered watch glass, and the gonads examined.

None showed any signs of maturity. The testes, two or three located in a whorl well to the distal portion of the column, appeared as small hemispherical elevations devoid of nippleB

and without motile sperm.

Each of the specimens was fed two or three young Dapbnia magna. Some of the hydras seized a daphnid at once. All

the individuals were whitish at the time of feeding. The

specimens were then placed in the constant temperature cabi net set at 16°. They were held at this temperature, which

approximated that of the lake, while observations were made

at twenty-four hour Intervals during the next four days. At each examination, the water was completely changed with fil

tered lake water to prevent fouling. The specimens were fed

as just described. During these manipulations, which required

about fifteen minutes per specimen removed separately to and from constant temperature, individuals were exposed to room o and illumination temperatures ranging from 20 to 25 . On the second day after collection (about 21| hours after

the first examination and feeding), inspection disclosed that

- m - ovaries were developing in 10 of the 22 specimens. Scallops of eggs were just appearing in the ovaries. They were basal- ly located pairs — at the zone where two bore a bud seen at collection. Testes were becoming more conical in shape in these individuals and an additional testis had appeared in several. But nipples had not yet formed on the testes of the individuals which had turned hermaphroditic or on the testes of the others. The Daphnia were dead in all the containers with the exception of two. These, numbered 3 and 6, happened to con tain males in which no ovaries had appeared. These individ uals were still white whereas all the others were turning a tannish hue. This change in color of hydras accompanies feeding on daphnids, as established in the discussion of food reactions.

It occurred to me that the capacity for self-fertiliza tion might be tested in the hermaphrodites. Accordingly, special precautions were taken to assure that no hydra spermatazoa would be present in the filtered lake water used for changing the watch glass cultures or washing the daphnids introduced for feeding. It was subjected to boiling for five minutes and then aerated for fifteen minutes. The same pro cedure was used in handling water for subsequent changes. On the third day five additional specimens had turned hermaphroditic. One of these had a bud appearing at the zone where two ovaries were forming. Of the oldest hermaphrodites,

- 2 k $ - seven of the ten were approaching sexual maturity: nipples had formed on most of the testes; motile sperm were observ able in these; eggs were approaching the extrusion stage in four, and three bore eggs extruded and ready for fertiliza tion. Motile sperm were observed in the testes of these three. Fifteen of the 22 hydras had now turned hermaphro ditic. The two males, which had not been feeding, were still white; they were contracted, refused food, and otherwise showed the symptoms of depression. So they were squashed.

Their stenoteles ranged up to 22 microns, definitely deter mining them as H. americana.

On the fourth day, a thecated egg was seen on one of the three hermaphrodites bearing extruded eggs the day before. This individual bore a second extruded egg opposite, and a third egg was forming in the scalloped phase. The sperms in the most proximal of the four testes were active. Two of the remaining five individuals had now turned hermaphroditic; their gonads showed signs of synchronous ripening.

Unfortunately, on October 16 it was necessary to trans port the specimens to Detroit. A car refrigerator iced at around 10° was used in transporting the specimens to the Cranbrook Institute of Science laboratory where they were o held in a refrigerator at temperatures varying from 6 to 10 .

When the specimens were examined three days later, two more individuals, each bearing a thecated egg, were found among the oldest hermaphrodites. The thecated egg seen on

•* 2I4.6 — October 16 lay free in the watchglass; but the other two eggs were disintegrating; the parent was undergoing depres sion and the testes had been resorbed. Examination of the eggs produced by the three self-fertilizing individuals dis closed an embryonic theca almost identical with those of the fertilized eggs from females as figured by Hyman (1929» pi* 30, Pigs. 11 and 12). A study of the nematocysts left no doubt that the self-fertilizing hydras were H. americana.

Nothing can be said about the inheritance of hermaphro ditism in these monoecious strains. All attempts to clone culture the individuals failed, despite use of aerated Lake

Erie water. Probably because of temperature changes or other trauma, sexual development ceased; as observed in the field, the maximum number of eggs borne by an individual was three, the number of testes four. Though hand-feeding was employed and some individuals accepted crushed daphnids or copepods, buds did not develop. Despite use of aerated Lake

Erie water for changes after feeding, the animals were in depression by November 11, 1953. Each specimen was then squashed for nematocyst examinations; these definitely estab lished that all of the individuals in the group studied belonged to the same species.

These laboratory observations show* (1) that self- fertilization can occur in hermaphroditic individuals of the species H. americana; (2) that the embryonic thecae appearing after self-fertilization are identical with those seen in

- 2k7 -

/ dioecious individuals; (3 ) that testes of hermaphrodites, though fewer in number and restricted to the distal column, are identical with those observed in dioecious individuals;

(I4.) that in monoecious strains, though the testes appear distally before ovaries are formed, spermatozoa and ova are functional synchronously; (5 ) that sexual individuals will feed except when in a state of depression; (6 ) that hermaph rodites, like males and females observed in the field, may bear buds while still retaining gonads. It should be mentioned here that, although I did happen to see sperm escaping from the testis of one of the isolated hermaphrodites whose extruded egg developed an embryonic theca, I did not actually witness a sperm penetrating an egg. Barring the possibility of parthenogenetic development — a phenomenon unknown in the Hydridae — the conditions main tained during the observations were such that the stimulus for embryonic development had to be sperm produced by the same individual which produced the egg. Thus the capacity for self-fertilization In the species has been demonstrated.

It is interesting to note that self-fertilization in hydras was first observed a hundred years ago by Max Schultze.

His observations were later confirmed by Zoja. In reviewing their work, Paul Schulze (1917, p. 81) establishes that both

Investigators were studying H. vulgaris, the European species which so closely resembles our indigenous H. americana; he considers Zoja*s demonstration conclusive and quotes part of

- 21+8 - the detailed account of the behavior of the animal during the act of self-fertilization: "Der polyp krtimmt den oberen

Teil seines Kbrpers gegen den unteren, driickt und schiebt die Eier so, dass eine Menge der sich lebhaft bewegenden

Samentierchen ausder an der Spitze des Hodens befindlichen Offnung herausgedruckt wird, u.s.w."

I did not see any such movements which might be related to self-fertilization behavior in the mature hermaphrodites of our species. Tannreuther (1909a), in reporting his demon stration of self-fertilization in the monoecious Chlorohydra viridissima, makes no mention of such behavior. While veri fying Tannreuther> s work with mature hermaphrodites from the

Pelee Island green hydra culture, I observed the escape of spermatozoa from the nipples of the distally located testes, of several individuals; but this occurred without any press ing of the testes against the extruded eggs. As Tannreuther reports, the sperm may remain active from one to three days after they escape. This vitality insures penetration of the egg under experimental conditions.

Closer attention to the life history of H. americana in other localities may reveal that hermaphroditism is wide spread in this species. If such proves to be the case, separateness of the sexes loses its diagnostic value; but there are other constant characters which distinguish the species from the highly protandrous H. came a and the mono ecious H. utahensis and H. hymanae of this continent or the

- 2k9 - monoecious H. vulgaris and H. stellata of the European con tinent (see section on taxonomy). At present, the scanty evidence available indicates that in populations of some localities only males appear one season, and only females appear in another; in some localities both males and females appear during the season; in Lake Erie populations males, females and hermaphrodites appear during a season. Hermaph rodites exhibit a slight degree of protandry, which is common in the monoecious forms of hydras as Schulze (1917, p. 81} points out, but there is no evidence of true protandry exist ing in the species.

H. oligactls and H. pseudoligactls Compared

The only statements which can be made with any degree of certitude about sexuality, as observed in these two stalked species of Hydra living in close association on the leaves of water plants in the bay, follow: (1 ) the sexual periods of both occur later in the fall than in the other two species inhabiting the bay; (2} gonads develop in H. oligactis before they appear in H. pseudoligactis; (3) both species are strictly dioecious; (l\.) in males of H. oligactls nipples never form on the testes whereas in mature males of

H. pseudoligactis distinct nipples are present on the testes, which are characteristically pumpkin-shaped and closely clus tered on the column; (5 } females of both species produce more eggs than do those of other species, the ovaries ripening

- 250 - simultaneously and extruding numerous spherical white eggs;

(6) the fertilized eggs of the two species are practically identical in appearance, being inclosed in a thin theca devoid of spines; (7) the thecae of the embryos of these species, unlike those of the other two observed in Lake Erie, become fastened to substrata, especially dead tree leaves, where they can be found from late fall to early spring; (8) the period of sexual activity in both species extends well into the winter and may continue in some individuals at tem peratures approaching the freezing point of water.

Duration of the sexual period is estimated primarily from the data obtained during 1952-^3 by weekly collections from the slide rack.and from the Myriophyllum plants at station 5. Male H. oligactls appeared first at the beginning o of November when the water was around o . The first female was found about two weeks later; the ovaries were just form ing. Only two more females were found up to the time slush ies formed at the end of December. These bore fertilized eggs. Among the few hydras collected during January, an occasional male with motile sperm was taken. A few eggs adhering to dead tree leaves were seen, but no additional females were encountered during January or February.

Females of H. pseudoligactis were not found until the water was approaching the freezing point. They also were scarce. Only four were found; the last one, observed in

January, bore thecated eggs. Mature males appeared about

- 2 $ 1 - the middle of November, and a few were found in collections up through January.

The number of individuals turning sexual in the oligao- ti s-pseudoligactls population during the 1 9 $ 3 - 1 9 season may have been greater. Several maturing females and a number of males of both species were found on dead tree leaves during a collection made early in December.

The potentiality for sexual activity to persist in some individuals at extremely low temperatures (0.2°) is indicated by the occurrence of a mature male oligactls among 27 hydras obtained from a collection of leaves made through the ice- cover at station 5 on February 3, 195^. This individual was anomalous in that it possessed ten tentacles. It bore two well-developed buds. From the budding zone, eight testes, characteristically low and devoid of nipples, spiraled up the column. Motile sperm were seen in some of the testes.

Squashes of the detached buds showed the typical holotrichous iBorhizas of H. oligactis.

That sexuality does not persist in these species through the late winter or reoccur again in the early spring is shown by additional 1 9 ^ data to be treated later. The experience with the Lake Erie oligactis-pseudoligactis sexual cycle resembles that of Miller (1936, pp. l£0- l£2) with the Douglas Lake population. Among the large num bers of hydras growing on slides in several racks suspended in the lake, he found no sexual specimens until October 30

- Z$Z - when the temperature was 6°. These were males. Females did not appear until November l]j>, when the ice-cover was forming. (Bazuin also records November 1£> as the earliest date of finding females in the locality of Grand Rapids,

Michigan according to Hyman, 1931b.) No females were present after January 20. During this interval only fourteen devel oped ovaries or eggs, whereas many more males developed gonads (as high as 27 per cent of the l+,000 individuals on the racks a month and a half after the sexual period began).

No males were present after January 1. The sexual period, therefore, extended over a period of about three months.

Miller did not attempt to distinguish between the two species in the mixed population, although this is easily done when the animals are in the sexual state. His paper contains no details on the development of the sexual organs. The pioneer study of the Douglas Lake hydras done by

Welch and Loomis (1918-1921+) was confined to the summer sea sons; their paper does not cover sexual reproduction. In his year-round study of the H. oligactis population of Kirk patricks Lake (191+8-191+9)* Bryden encountered no sexual hydras although temperatures in late November and December ranged from 7 to 1+°.

Besides Miller1s excellent work (1931-1933) the only other field study dealing with sexual reproduction of hydras is the one done by Boecker from 19l£ to 1918 on H. oligactis.

His data, though based on small numbers of hydras, collected

- 253 - chiefly from the rivers O m e and Elbe in Germany, are valu able. His careful record of observations on the sequence of gonad development in relation to changing water temperatures leave no doubt that the sexual cycle of the species in

Europe follows the same course as the cycle of the species in our country. Findings on the sexual organs and embryonic thecae, summarized in statements 3 through 6 above, confirm those of

Hyman (1930, 1931b), who studied the two species thoroughly in laboratory cultures over a period of several years.

In the case of the ubiquitous Hydra oligactls, which, like Chlorohydra viridi3slma, appears to have a cosmopolitan distribution, there can be little doubt that the American species is identical with the European oligactis. In the section on taxonomy, it is pointed out that, with the excep tion of lack of any true spination in the embryonic theca, the characters of the Lake Erie specimens agree most closely with those described and figured by Schulze (1917) from European specimens. Hyman (1930, p. 326) mentions that”the degree of spinescence is the one point in which the American oligactls seems to differ slightly from the European.” She speaks of difficulty in finding females, and states: ”1 have seen only two or three fertilized eggs of oligactls and the character may be subject to variation.” Females were also rare in collections from Lake Erie, and in my cultures. I was able to find only nine individuals

- - — five from collections and four from cultures — which bore fertilized eggs, so the extent of variation in this differing character could not be studied. Examination of 19 ferti lized eggs obtained from these specimens, however, showed the embryonic theca to be as Hyman describes it (p. 326):

thin and scarcely spiny at all ... practically spine less ... merely wavy.” In general, the thecae were as thin and spineless as those inclosing fertilized eggs seen attached to H. pseudoligactis. Some thecae displayed varying degrees of wavy irregularities, but none exhibited the distinct

"kleine Hbckern" Schulze (1917* p* 106) describes and figures. Schulze does not state how many eggs he examined; and neither he nor Eymaxi give any dimensions of the eggs. Boecker (1918) is quite explicit on this matter, giving the range in diameter as 0.1|.5> to mm. for the eight eggs measured on the five mature females he was able to find.

(Collections numbered 68 and 70, p. lj.88.) Boecker, although quite critical of Schulze's newly published monograph, is vague in his account of the fertilized eggs. He describes them as being spherical, pale brownish in color like horn, with a knobby (hbckerig) surface (p. i|.93). I can find no other valid information concerning the thecated eggs of this supposedly well-known species in the literature.

The thecated eggs are spherical as Schulze, Boecker, and Hyman state. The 19 I examined varied in diameter from 0*36 to 0*£f.6 mm., the majority measuring close to 0.i|.f> m m .

- 2$$ - These dimensions fall within the range of 15 eggs measured from five specimens of H. pseudoligactis. The thecae In both species remain a pale yellow, and are so thin they are hard to distinguish from the white sphere of the embryonic tissue they inclose. Eggs found attached to dead tree leaves varied in size within the diameter ranges given above; they were also spherical in shape, except for a slight flattening of the theca where it adhered to the leaf epidermis. Because of the similarity in appearance of the eggs of the two spe cies, one can be certain of the species of deposited embryos only by study of the nematocysts of hydras which may hatch from them.

McConnell (1935) maintains that the embryothecae of eggs of H. attentuata on leaves differ from those attached to the parent. I did not find this condition to obtain in the observations made at the time of collection. It should be noted that McConnell made his observations on aquaria specimens where conditions were not properly controlled.

Bis conclusion that the structure of theca may vary so much by place, oxygen, and amount of dissolved material that nit is not a safe procedure to use the characteristics of the shell as a taxonomic feature in the study of Hydra'1 is not warranted.

Females of the two species are easily recognized. In

H. pseudoligactls ovary formation begins with a swelling of the whole region of the distal column where numerous eggs

- 256 - appear in. the scalloped, phase, round up and are extruded into epidermal cups. (Hyman, 1931b, pi. 29, Pig. 5? pi. 30, Pig. 7.) Five mature females taken from the lake bore from ten to twenty extruded eggs. It appears that fewer eggs are produced by H. oilgactls. One of the five specimens col lected from the lake bore eight extruded eggs, the others bore only four or five; the four specimens found in cultures bore from three to nine. Ovaries, ova, and zygotes were white in all cases, regardless of whether the parent was colored brown from a daphnid diet or orange-red from eating other food. In general these findings agree with those of

Schulze (1917, p. 106); he found that oligactis females pro duce from four to ten eggs and established that carotenoid pigments coloring the parent do not invade the white ovaries or eggs. Boecker (1916, p. lj-93) reports that the largest number of eggs found on females in his collections was five.

INFLUENCE OF EXTERNAL FACTORS ON GONAD FORMATION From the evidence at hand it is clear that gonads form in the hydras under investigation when the water temperatures, are declining rapidly during autumn (see Fig. 5). The spe cies can be arrayed in sequence of onset of gonad formation and probable extent of their sexual periods in western Lake Erie (1952) as follows:

- 257 - H. americana ...... September 26 - October 27 (one month(?): 21 - 1 0 ° )

H. littoralis ...... October 13 - December 28 ' (two and a half months: 13 - 0.5 )

H. oligactls ...... November 1 - February 3 (three months: 8 - 0.2°)

H. pseudollgac tis ... November l8 - January 31 (two and a half months: 6 - 0.2 )

The findings also show that there is only one sexual period among the Lake Erie hydras; no repetition of gonad formation occurs with the wanning of the winter waters. In: some localities the sexual period of H. americana and H. lit toral! s may occur in spring. The reports of such incidences, reviewed in previous sections, do not make any mention of actual observation of a second sexual period in the popula tions during autumn. In view of lack of evidence from ob servations made in the field on the same population, Hyman’s conclusion that in H. littoral is there must be two sexual periods in nature does not appear justifiable. The complete quotation from Hyman (1938, p. 7) follows:

In Chicago, male specimens were found in nature in the late fall, and consequently it was concluded that sexuality is induced in this species as in most other hydras by a falling temperature. Trowbridge, Bragg, and Self (1936), however, found young males in March, and the culture which I started from specimens brought to me from Pomp ton Lake ‘New Jersey1 in March soon became sexual. Later this same culture on being returned to room temperature after a short exposure to low temperature (5°C) again displayed sexual activity. It therefore appears that in Hydra llttoralis sexuality is induced by either a falling or a rising temperature and there must be two sexual periods annually in nature.

- 258 - All authors agree that there Is only one sexual period

In H« oligactis and H. pseudoligactis and that It occurs in

nature with falling water temperatures. Frischholz (1909)

was the first to demonstrate that the optimum temperature

for the formation of gonads was about 1 0 °C. Schulze (1917,

p. 105) points out that Nussbaum's claim of 1893 and 1909

that gonads form only after a period of hunger was proved false by the work of Krapfenbauer In 1908 and Koch’s experi ments of 1911. (See Schulze’s bibliography for literature

cited.) Schulze confirmed Frischholz's results. He placed half the animals from a room-temperature culture in a cellar where the temperature was about 1 2 ° . After ten days, when

the temperature was 10°, testes began to appear in almost all the animals; but only two produced ovaries. The animals

held at room temperature remained asexual. The optimum tem perature for sexual maturity (Geschlechtsreife) was given such weight by Schulze (1917, pp. 35-36) that he included it

among the criteria used in erecting the genus Pelmatohydra.

This optimum is about 1 0 ° . Hyman (1928) in confirming

the results of Frischholz and Schulze induced sex organs in

H. oligactis in about three weeks by lowering the temperature

to around 1 0 ° . She also proved by clone culture experiments that in dioecious species of Hydra, the sex of asexually pro duced offspring is the same as that of the parent.

The best review of the whole subject of gonad induction is contained in Boecker’s report of field work on sexual

- 259 - reproduction in the German hydras, which appeared a year

after Schulze*s taxonomic monograph. He severely criticizes

Nussbaum *s experimental procedures, maintaining his conten

tion that inanition rather than temperature was the most important external influence in producing gonads was entirely

unwarranted. He evaluates the evidence contained in the

works published by R. Hertwig, Krapfenbauer, Frischholz, and Koch (1906 to 1911) from the Munich Zoological institute as

conclusively demonstrating that the state of nutrition is of

importance only to the extent that a greater percentage of individuals becomes sexual in cultures abundantly supplied

with food than in those moderately supplied with food or not

fed at all. Boecker harmonized the results of the Munich group with his observations of natural populations, and his

own observations of H. oligactls in cultures. He sums up by

stating (p. 1+80): "Von extremen Hungerzust&nden abgesehen 1st der Eintritt der Geschlechtsreife in bei geeigneter

Temperature gehalten Kulturen von der Masse der Flitterung unabh&ngig."

The earlier assertion by Nussbaum, namely, that male

hydras can be transformed by external influences to females

and changed back has been questioned on grounds of faulty

experimental procedures by his contemporaries in Gemany,

and was later disproved in this country by Hyman (1928).

Likewise, as Hyman (1928, p. 79) points out, the claims made by Nussbaum that rich feeding induces females and starvation

- 260 - males has not been verified by others. All workers with hydras are in agreement that male and female gonads are in duced by the same external factors, not opposite ones.

Mason (191+9) apparently has not familiarized himself with the literature; he includes hydra among those animals for which "there is strong evidence of nutritional sex determina tion, inanition favoring a preponderance of males."

Hyman in her 1928 literature review of sexual reproduc tion— the only one available in English up to the present— cites Whitney's work (1907) with the green hydra but does not review it. Somehow Boecker's report on £. virldissima in

G e m a n y was overlooked. It remained for Miller (1936) to call our attention to this field study, but he does not men tion Whitney's paper nor review Boecker or the other German authors cited with respect to sexual reproduction. In re porting our work in progress with the Lake Erie hydras,

Lahglois (195U, PP« 110-112) raises the question of the effect of environmental influences on sexuality in natural populations, and places considerable emphasis on Whitney's experimental results. Accordingly, Whitney's findings are being examined in some detail with special reference to con flicting evidence obtained from observations of Pelee Island C. vlridisslma cultures.

Whitney observed that under natural conditions the green hydra became sexual only in the spring months of April, May, and June, producing testes and eggs. If kept at a temperature

- 261 - of 15>-22°C throughout the year sexual organs very seldom formed. At the suggestion of T. H. Morgan, Whitney conducted a long series of experiments from 190i|.-1907» making observa tions simultaneously under field and laboratory conditions to determine the influence of external factors in causing the formation of gonads in “Hydra viridis.” He carried out some of the experiments at Columbia University during the winter with animals from a frozen pool near Fort Lee, New

Jersey, but the most of the work was done with a population from the spring-fed pool at the Biological Laboratory, Cold Spring Harbor, New York. Here the hydras had an abundant supply of food from beef fragments fed the trout in the fish pens, and the water remained at an almost constant tempera ture of 12° throughout the year. A summary of Whitney’s results follows:

;(1) In a majority of the hydras which were kept at a freezing temperature under the ice in the pool near Fort Lee, or were at a constant temperature of 12° in the Cold Spring Harbor fish pen for several weeks, and were then placed at a higher temperature of 18-26” with out food, testes developed within 6-II4. days and eggs within 10-20 days (i.e., the sexual animals all became hermaphrodites).

(2) Control animals kept at room temperature did not turn sexual when food was withdrawn.

(V) With longer exposure to lower temperature eggs and testes formed, with shorter only the latter. (5) Only the largest individuals produced eggs as well as testes; the smaller Individuals produced only testes; no individuals were found which produced only eggs. - 262 - (6 ) Low temperature followed by a higher temperature caused rapid bud formation irrespective of food con ditions. A careful reading of Whitney's paper in the light of what was known of the biology of hydras at the time discloses

the author was under the impression that both monoecious and dioecious forms existed in £. viridissima. Subsequent

studies have established that the species is hermaphroditic, often being protandrous (Tannreuther, 1909a; Schulze, 1917>

1922a, 1927; Boecker, 1918; Hyman, 1929, 1931b; Ewer, 19lf8). Not knowing that some degree of protandry is common in all monoecious forms of hydras, Whitney placed undue emphasis on the time lag between the appearance of testes and eggs in evaluating the results of his experiments.

No attempt to duplicate Whitney's experiments was made with the green hydras obtained from Felee Island because comparable conditions for field experimentation were lacking.

Certain observations made on a culture maintained in the laboratory, however, are at variance with some of Whitney's findings; so the history of the culture will be briefly reviewed.

As previously stated (see section on habitats), when the collection containing the green hydras was made from the Pelee Island swamp on October Ilf, 193>3» "the water tem perature was l£°, the air temperature about 20°. The water from the surface collected with the duckweed contained only a few copepods. The duckweeds (Lerana and Spirodella) [ ' - - 263 - supported a scanty number of oligochaetes and rhabdocoeles.

Duckweeds were placed In a battery jar in water from the habitat and allowed to stand overnight in the laboratory.

In the morning, several hydras (later identified as H. carnea) and two green hydras were seen on the sides of the container. None of the hydra were sexual. The battery jar was placed in the constant temperature cabinet at 15> degrees overnight, and transported to Detroit in an iced car refrigerator. The next morning on arrival at the Cranbrook Institute of Science, the temperature of the culture was 13°. Two green hydras were seen at the time, and several of the other species. The jar was placed in the lower compartment of the refrig- o erator where the temperature was about 10 and left undis turbed for the next six days. Because the temperatures in the laboratory sometimes rose to 29°, the jar was placed in the photographic room in a mild light where the temperatures varied between 17 and 20° as recorded by the maximum-minimum thermometer.

No food was introduced into the culture at any time, and the only food animals available were a scanty supply of oligochaetes and copepods originating from the habitat water.

The water level in the jar was maintained by adding Cran brook well water.

The culture began to flourish toward the end of the month, and by the middle of December healthy 0. viridlssima and H. carnea were abundant. They could be seen fully

- 262+ - extended on the duckweed roots, the debris in the bottom, and the sides of the container. The bulk of the green hydra

population was concentrated in the upper inch of the water on the glass of the jar. Inspection of 106 green hydras

adhering to the sides of the glass was made with a 10 x hand

lens. About two-thirds bore one or two buds. No sexual organs were seen. Nor was any evidence of gonads on the

green hydras observed in samples from the jar up to this

time -under higher magnifications of the stereoscopic micro scope. It was noted that the hydras were stout but very

small, the column length of full-extended specimens measuring

around two millimeters.

On December l£ the culture was removed to the laboratory where some crude experiments were made to determine differ

ences in response to light between the Chiore11a-bearing green hydra and the brown H. carnea. The culture was sub jected to direct sunlight which caused the temperature to o rise to 30 at times. After the four days when the experi ments were in progress the culture was kept on the laboratory

table where it received a mild light from the north window.

Temperatures were now stabilized in the laboratory; they varied from 16° in the morning to 23° in the afternoon. During the night when the heat was shut off the room tempera ture dropped to around 12°.

The hydras turned sexual toward the end of the month —

a period of two months and a half after collection. H. carnea males and females were first seen on December 18. The first sexual C. viridissima was found on December 30. This speci men bore two testes in a position which further study proved to be characteristic: they were situated extremely distal on the column, close to the base of tentacles, and oppositely located. Motile sperm were seen erupting from the nipple of one testis. On January 2, li|. males were seen on the glass; and two days later males, females, and individuals bearing both testes and eggs were observed in a sampling of the jar. At this time the copepods in the culture had become fairly abundant; but few hydras were budding.

That the C. viridissima in the Pelee Island culture did not represent monoecious and dioecious clones, but was a highly protandrous hermaphroditic form was shown by the his tories of 71 individuals. These specimens were selected at random from among individuals which had formed testes in the culture during January and February. Each was isolated in a watch glass and covered with strained Lake Erie water. About a dozen Cyclops cultured from the pelee Island source were pipetted into the containers every third day when the specimens were examined under the microscope for signs of ovaries. Only rarely did any of the animals ingest more than one of the copepods, which they could easily paralyze.

After the feeding operation the water was changed to prevent fouling. The lake water used had been strained and bottled and was aerated. The cultures were covered with glass and - 266 - kept on the table with the mass culture. Twelve of the individuals became depressed and were discarded while still in the "male" phase. The remaining 5>9 continued to remain healthy, their color and posture resembling that of the animals in the mass culture. All of these developed into hermaphrodites, in 35 the testes and eggs were ripe synchro nously. In 2ij. the testes were being resorbed, the sperm becoming non-functional before the eggs were extruded.

These results indicate that the viridissima originating from Pelee Island were completely protandrous — a good percentage, in fact, extremely protandrous.

In those specimens where sperms and eggs were ripe simultaneously, the ovaries began to appear the first day or second day after the specimen was isolated as whitish scal loped swellings of epidermis opposite each other at the middle of the column. The eggs, never more than two in num ber, rounded up and were extruded four days later. While the eggs were retained in the epidermal cup, motile sperm could be seen in at least one of two testes of these individuals.

Several instances of emission of the spermatozoa were ob served, and in seven cases individuals produced thecated embryos. The embryo-thecae were characteristic of C. viri dissima, the only species in which the theca is composed of small polygonal plates. Under the conditions maintained, it is evident that the eggs were fertilized by sperm from the same individual. This verifies Tannreuther * s earlier - 267 - conclusion (1909a) that self-fertilization can occur in the green hydras. In the more highly protandrous group, ovaries did not start developing for three or four days; the egg took about the same length to mature, but before it was ripe the testes were being resorbed and the spermatozoa were non functional.

Two samples of 100 hydras, each selected at random from the culture, reflected the protandrous condition of the popu lation. The first sample taken eight days after testes bear ing individuals appeared in culture contained: 30 asexual individuals, about half, with one or two buds; I4I4. bearing testes (10 with only one testis formed, the rest with two testes, excepting one which had three); 10 with one or two extruded eggs, some thecated; 16 with well developed testes and eggs in varying phases of maturity. Three of the "males" had a bud, but none of the other sexual specimens were bud ding. Lack of budding while in the sexual state was also ob served in the animals observed in the watch-glass cultures.

The second sample taken two months later was composed of 10 asexual individuals, l£ "males," 35 "females," and lj.0 hermaphrodites. Several thecated embryos were found in the debris. No copepods were seen in the culture, and a bac terial scum had formed on the surface of the water. Yet the J hydras appeared quite healthy and were fully extended on the glass close to the surface.

- 268 - Extreme protandry observed In populations of some her maphroditic forms may be an indication that the species is

tending toward dioeciousness. Thus, the specimens of

H. ca m e a studied from the culture were seen mostly In the

"male" and ’'female” phases, and no true hermaphrodites were

seen in the culture. Several individuals reared in watch glasses formed only abortive testes and later became mature

females. H. carnea remained in the sexual phases In the cul

ture for about a month and a half. After the middle of Feb

ruary, none could be found. This was probably due to the

sparse food supply or other conditions in the culture. On

the other hand the green hydra, as Is well-known, can sur vive In cultures that have become quite foul.

Shortly after the last sample of the green hydras was

taken, numbers of the protozoan parasite Hy dr amoeba hydroxena were observed infesting sexual specimens squashed for nematocyst examination. No hydras could be found in the culture

after the middle of March— due probably to the pathogenecity of the parasite.

A clone subcultured from one of the £. viridissima

hermaphrodites was maintained In a balanced aquarium kept in

the laboratory from February to May 1951+. The animals were fed only once a week from the copepod culture. None of the members of the clone were observed to turn sexual again.

Certain findings in the work with the Pelee Island

£L* vlrldissima clearly conflict with some of Whitney's

- 269 - findings from his experiments with the New York green hydras. These are summarized with reference to Whitney's results as follows:

(1) Temperature. The hydras formed gonads at room tem peratures without being subjected previously to near freezing or continuous low temperatures for several weeks. In the swamp the water had just attained the 1J?° temperature.at the time the stock for the culture was collected; temperatures during September of 19^3 were abnormally high (Fig. $). It o is doubtful that holding the culture at 10 for a week after it was started had any effect on gonad formation. Only a few hydras were present. The culture did not flourish until it was removed from the refrigerator, and the numerous hydras which became sexual did not develop testes until exposed to room temperatures for about a month. In fact, unlike Whit ney's animals, our animals reacted more according to the general observation about the green hydras summarized by

Morgan (1907, p. 311): "In the spring, sperm and eggs are produced. If the hydras are brought into a warm place in the autumn or in the winter, they will produce sperm and eggs in a few weeks.”

(2) Gonad Sequence. The lag between the appearance of testes and eggs emphasized by Whitney has no relation to nutrition. It is explainable by the protandrous nature of the species. The cases reviewed above demonstrate that some populations may be highly protandrous.

- 2 7 0 - (3) Size and Sex. Regardless of differences in size, the individuals which become sexual produced both testes and eggs. Whitney gives no measurements of his specimens, simply stating (p. 536): "Throughout all the experiments it was noticeable that only the larger individuals produced eggs as well as testes while the smaller produced only tes tes. No individuals were found which produced only eggs." The animals whose histories were followed in watch glasses ranged in column length when fully extended from 1.5 mm. to

2.8 mm. The mean size was about 2.5 mm. the tentacles being about half the length of the column. They were comparatively thick, having a column diameter of about 1.5 mm. None of the specimens measured during the history of the culture ex ceeded 3 mm. in column length. These were very small hydras. Ewer (191+8) gives column length up to 30 mm. from species records. That the small size was not due to inanition is indicated by Lashley*s careful statistical analysis of size relations of two clones kept under similar conditions and fed abundantly with copepods. These green hydras were also small, the mean length of column in the two clones being about 2 mm. in the one, and only 1 mm. in the other (Lashley,

19l5» pp. 180-188). it is interesting to note that Lashley failed to obtain sexual reproduction under varying environ mental conditions during the long course of his experiments.

(lj.) Nutrition. The conditions of Whitney*s experiments with respect to food deprivation are quite clearly stated.

- 271 - He fails to establish, however, that his experimental animals were actually ingesting prey when abundantly fed in aquaria.

Some of the animals mentioned as the principal source of food in the experiments are ciliates, rhabdocoeles, ostracods, and rotifers. These are generally not eaten by any hydras. Such forms appeared in the Pelee island culture, but the hydras rejected them as prey. The food animals gen erally used in culturing the green hydra are copepods. And at times the hydras may not capture and eat even the small species of Cyclops which are considered especially good.

No mention of copepods as a food source in either of the collection sites or in the culture experiments is made in Whitney's paper. In general, the evidence is weak for the conclusion stated (p. 536): "An abundance of food following the period of low temperature suppresses the formation of testes and eggs." The pelee Island hydra Isolated in watch glasses were fed with Cyclops, and the majority became her maphrodites, Whitney's conclusion that food is an environmental fac tor In causing gonads to form is difficult to harmonize with what we know about the sexual periodicity of the species In nature. C. viridlssima, as Whitney states, usually has its sexual period in late spring. Boecker (1918) observed that sexual maturity of green hydra populations in Geiroany also occur In the spring months. In May and June the supply of the copepod food animals is generally abundant in the pools,

- 272 - ditches, and ponds where the species is most commonly found. Apparently in nature a period of low temperature is an im portant prior condition for gonad formation. As in some other species, the sexual period occurs with rising tempera tures. It seems questionable, however, in view of what we know of the ecology of the other hydra species, that the nutritional state of the animal has a bearing on gonad forma tion in £.• viridisBima. In general the nutritional mutualism existing between the zoochlorellae and the polyp makes the green hydra a poor subject to choose for experimentation In the food-temperature problem. Sunlight obviously plays a role in the symbiotic relationship. It is interesting to note here that Rowan

(1930) suggests that seasonal light intensity rather than temperature may be the controlling factor in the sexual periodicity of the hydras. Gonadotrophic hormones regulating reproduction are not present in the Invertebrates, however, according to Hanstrftm’s review of the subject (1939).

Contemporary general embryologists apparently consider the experimental attack on the problem of gonad induction a futile endeavor. The assertion originating from the Odessa

State University laboratory (Wlshnyewsik, 1937) that pro longed starvation always induces the formation of gonads in hydra can hardly be given serious consideration since neither experimental conditions nor species used are designated.

The report of the most recent work on sex and nutrition in the hydras, done by ito (1951), is unfortunately published

- 273 - in Japanese. The abstract in English by Y. Watanabe states that I to, from results of experiments with two new Japanese species of Hydra, found little possibility of gonads arising with changes in quantity and quality of food, dissolved oxy gen or organic substances in the media. He reports that gonads are Induced in both species by raising the temperature from 8 to 21°. They are not induced by lowering the tempera ture, or by cultivating the animals continuously at a lower temperature under different food conditions in the media.

SURVIVAL, DISPERSAL, AND HATCHING OP THE EGGS The German authors claim that the female oligactis exhibits a characteristic behavior in depositing Its eggs.

"Eiablage," as they term the act, is accomplished by the animal contracting to the extent that the most distally located eggs are brought into contact with the substratum to which they adhere by sticky secretions. The female may re main in the same spot, contracted into a flask-like shape, for a couple of weeks. Schulze (1917) portrays Eiablage by H. oligactis (Pig. 70) after drawings made by Laurent (l81jlj.) and summarizes Brauer's confirming observations (1908).

(See Schulze’s bibliography for authors cited.)

Boecker (1918, pp. lj.93-il.9ll-) gives a detailed account of Eiablage observed in specimens attached to Potamogeton leaves from the O m e and the Altwassers des Mad. He confirms the earlier observations, but states that in all cases

- 27k - females found In "Wochenbett" showed a dark-colored thicken ing of the ectoderm so that the transformation into the flask-like form could not he considered a simple contraction of the mother animal, as assumed by Laurent and Brauer. I quite agree with Boecker on this point. In fact, it may be that what Boecker and the other German workers interpreted as egg-depositing behavior represented onset and recovery from the state of depression in the few animals they observed.

As Boecker himself points out (p. ij.82), freshly collec ted hydras frequently show symptoms of depression, and it is my experience that egg-bearing oligactis are especially sus ceptible to depression even under the best culture conditions, in three cases of so-called egg deposition I happened to ob serve in Lake Erie specimens, the animals were undergoing depression. They did not feed, their long tentacles had become greatly shortened— just as described by Boecker and shown in Laurent»s drawings; the epithelial tissues were brown and bloated, and the whole body remained contracted in a squat, stiff flask-shape. After three days two of the animals recovered, ingested crushed daphnids, and became active. The third animal disintegrated. In each case, embryos were found in the watch glass fastened on the frag ment of dead tree leaf to which the parent had been attached. As Boecker noted, the newly formed egg shells are sticky.

Thus, the eggs may adhere to a suitable substratum when brought into contact with it by the fairly frequent contrao-

- 275 - tlons and bendings of the normal animal. Such proved to be

the case, at least, during observations made upon three

active females, each bearing four fertilized eggs. The

specimens, attached to a clean piece of dead tree leaf upon

which they had been collected, were isolated in watch glasses.

During the hourly observations, at least one egg in each case was seen to adhere to the leaf surface upon random con

tact with it. After twelve hours, two or three eggs were

found fastened to the leaf epidermis in each watch glass. The parent hydras did not exhibit the prolonged contractions

of the body or shortening of the tentacles, which are symp

tomatic of depression. They were able to ingest daphnids, and were subsequently clone-cultured until some of their off

spring again formed ovaries when the cultures were placed in

the refrigerator. The newly-formed egg shell of H. pseudoligactis is

apparently more adherent than that of H. oligactis. Extruded

eggs can be removed from their epidermal cups by touching the sticky theca with a suitable surface, such as a clean piece

of dead tree leaf. Bazuin has found many clusters of_eggs

attached to algae and other substrata in a lake in the vicin ity of Grand Rapids, Michigan (Hyman, 1931b, p. 303}. The

adherence of Pelmatohydra eggs to leaves of water plants and

to slides suspended in racks in Lake Washington, Minn, and Douglas Lake, Mich, is reported by Miller (1936, pp. lij.8-152;

species not determined), in Lake Erie, the eggs of pseud-

- 276 - oligactis, like those of oligactis, were found fastened to the epidermis of dead tree leaves, in rare instances on the underside of a Potamogeton leaf. Apparently, the eggs become fastened to substrata while the theca is still sticky, as suggested by Hyman (1931 b). In one case, five eggs from a

Lake Erie pseudoligaotis were seen to adhere to the leaf sur face when brought into contact with it by random movements of the female’s body.

When hatching of the embryos of H. oligactis and H. pseudoligaotis takes place in Lake Erie was not definitely ascertained. Field observations made during the early spring of 19£1|. indicate the embryos probably do not hatch until the water temperature rises to about 10°. A series of collec tions between April 6 and April 17 (temperatures 5 to 7°), yielded numerous stalked orange-colored hydras bearing many buds (see Table VIII). Examination of samples of f>0 easily obtained from a quart jar of Myriophyllum disclosed no evi dence of gonads. Eggs were not found in the washings from the milfoil leaves. Dead leaves dredged from the bottom in daily collections or removed from the twine of a hoop net set near station 5 , also yielded hydras in the same condition. Attached to some of the dead leaves were found the minute eggs described above. Their whitish color made them quite easy to see against the dark surface of the dead leaf. The eggs were attached separately but tended to cluster along the veins on the underside of the leaf. This is the - 277 - place where egg-bearing females of H. oligactis and H. pseudoligactis were first seen in early December. No embryos were found on leaves at that time. In the collection made through the ice-cover at the beginning of February no females were found, but some embryos were observed attached to leaves dredged from the bottom (temperature 0.2°). Possibly the majority of the eggs found on the leaves during April were produced during the period between the beginning of December (temperature 5°) and the beginning of January when the ice- cover formed.

Eggs were found on every species of tree leaf appearing in the collections r Q,uercus alba, Q,uereus Muehlenbergl 1 , Ulmus americana, plat anus occidentalis. The majority appeared on the leaves of the white oak or the Bycamore which made up the bulk of the five-quart samples of leaves. The number of eggs were obtained in the collection of April 6 , when 60 were found. It should be noted that the leaves were kept in changes of lake water during examination to prevent damage to the eggs, and that no hatching hydras or empty egg shells were observed by inspections made with a 10 x hand lens. The eggs were firmly attached to the leaf epidermis by the periphery of the theca, but could be easily isolated without injury by removing them with a bit of the epidermis grasped in jeweler's tweezers.

To find out something about the conditions under which embryos might hatch, the following experiment was performed.

- 278 - The sixty eggs obtained in the collection of April 6 were removed from the leaves in the manner just described. They were rinsed in fresh lake water at 5°, and segregated in groups of twenty into three petri dishes containing filtered lake water. One dish was placed in the constant temperature; cabinet at £>°, the other on the table at room temperatures which fluctuated between 19 and 22°; the third was placed in a deep-freeze where the eggs were quickly frozen in the water. After two hours the eggs were removed from the deep freeze and immediately transferred to the constant tempera ture cabinet where the ice encasing them gradually thawed. After six days, seven of the twenty embryos left at room temperature had hatched. During the period temperatures ranged from 18 to 2lj.°; water was changed every other day to prevent fouling. Squashes of the young hydras showed that four were H. oligactis and three were H. pseudoligaotis.

None of the embryos held at $° had hatched. The temperature in the cabinet was gradually Increased to 10° and the water in the containers was changed. Six days later, on April 18 when the experiment had to be terminated, the results were as follows: none of the twenty embryos subjected to sharp freezing had hatched; three of the twenty embryos originally held at constant temperature of £>° had hatched (one was P. oligactis, the other two H. oligactis); the remaining fourteen eggs left at room temperature had fungused badly through failure to change water, and no additional embryos

- 279 - had hatched.

Results of field observations made the following month between May 7 and May 18 lend support to the hypothesis that the hatching temperature for embryos of the two species lies around 10°. The lake had warmed to 10° by the first of May and water temperatures during the period of collections were around 12:°. The same sampling methods used in April were followed. Very few eggs could be found, however. In several

samples no eggs were found, on some of the leaves empty

thecae were observed. Many small hydras were on the leaves; but which were newly detached buds or newly hatched hydras, I could not determine. Whereas a five-quart sample of dead leaves yielded sixty embryos at the beginning of April, only ten could be found in such a sample examined on May 7* On

May 18, twelve embryos were found after examination of $00 leaves.

The ten embryos taken in the first collection were placed and held at 12°. Ten days later six had hatched; two were H. pseudoligaotis, four H. oligactis. The twelve eggs taken in the last collection were allowed to dry on the bit of dead leaf epidermal tissue, and transported to Detroit at air temper attire s. They were covered with filtered Lake Erie water five days later (May 23} and left at room temperatures.

The water was changed daily. During a period of twenty days no embryos hatched, so observations were not continued.

- 280 - Whether embryos of the H, oligaotis and H. pseudoli gaotis can survive desiccation and freezing remains to be conclusively demonstrated. Miller (1936, p. l£2) in his study of Pelmatohydra (H. oligactis and H. pseudoligaotis) merely mentions that certain preliminary experiments indi cate that the eggs cannot withstand either severe freezing or drying. Apparently he did not publish the results of his experiments.

The length of the dormant period of the embryos and the stimulus for hatching also require further investigation.

Miller succeeded in getting good hatches of hydras from eggs of the species found on substrata beneath the ice-cover in from six to eight days at room temperatures: 8 from 11 eggs (Lake Washington, Minn., p. llj.8); ij. from 5 eggs (Douglas

Lake, Mich., p. 1£>0). it will be noted that the hatch was greater in about the same period than that obtained from the

Lake Brie eggs; also that the embryos tolerated an even more drastic step-up in temperature.

The hypothesis advanced by Miller (p. 152) that the hydras eggs hatch before the ice-cover leaves the lakes does not appear tenable in view of the Lake Brie findings.

Certainly Miller's results on hatching at room temperatures do not support his hypothesis; and he performed no laboratory experiments to demonstrate that embryos of H. oligactis or

H. pseudoligaotis would hatch at from 0.9 to 1.7°, the tem peratures of the water under the ice-cover. Miller's

- 281 - tentative conclusion was based primarily on the observation that egg clusters did not appear on his slides until the ice- cover was forming and that they were absent after the ice- cover left; however, the few eggs that were attached to the slides (pp. ll|.9, 1£>1) may have washed off when the ice went out. It is my experience that the thecae do not adhere closely to a glass substratum as they do to the clean sur face of a dead tree leaf. Miller mentions examining materi als dredged from the bottom for hydra eggs (p. 129), but reports no findings.

Like Miller, I cannot give a valid account of the hatch ing of the embryos from the eggs. The sequence of events was not followed closely enough in the work with the oligac tis and pseudoligactis eggs. By chance the emergence of an embryo from an americana egg and from a 11 tt or alls egg was observed. The few eggs available from these species made experimentation unfeasible. I did not observe the hatching of embryos from the eggs of H. carnea or C. viridissima; eggs obtained from room-temperature cultures of these species were used to test the effects of freezing and desiccation on the embryos; but the results were negative.

- Accounts of theca formation and hatching available in the literature are somewhat confusing. Statements of experi mental conditions are vague and in some cases the accuracy of the species Identification Is questionable.

- 282 - The first study of the development of H. oligactis in this country was done by Tannreuther (1908). From the in ternal evidence it is quite certain he was working with the same species used by Downing in his spermatogenesis study of

190£, and erroneously named by him H. dioecia at that time

(Hyman, 1930, p. 329). Tannreuther established that the egg disintegrates within 214. to 30 hours after extrusion if not fertilized; -under laboratory conditions, fertilization occurs within two or three hours after the egg breaks through the epidermis. He states (p. 270): "After the membranes are formed the eggs are glued to the object on which the parent rests. This, however, is not always the case. In a few instances observed, the embryos hatched out while the eggs were yet attached to the parent."

Schulze overlooked Tannreuther ’ s work in his review of the literature on hatching. According to early observations of Haacke (1880) tentacles in the embryo, unlike those in the bud, arise simultaneously. Frischholz (1909) says that the embryo already has a stalk. Koelitz (1910) states that on leaving the egg, the embryo possesses no tentacles; these arise one or two days later, usually four simultaneously, the rest singly. (See Schulze, 1917, pp. 106-107, and his bibliography for authors cited.)

Schulze mentions he did not observe hatching of H. oli gactis embryos himself. I saw only one oligactis and one pseudoligaotis embryo hatch; these had no tentacles or stalk, - 283 - but the tentacles formed as described by Koelitz. Nemato- cysts of the recently hatched hydras were characteristic of

those of the adults.

In 1938* McConnell published a completely illustrated

paper entitled "The hatching of Pelmatohydra oligactis eggs."

This account appears to be the sole source for statements in

current treatises (e.g. Hyman, 194°» P* kk°)t pertaining to the length of the embryo*s dormant period, its capacity to

survive drying, and the hatching process. It seems worth

while, accordingly, to review McConnell’s work quite complete

ly. His findings are based upon the histories of 30 embryos

which were closely watched from the time of theca formation

and dropping off from isolated aquarium specimens up through emergence from the egg and the outgrowth of the tentacles. The time required for hatching ranged from 35 to 70 days, the average being 57 days. The embryos were not reared or inspected under constant temperature conditions, as implied

by the author in his introduction. (See table, pp. 172-173;

temperatures of laboratory recorded at time of theca foraia- tion ranged from 1 If to 22°, at hatching from 5 to 23°.) At

the time of hatching, the chitin-like substance of the theca is softened by some secretion from the embryo which McConnell believes is an enzyme. The shell splits open and the embryo,

swelling due to imbibition of water, emerges from the egg. At hatching, a gastrovascular cavity and rudiments of some

tentacles have formed; cnidoblasts are already differentiated

- 28I4. - in the region of the hypostome. No mention is made of the work done by Haacke, Frischholz, and Koelitz on H. oligactis development; nor are pertinent papers by Hyman or Schulze cited.

The validity of McConnell's findings cannot be ques tioned. It becomes apparent, however, even upon casual examination of his paper, that he was not dealing exclusiveily

— if at all— with H. oligactis or with H. pseudoligaotis.

One first gains this impression from the very clear photo micrographic illustration captioned ’’Fig. ij.. 970 x. Highly magnified section of newly formed theca.— Note the heavy spines." The thick theca and its spination pattern show a striking resemblance to these features as seen in the egg of

H. americana (compare with Figs. 11 and 12, pi. 30, Hyman,

1929). The other photographs exhibit thecae of thicknesses which greatly exceed those of the extremely thin thecae characterizing oligaotis or pseudoligaotis eggs. Spines as well as thick thecae are mentioned throughout the discussion. Moreover, the theca is not described as sticky at formation; the embryos dropped off the parent and did not adhere to the watch glasses in which they were reared. This is certainly not characteristic of oligactis or pseudoligaotis eggs.

What the author says about the capacity of the embryo to survive drying is quoted in context:

When newly formed the theca material is rather elastic. This elasticity is lost in a very short time and this material becomes very brittle and firm.

- 285 - These thecated embryos can in this stage withstand desiccation for long periods of time. Several from the control materials were allowed to dry for from 8 to 20 days and yet when placed in water hatched and produced normal embryos. It may be that embryos protected by a heavy shell, such as those reared by McConnell, can withstand drying. For example, Griffin and Peters (1939) in their description of H. oregona, mention that the pond near Portland where the new species was discovered dries during late summer; the heavily thecated eggs produced in June are the means by which the population survives in the type locality.

Chlorohydra viridissima populations probably maintain themselves in pools which freeze solid during winter or dry during periods of drouth through the capacity of their eggs to survive. Boecker (1918) studied sexual reproduction of green hydras whose embryos apparently survived these condi tions during three seasons in swampy pools and ditches near

Berlin. The swamp on Pelee Island where the asexual animals for my C. viridissima and H. came a cultures were collected freezes solid in winter and may dry up during seasons of drouth. The possibility that adults might be transported into the habitat on duckweed clinging to the feet of water- birds cannot be ruled out. It seems more likely, however, that the eggs, which are of the non-adhesive type, drop into muck. Like the parents, the embryos tolerate the high tem peratures and foul waters of the swamp, and in addition may

- 286 - be resistant to freezing and drying.

My failure to hatch, the eggs of C. viridissima and

H. carnea, which dropped from the sexual animals under ob servation, can probably be attributed to faulty procedure.

At first, attempts were made to hatch the eggs in individual watch glasses at room temperatures, using water from the stock cultures. The eggs — about ten from each species — fungused although the water was changed daily. Next two series of twelve were cultured in water transported from Lake Erie, strained and stored in the refrigerator. Clean watch glasses and fresh water were used daily,' but none of the eggs hatched during a three-week period. Consequently, half of the eggs — six for each species — were frozen in their culture water in the ice-compartment of the refrig erator for about 2lj. hours. When the ice in which they were frozen had thawed, the culture dishes were removed to the laboratory table. None of the eggs hatched. The remaining eggs were dried by allowing the water to evaporate from the watch glasses. After ten days, the eggs were covered with water. The H. carnea eggs had shrivelled much more than the green hydra eggs; but rounding up of the theca to the spheri cal shape normal for these species was observed under the microscope when the eggs were covered with water. None of embryos, however, hatched from these eggs. Capacity of hydra embryos to withstand temperatures close to the freezing point for prolonged periods appears

- 287 - to be the sole mode of continuing populations of some species

in the northern latitudes of our continent. Rowan (1930)

reports that H. canadensis is killed by serious frosts in

early fall; in the lakes of the type locality near Edmonton,

Alberta, none of the animals could be found after October.

as H. carnea), found in the same habitat, does not survive the cold as long as the giant new species.

The experience with H. llttoralis and H. americana in the Lake Erie study area suggests that few of these animals

survive the winter, and that the embryos which hatch may be

the source of entirely new clones appearing when the waters warm and asexual reproduction begins. Should investigation disclose that winter-kill prevails in populations of the upper lakes, then the role of sexual reproduction in main taining these species in the Great Lakes can be considered

quite important.

Prom the foregoing observations and reviews, it is ob vious that much work in field and laboratory needs to be done on the whole subject under discussion. We do not yet possess a complete account of the hatching of the eggs of

'P * the much-invest!gated H. oligactis. As to the duration of the dormant period of the embryo of this species and of

H. pseudoligaotis, field observations in Lake Erie and in Douglas Lake indicate that the length is from four to five months. During this period the embryos are subjected to - 288 - near freezing temperatures — in Lake Erie under the ice- cover to 0.2.°. The possible duration of dormancy in the other species cannot even be estimated. What the hatching stimulus is remains to be determined.

Field observations indicate that heavily-thecated embryos may be able to withstand prolonged drying or freezing; but adequate demonstrations of this capacity through proper experimentation have not been made. As to dispersal, there is no question that eggs with a sticky theca, such as those of H. oligactis and H. pseudoligaotis, are transported by the substrata to which they adhere. In the present study, it has been shown the main vehicles of dispersal are dead tree leaves.

SPECIATION PROBLEMS In s\jmming up the current outlook toward furthering our knowledge of speciation, Ernst Mayr calls attention to the need for studies of the process in certain groups. What he says (191+9, p. 296-297) is of special interest to students of the Hydridae:

The process of speciation in ordinary terrestial bisexual animals is fairly well established. There are, however, large groups of animals of whose specia tion nothing is known. This is particularly true of all groups with aberrant reproductive mechanisms. A study of the speciation pattern among animals with temporary or permanent parthenogenesis, self-fertilizing hermaphroditism, and various forms of temporary or permanent asexual reproduction is badly needed. Such studies should first determine whether or not there are

- 2 8 9 - well-defined populations within species in these groups, and whether or not some of these populations have the earmarks of incipient species. The sub species concept has never been consistently employed in any of these groups.

Mayr's statement is certainly applicable to the hydras.

Populations of the species are characterized by long periods of asexual reproduction. In some populations, as emphasized in the foregoing review, the species does not enter the sexual phase during the seasonal cycle at all. In other populations only males appear over a period of several sea sons. Thus asexual reproduction may be of long duration.

Self-fertilization can occur in some of the monoecious spe cies. In nature, of course, none of these hydras are obli gatory hermaphrodites. In populations of certain highly protandrous species, however, considerable reproductive aberrancy may exist.

That no subspecies of Hydra have been designated by contemporary specialists in the group does not reflect lack of recognition of the importance of the subspecies concept. There is no question that the theoretical criterion of reproductive isolation, which has emerged primarily from the work of Huxley, Dobzhansky, and Mayr in the "new systematics,' forms the basis of a sound verbalization of the species concept: "Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups." (Mayr, 19^2., p. 120.)

- 290 - Interbreeding as the ultimate test of conspecificity

Is recognized by Ewer in the review of the Hydridae (191+8).

However, because of the incompleteness of descriptions of many species and our lack of knowledge of their geographical ranges, specialists working with the hydras believe, like

Ewer, that it Is best to postpone subspecific classifica tion. Accordingly, any form which can be clearly identified and distinguished from all other species on the basis of morphological criteria is regarded as a species. Even with the present array of monotypic species, the ecologist is confronted with taxonomic difficulties when he encounters a population of mixed species (see Part I).

Eventually, as In the history of other groups, we may learn enough about the biology of hydras to establish poly typic species and make some attack on problems of speciation In the group. First, we must find out much more about the geographical ranges and the ecological structure of the known species. Hiatuses of information about world distribution are common for every species. The single records for some species given by Ewer In the 191+8 review in no way Indicates that these species are geographically isolated, although It is theoretically possible that allopatric speciation may have occurred in certain of these species.

When the ranges of the commonest hydra species are worked out, specialists in the group should have no great difficulty in recognizing an assemblage of populations which

- 291 - differ taxonomically enough to be considered subspecies.

Application of sound principles and methods of systematica in other groups has established that populations of the most distinct subspecies usually are found along the periphery of the species range. (Mayr, Linsley, and usinger, 1953.) Polytypic species are now regarded as aggregations of poten tially incipient species, and ecological differences in populations found at the periphery of a species may throw light on speciation process.

Because of our present state of ignorance regarding the ecological structure of hydra populations, we can only specu late about the nature of the speciation in the Hydridae.

Hydras are among the most sedentary of animals living in the fresh waters. They are also among the most ancient. Unlike marine hydroids or the fresh-water jellyfish, their life cycle lacks a motile sexual generation. Studies of other sedentary species have indicated that they exhibit a greater tendency than the mobile species to speciate into ecological or geographical races. One would expect to find accordingly a greater diversity of species in the hydra group. Has speciation ceased long ago? Or is it possible that because of the high asexual reproductive potential of a single indi vidual, mutant clones can build up large populations which survive as incipient new species? How great a factor is the phoretic relationship with molluscs or arthropods and the anabiotic capacities of the hydras in dispersal of incipient species? - 292 - How does one account for the cosmopolitanism of two

species, H. oligactis and C. viridissima? Apparently geo

graphical barriers do not exist for the common "brown hydra"

and the "green hydra." Their eggs probably withstand desic

cation and, like those of other cosmopolitan microorganisms,

are dispersed by windB and birds, is it possible that

Incipient species in populations are continuously swamped

by the air-borne immigrants?

The green hydra represents a special case peculiar to the Hydridae. No one, according to Ewer's review (19^8), has

succeeded in inoculating other hydra species with zoochlorel-

lae and effecting a true symbiotic relationship. Chlorohydra viridissima stands as a monospecific genus, pending further

investigation. It is interesting to note here that Lashley

(1915j 1916) established by clone culturing and careful sta tistical analysis that wild populations of C. viridissima

consist of distinct strains which differ in initial number

of tentacles, size of body, color, and age at which asexual reproduction begins. Such temporary clones are now recog nized as "races" or "microspecies." In natural populations,

such lines lose their genetic identity by absorption in the joint gene pool of the population during the sexual period of the parent species. The "species problem" In the green hydras

Is being studied currently in this country by Hadley and Forrest (personal communication).

- 293 - Our presentation of the findings on sexual reproduction as observed in the hydras of a population in Lake Erie indi cate that the local species are sympatric. The species in the habitat reach sexual maturity quite synchronic ally.

Even in the absence of evidence from cross-fertilization experiments, there is little doubt that mechanisms of re productive isolation are operative. Incipient cases of syrapatric speciation are unknown excepting polyploidy and other aberrant phenomena. (Mayr, 191+9, p. 29S>.) It might be worth while to compare the ecological structure of these species in the population as it exists in the upper Great Lakes — especially Lake Superior. Should temperature prove to be an ecological barrier segregating populations spatial ly, then it would be worth the trouble to carry out experi ments to test Interbreeding potentialities of the two popu lations. The few breeding experiments which have been performed with hydra species (Schulze, 1917; Ewer, 191+8) were abortive. To have biological significance such experi ments should be made with a series of specimens sampled from populations where conspecificity may be in question.

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Allee, W. C., Orlando Park, Alfred E. Emerson, Thomas park, and Karl P. Schmidt. 191+9. Principles of animal ecology. W. B. Saunders Company, Philadelphia, xii, 837 PP*» 263 figs.

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1951+* Pseudo tides in Lake Erie. Inland Seas 10 (i+): 291+ (with two photographs, p. 2 6 5 ). • 1955a. Stratification in western Lake Erie in summer of 1953: effects on the Hexagenia (Ephemeroptera) popula tion. Ecology 36 (2): 239-21+1+.

• 1955'b. New methods of collecting bottom fauna from shoals or rubble bottoms of lakes and streams. Ecology 36 (3): 52U-525. Bryden, Robert R. 1950. Survey of the hydras of central Tennessee, a preliminary report. Jour. Tenn. Acad. Sci. 25 (1): 82-83. - 1952. Ecology of Pelmatohydra oligactis in Kirk patricks Lake, Tennessee"! Ecol. Monog. 22 (1): 1+5-68. - 296 - Caullery, Maurice. 1952. (Translated from the French by Averil M. Laysaght.) Parasitism and symbiosis. Sidgwick and Jackson Limited, London, xii, 31+0 pp., 80 figs.

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19l+2a. II. Light penetration and its relation to turbidity. Ecology 23 (1): 1+1-52. 19l+2b. III. Phytoplankton and physical-chemical data from November, 1939, to November, 191+0. Ohio Jour. Sci. 1+2 (1): 21+-1+1+.

19i+i+. IV. Relation of limnological and climatic factors to the phytoplankton of 191+1. Trans. Araer. Micros. Soc. 63 (3): 203-236.

Chandler, David C. and Owen B. Weeks. 191+5. V. Relation of limnological and meteorological conditions to the produc tion of phytoplankton in 191+2. Ecol. Monog. 15: 1+35-1+57. Chang, Joseph T., H. H. Hsieh, and D. D. Liu. 1952. Obser vations on hydra, with special reference to abnormal forms and bud formation. Physiol. Zool. 25 (1): 1-10.

Child, Charles Manning. 1915* Senescence and rejuvenes cence. The University of Chicago, Chicago, xi, !+8 l pp., 2 0 1 figs.

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Clark, H. Walton and Charles B. Wilson. 1912. The mussel fauna of the Maumee River. U.S. Bureau of Fisheries Doc. No. 757, 72 pp.

Clemens, Wilbert A. 1922. Hydra In Lake Erie. Science 55 (11+26) s i+1+5 -1+1+6 .

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- 307 - APPENDIX

(TABLES I-IX)

- 308 - Table I. Mean sizes in microns of the species of from Lalce Erie Based upon general averages of means derived from measurements of major axes of undischarged nemato cysts sampled in thirty individuals of each species. Ranges of major and minor axis dimensions of indivi dual nematocysts are given below mean values.

Species Stenotele Holotrichous Atrichous Desmoneme isorhiza isorhiza oligactis 10.2 10.5 7.6 5.5 (9x7-13x11) (9x1*-12x5) (6xl*-9xU) (I*x3-7x5) pseudoligactis 12.7 11.6 8.7 7.2 (10x8-15x13) (10xU-13x6) (Sxl*-10x6). (6xl*-8x6) americana 17.1 9.3 7 2 8.0 (13x11-22x16) (8x6-11x7) (6xU-9x5) (7x5-9x7) littoralis 11.9 10.1 7.9 5.9 (9x7-llixl0) (9xU-12x6) (6x3-9x5) (5x U-7x 5) carnea 13.7 10.1 8.5 6.8 (9x8-18x15) (9x5-12x5) (7x1*-9x5) (5xli-8x6)

- 309 - table XI. Nematocyst measurements In microns from L. H. Hyman's descriptions of species of Hydra

Tabulated from papers cited in text for comparison with measurements of Lake Erie specimens (Table I). Mean values are designated by an asterisk. The other values given by Hyman are ranges of major axis measure ments of the individuals sampled, number not stated.

Species Stenotele Holotrichous Atrichous Desmoneme Isorhiza isorhiza oligactis 10-13 10 - 12 8 - 9 5 - 6 pseudoligactis 10 - 17 10 - 12 8 - 10 6.U - 8

americana 13 - 21 7.5 - 10 6.5 - 7.3 8.3 - 9.5 9 littoralis 10-17 10 *8 5 - 6

carnea 9 - 19 9.U - 10.8 *9 6 - 8 *10

- 310 - Table III. Mean numbers of hydras per square meter of rubble bottom* Gibraltar Island shore* Fishery Bay* 1951-1952, as deter mined by Britt1s concrete block-chemical bath sampling method*

Station - Depth in meters Temp. Total Date A - 1m. B - 2m. C - 3m. D - 5m. Number ' o_ ; C of Total Mean Total Mean Total Mean Total Mean •hydras

1951 10/15 lh.0 15 50 37 92 56 lbo U73 2365 581 12/5 U.6 1 3 36 90 31 78 11 28 . 79 1952 3/27 2.0 — 0 0 1 5 7 8

5/3 10.0 0 0 0 0 7 36 28 70 1 35

6/29 22.0 11 27 187 623 25 83 3U9 111*5 572 9/10 21.6 j 1 3 5 12 52 173 15 50 73

* For complete description of this new method* see paper by N. Wilson Britt (1955b). Counts of hydras (species predominantly H. littoralls) were made from living collections removed by a weak alcohol - HCl bath from the series of concrete blocks lifted and reset on the dates listed. Means were based on collections from four blocks (0.1 m. square* 8 cm. thick) except in the following casest 10/l5j A-3, D-2; 3/27: A -- blocks gone (reset) and C-2j 6/29: B,C*D - 3j 9/10 C - 3j D - 2. Britt's concrete-block stations can be located with reference to the map (Fig.2): A and B were in shallow water in the vicinity of station 3} C lay between stations 3 and 2* and D in the deepest water about 100 feet north of station 2,

- 311- Table IV. Summary of 1952 seasonal data from stone-anchor collections; numbers of hydras (H. littoralis) colonizing stone anchor at rubble-bottom stations (la, lb, lc, 2, 3, U, 6), Fishery Bay

Date* Immersion Number Total Mean Mean Temp. interval of number number number 1952 in days stations of hydras of hydras with- buds °C.

8/11 7 6 12 2.0 0.3 23.0

8/17 6-7 h 9 2.2 1.3. 2lu0 8/25 7-8 7 16 2.3 0.7 23.0

9/1 7 7 25 3.6 o.U 23.0

9/9 8-10 7 12U 17.7 6.5 20.5

9/15 5-6 7 78 11.1 2.0 21.5

9/23 6-8 7 62 8.8 3.1* 21.5

9/29 5-7 7 117 16.7 lull 18.0 10/6 6 5 29 5.8 1.8 15.5

10/13** 7-8 6 80 13.3 5.8 13.5

10/18 7 2 13 21.5 5.0 11.0

10/27 1U 5 128 25.6 6.2 10.0 11/U 7 6 Uo 6.6 0.8 7.U ll/ll 7 7 m 16.3 3.3 6.0 11/18 7 7 U7 6.7 1.0 7.5

11/25 7 6 7 1.1 0.3 6.5 12/2 7-8 5 5 1.0 0.2 3.0 12/28 26 5 26 5.2 1.0 0.5 Greatest variation in any station from collection date listed was plus or minus four days; any variation in immersion intervals included are shown by the range.

Gonadal hydras were found in collections beginning Oct. 13s for numbers and sex in collections to end of year, see Table IX. - 312 - Table V. Summary of 1952 seasonal data from slide-rack collections: numbers of hydras (H. littoralis) colonizing 18 slides at rubble-bottom stations (la, lb, lc, 2, 3> 1*> 6),.Fishery Bay.

Date* Immersion Number Total Mean Mean Temp. interval of number number number 1952 in days stations of hydras of hydras with buds °C.

8/11 7 1* 26 6.5 2.3 23.0

8/17 6-7 U 29 7.3 2.7 2U.0 8/25 7-8 7 38 5.1* 2.1* 23.0 9/1 7 7 23 3.3 0.8 23.0

9/9 8-10 7 72 10.3 5.U 20.5 H CM 9/15 5-6 7 1*7 6.7 • 21.5

9/23 6 - 8 7 31 i*.U 1.7 21.5

9/29 5-7 7 1*1* 6.3 1.0 18.0

10/6 6-7 6 1*9 8.1 2.6 15.5

10/13** 7-8 6 35 5.8 2.8 13.5 10/18 7 2 6 3.0 0.5 11.0

10/27 U* 5 50 10.0 1.1* 10.0

ll/U 7 6 11 1.8 0.5 7.1* 11/11 7 7 1*8 6.8 1.8 6.0

11/18 7 7 16 2.3 0.7 7.5

11/25 7 7 12 1.7 0. 1 * 6.5 12/2 7-8 5 5 1.0 0.2 3.0

12/28 26 5 13 2.6 0.8 0.5 * See footnote - Table IV.

** See footnote, Table IVj gonadal hydras from stone and slides com bined in totals shown in Table X. - 313 - Table VI* Summary of 1953 seasonal data from slide-rack collections: numbers of hydras (H. 11ttoralls) colonizing 18 slides at rubble-bottom station (l, 2, 3, U) ,* Fishery Bay.

Date** Immersion Number Total Mean Mean Temp. intervals of number number number 1953 in days stations of hydras of hydras with buds °C.

7/11 11,12, 7, 9 It 255 63.8 3lt.2 23.lt 7/22 11,10,10, 7 U 37 9.3 3.3 25.5 7/30 7, 7, 7, 7 U 55 13.8 5.5 2lt.8

8/7 8, 8, 8, 7 it 233 58.2 30.6 2lt.5 8/13 5, 5, 5, 7 It 106 lOlt.O 37.8 26.2 8/20 7, 7, 7, 7 It 181 U5.2 16.7 25.0

8/27 7, 6, 7, 7 it 6lt 16.0 lt.5 26.2

9/3 7, 6, 6, 7 it 16 lt.O 0.0 27.6

10/12*** 38,38,39,39 it 388 97.0 13.0 16.2

12/7*** 52,55,57,52 It 116 29.0 11.2 5.U

* Station 6 is not included because of inadequate data resulting from loss of rigs through vandalism.

** Collections were made at majority of stations on dates listed; immersion intervals are listed in station number sequence. Gonadal hydras; see discussion in text.

- 311+ - Table VII* Suranary of 1954 seasonal data from slide-rack collections: numbers of hydras (H. littoralis) colonising 16 slides at rubble-bottom stations (1, 2, 3, k, 6), Fishery Bay

Date* Immersion Number Total Mean Mean Temp. intervals of number number number 195k in days stations of hydras of hydras with buds °C.

2/3 56-58 3 0 0 0 0.2

4/6 62-63 3 0 0 0 5.0

k/17 9-10 5 0 0 0 7.8 5/8 20-21 5 108 21.6 14.6 11.0

5/16 8 5 70 H u O 6.2 11.4

6/17 33 k 587 146.7 37.7 19.4 6/25 7 5 492 98.4 47.8 21.6 7/30 36 3 2kl 80.3 16.0 24.6 8/8 8 5 103 20.6 7.8 23.8

Greatest variation of any station from collection date listed is plus or minus one day; any variations in immersion intervals listed are shown by the range; the collection of April 6 included an inter val of 117 days for collection from rack found at station 3. Ice action caused loss of collections at stations 1 and 1*. Vandalism caused loss of 33-36 day interval records at station 1 (July 30) and station 6 (June 17, July 30).

- 315 - Table VIII. Number of buds per hydra during spring pulse as determined by counts from sanqples of $0 speci mens from mixed H. oligactis - H. pseudoligactia population* at station 5 “

Date Number Number with buds Temp. without 1951* buds One Two Three Pour Five °C.

l*/8 31 10 8 1 5.0

h/12 28 10 9 2 1 5.2

l*/ll* 19 7 8 11 3 2 6.1*

5/8 18 ! 10 8 6 6 2 11.0

5/13 17 16 7 3 7 £*# 11.2 5/16 19 17 6 5 2 1 11.1*

5/17 20 17 6 1* 2 1 12.0

5/18 22 Us 7 6 1 12.0

6/19 26 .16 7 1 18.8

6/27 35 13 2 21.6

* Ratio of oligactis to pseudoligactis was about 10si. No signi ficant differences in number of buds per individual, bud pattern, or column size between the two species, which are identical in body form, were found.

**Includes two specimens with six buds.

- 316 - Table IX. Occurrence of gonadal individuals of H. littoralis In collections from slide-rack and "stone-anchor rigs made during sexual period, 1952, Fishery Bay

Date* Temp. Total number Number of gonadal hydras of 1952 °C hydras Total Males Females

10/13 13.5 115 2 2 0 10/18 11.0 1+9 1 1 0

10/27 10.0 178 9 7 2

l l A 7.1* 1 5i 15 10 5 11/11 6.0 : 162 . 32 22 10

11/18 7.5 63 15 1+ u

11/25 6.5 19 3 1 2 12/2 3.0 10 0 0 0

12/28 0.5 39 2 1 1

Totals 686 j 79 1*8 31

* period between inclusive dates approximates immersion inter vals; see Tables IV and V for details.

- 317 - AUTOBIOGRAPHY

I, Louis Burrell Carrick, was born September 22, 190i{.» in Pittsburgh, Pennsylvania, the birthplace of my father and mother. I attended public schools in Ohio and Michigan, graduating from high school in Detroit and receiving the degree Bachelor of Arts from the College of the City of

Detroit in 192£. Then I pursued graduate studies in zoology at the University of Michigan while working as a teaching assistant in the laboratory and doing research under the di rection of Carl L. Hubbs. Prom the University of Michigan, I received the degree Master of Science in 1929. I did not return to the field of academic biology until 19^6. During the interim, I wcLs employed by industrial and governmental organizations in various capacities which required scientific training. The summer of 19ij-6 I studied ecology at the Franz Theodore Stone Laboratory where I served as research assist ant. I was then employed as an instructor in zoology at

Wayne State University until the summer of 19^1, at which time I undertook work toward the degree Doctor of Philosophy in residence until 19£>Ij- at the Franz Theodore Stone Labora tory of the Ohio State University. While completing require ments for this degree, I have been on the research staff of the Cranbrook Institute of Science as guest investigator and have served as a member of the part-time faculty of the Department of Biology at Wayne State University.

- 318 -