PREFACE
During the Meeting of the International Council for the Exploration of the Sea at the end of May 1935 a number of lectures on various subjects were given at two Special Meetings held during the week of the session. On Monday, May 27th, a Special Plankton Meeting was held at “Eltham” on which occasion Mr. F. S. Russell of Plymouth, Professor A. C. Hardy of Hull and Mr. S. Landberg of Bornø lectured on the questions recorded in the list of contents below. On May 31st a Meeting was held at the premises of the “Kongelige Danske Videnskabernes Selskab” ; on this occasion Professor B. Schulz of Hamburg reported on some results obtained from the Kattegat investigations of internal waves undertaken in 1931 (a summary of this lecture also appears in the following pages). Further Professor Johan Hjort of Oslo lectured on “Fishing Experiments and Administrative Problems”. Professor Hans Pettersson of Gothenburg demonstrated a film showing internal waves originating from the action of an oscillating magnetic force in a paramagnetic fluid. Professor N. Z e i 1 o n of Lund demonstrated internal waves produced in a whipping trough with water layers of different densities.
TABLE OF CONTENTS
Page I. A Review of some Aspects of Zooplankton Research, by F. S. Russell, Plymouth ...... 3 II. Further Investigations upon the Photosynthesis of Phyto-plankton by constant Illumination, by H. Hög lund and S. Landberg, Bomø .. 31 III. The continuous Plankton Recorder: a new Method of Survey, by A. C. Hardy, H u ll...... 35 IV. Die Ergebnisse der internationalen hydrographischen Beobachtungen im Kattegat im August 1931, by B. Schulz, H am burg...... 49 A REVIEW OF SOME ASPECTS OF ZOOPLANKTON RESEARCH.
BY
F. S. RUSSELL, Naturalist at the Plymouth Laboratory of the Marine Biological Association of the United Kingdom.
T is now nearly one hundred years since the In reviewing the present position I shall try to tow-net was first introduced by Johannes embrace all the aspects of the situation. It is Müller (1846). Zoology was still in its necessary to take the wide view without unduly I early descriptive stage and a rich ground for the stressing any particular problem, because we have investigation of new species was thus opened up. suffered in the nast from a tendency to pursue About that time also the theory of evolution was certain directions of enquiry to a disproportionate assuming concrete form, to be firmly established extent. As a result plankton research has become by the publication of Darwi n’s “Origin of somewhat uncoordinated, and advance has been Species” in 1859. The evolutionary theory was the held up by a lack of consideration as to which are main directing force for the study of embryology, the problems that require special attention in order and in the development of this branch of zoology, to bring our various fields of knowledge into line. research on plankton animals has played a greater With such a wealth of data scattered amongst part than that on any other population of the the biological literature of the world the task of animal kingdom. We owe to the labours of the review becomes impossible unless we first fix upon zoologists of the latter half of last century most certain outstanding events around which the main of our present knowledge on the affinities and sequence of the review can be evolved. natural classification of the major groups of in I have adopted the following landmarks as vertebrate animals. With the exception of insects, representative of the sequence of events after the the invertebrates are essentially inhabitants of the introduction of the quantitative method by Hensen, sea, and almost all spend the earlier stages of their leading to the publication of his Methodik (1895) lives drifting in the water layers in the plankton community. 1. The Plankton Expedition in the “National” Among the first workers in this line of research under the leadership of Hensen in 1889. was T. H. Huxley who sailed in 1846 in the 2. The comprehensive scheme for the collection “Rattlesnake” to Australian waters. In these days of plankton throughout the North Sea and of elaborate and costly apparatus it is well to bear adjacent waters of the Atlantic, Channel and in mind what far reaching results can be obtained Baltic, during the years 1902 to 1908 by the by the simplest means. Huxley used as his International Council. townet a bag of the bunting of which flags are 3. The completion byLohmannin 1906 of made. With this simple net he sieved the sea at a year’s detailed survey of the total plankton every available opportunity. As a result be production at Laboe in Kiel Bay. demonstrated the true position of the medusae and During these two latter periods plankton study siphonophores in the coelenterate kingdom, and received a constant leavening element in the in also traced the affinities of the pelagic tunicates tensely biological outlook of Professor Gran. to the sessile ascidians of our sea shores. Of even 4. The post-war period. more far reaching importance was the result of his examination of the medusae whereby he indicated Three publications previous to the last period the existence and importance of the germ-layers in should also be mentioned as having left their mark the development of metazoa. in the history of plankton research and stimulated But to-day I do not wish to take as my theme many workers, these were Adolf Steuer’s the examination of plankton animals with the “Planktonkunde” published in 1910; “Conditions ulterior aim of elucidating problems purely of Life in the Sea” by James Johnstone, in morphological or embryological in nature. I wish 1908 and the “Depths of the Ocean” by Murray rather to review those aspects of zooplankton and Hjort in 1912. research whose concern has been the study of I have taken these four periods because they plankton animals as members of a community, and illustrate the main problems that confront those for the part they play by their habits and concerned with zooplankton research. The first is abundance in the balance of life in the sea. The the necessity of knowing the distribution and review will be concerned only with metazoa. abundance of the plankton in the open ocean. - 6 —
IA.
1 £ • H - :- +■ V\ v*- li fv’v ^.-1 I
Fig. 1 Distribution regions of oceanic plankton organisms (after Steuer, 1933, p. 292, Fig. 8, for Copepods). For explanation see Text.
Secondly comes the desire for a similar knowledge biology and as regards the zooplankton much of of the conditions in the coastal waters whose origin our present knowledge has been built up from the and movements are so largely governed by the results of the collections of the great oceano oceanic circulation. In intimate connexion with graphical expeditions. Many years must elapse this is the use of our knowledge of the plankton before the description of all the species of plankton distribution as an aid to the interpretation of hydro- animals is brought to completion. We can however graphical data. Thirdly, knowing something of the say with gratitude to the workers in the past that animals’ distribution we need to know about their in many groups today the necessary knowledge is relations with their animate and inanimate environ reaching its final stages. Coincident with the ment, and the causes of their fluctuations in descriptive work our knowledge of geographical abundance. This is a work that has so far only distribution has grown and as the groups have been attempted in coastal waters where the been better known the distribution of the species necessary observations can be made at frequent among their faunistic regions has become more intervals throughout the year with the facilities of detailed. a laboratory close at hand. And finally the re It is probable that in ocean waters one of the search has shown the need for even more detailed main barriers to distribution is that of the tem observations, for a close study of life-histories and perature conditions. Animals adapted for success habits, and for the examination of the more ful reproduction in any given temperature range specialised problems that have arisen such as tbe will be carried in the water masses and ocean relations between the plankton and the fish that current systems so far as the necessary temperature are used as food by man. conditions are maintained. Before any work on the biology of animals can When the temperature conditions have been be attempted the animals that we wish to study taken into consideration the chief clues to the must be described and classified. This is a task geographical distribution of the species are to be that has gone on from the earliest days of marine found in the systems of oceanic circulation. The — 7 —
geographical distribution more complete. The data oceans can now be divided into definite masses of thus obtained will act as bench-marks for the study water with different characteristics. These different masses are constantly moving and are separated in years to come of the secular changes in the ^vate'’ movements of the ocean. With this idea in mina I from one another by regions of mixed water which have produced a preliminary table (Table I) giving may be wide or narrow according to the locality. the number of known species in the more important Such regions are the antarctic, the subtropical and plankton groups and their subdivision into broad the tropical convergences in the south, and the geographical areas. It has only been possible to transition regions in the north which vary from produce such a table with the cooperation of many narrow zones with well defined boundaries between specialists in plankton animal groups. I owe a warm and cold waters in the west to the wide debt of gratitude to the following workers for their spread mixing regions in the east. Each body of water has its own somewhat characteristic plankton very willing and painstaking assistance. Dr. S. T. B u r f i e 1 d (Chaetognatha) ; Mr. fauna at different levels. These faunas tend to mix G P. Farran (Copepoda) ; Professor P. Fau- at the boundaries, some species of the colder water vel (Tomopteridae); Miss A. B. Hastings surviving under favourable conditions found at (Tunicata) ; Dr. P. L. Kramp (Medusae) ; Pro deeper levels. Thus the distribution of the arctic fessor Th. Mortensen (Ctenophora) ; Dr. K. planktonic fauna dips beneath the surface layeis Stephensen ( Amphipoda) ; Professor^ W . M. towards the south. .... Tattersall (Euphausiacea) ; Mr. A. K. Tot- In a recent work on the geographical distribution of copepods Steuer (1933, p. 291) has adopted t o n (Siphonophora). The production of this table brings out some the following scheme, (Fig. 1). points of interest. I. Circum-polar Arctic. 1. The greater number of cold water species A. Sub-arctic (temperate, northern transi in the south as opposed to the north. tional. or boreal) 2. The phenomenon of epiplanktonic1 ) bi a. Atlantic. polarity. b. Pacific. 3. The small number of purely sub-antarctic II. Circum-equatorial tropical. species of copepods and perhaps other a. Atlantic. groups. b. Indo-pacific. 4. Differences between the faunas of the diffe A. Sub-tropical rent oceans. 1. Northern sub-tropical. 5. The inequality in the state of our knowledge a. Atlantic (and Mediterranean — on different groups. Black Sea) In all groups (except the meroplanktonic b. Pacific. medusae and the essentially bottom living Gammarid 2. Southern Sub-tropical. Amphipods) the preponderance of southern over a. Atlantic. northern cold water species is evident. This fact has b. Indian. been remarked on by Meisenheime r, (1905, c. Pacific. p. 90) for the pteropods. He suggests an obvious III. Circum-polar Antarctic. cause in pointing to the much greater area of con A. Circum-polar Sub-antarctic (southern fluence between warm and cold waters in the south transitional). than in the north as conducive to the opportunity It is never possible to confine living organisms for increased evolution of species from the warm within the limits of precise schemes, and while species-producing centre. But there is an added such a classification is convenient the boundaries factor that our increased knowledge of oceanic do not hold for all species. The most important circulation brings to light both on this problem limiting factor is probably temperature and each and that of bipolarity. Before proceeding it should species may have its own limits. There is thus be pointed out that Steuer (1933, p. 284) has some overlap in the limits of distribution of species suggested that there are no truly arctic Gymnoplean in the transition zones; this has been made clear Copepods, but that the arctic and sub-arctic regions by the work of the Discovery Expedition in the should be taken together. He gives as an example southern Atlantic (Mackintosh, 1934). \e t in Calanus hyperboreus, a species that has been regions where the transition in temperature is 1) In my original manuscript I had used the word abrupt, and amounts to several degrees, many “bipolar” ; it was however pointed out to me by Dr. H. B. species find their limits, and the faunas of the Bigelow that the original meaning of this word implied neighbouring water masses become clearly de discontinuous bipolarity (see also Stiasny, 1934); it is marcated. necessary to find some other term and I have used Since in many groups the majority of species “bipolar-epiplanktonic” as denoting that these species occur are now fully described it should be the aim of in the surface waters in north and south, though when future expeditions to make our knowledge of present in warm latitudes they are mesoplanktonic. (Continued on page 12). — 8 —
H H n r> ~a 2 o Ö C S; “13 C*1 o M i c« ^rD n~ 1-1 ^ ^ £ x § ^ ° Q- ~T H UÇ/5 ? Q ^3 2 5. •ö o a| ^ tr1 g 0 ^ - 0) > 3 »• 3 i A-S. M g B? ;T 3 » 'C ^ 03 M ►< _ _ s - £a - £“ ICfi f S o? oa ’ Ä.3 3 S | 3 *o M ■<œ -S'SÎ 2 a - 4 “ trj o “ CD rjp O ff- 3 O g --■5 -g. H n c ^ q - w z s; S S O en R, 5 ?•; » O § o is > S. g o o O ’=-•§ tå 2 c H H ;>* a 3 Cl. p ’ et> O s ? » 0 “c P > £a3 «-*■ > m H cr 05 rX p 2 M S 3 a M P a ” 1ft , = ® 2 A.'* > H 50 g. 3 3 > s 7T H >
4^ CO CD b o >—■ VO C O ^ C O »f* — 1 CC C o I OS to OV VC ^ 1 Warm 3 CO o O' s o C o CO CO 1 o vO o o •■c B 1 I 1 i bo vC 1 CO to t o CD 1 1 1 CO 1 •■O Co C o Cn 1 O s CO t o cn t—l o Deep Sea u K)
< 1 . C n b o H- O'. VC 4^- C n CO C O C n to C n C o CO CO C o C o C o TOTAL •—1 o t— * (—‘ C n - 1 O >£*■ o 4^ t O 4^ C n » cr n n 2 n ^ ^ G s : "e O ^ flQ 33 X O M *0 C/î Ö >* H— ^ o G -C 1 O d « r p a l c > H t“1 g 2 ’■3 n"5' S, S M Ï Ï - 2 . 2 '< 2 P *<« ^3ft, I—I 3 S r M § -< g - • S ’| k n o 3 n "a 3 "O ^ *t3 ^ & W C«o t- 0 O- ■o a o g g 5 -o' S a H H 33 43 -' M 3 P .S g > 6 . G ■§ 2 5. 0 a c/3 § sit- ° ° > O n * H •o tM 2 01 I - h s - a p SI cs3 n n o 2. > M © t! H «i 3. > a - > M 2 &• W > W > > » H S3 H > > EE H sO" > >
„ Cn 4* i—i to Co on (O' VO CO cn ^ 4*. Co On CO vO CO to O Cn In 3 oceans Co to CO CO tO K3 H- Atlantic and o g 1 _ H- 1 - 1 CO 4k Indian bo to Atlantic and 1 1 1 1 Os H-J CO 4^ to Co S 1 Co 1 bo CO Cn CT 1 Pacific CO Indian and 1 - 1 bo Os On to 4^ 1 ~ 3 1 Co o vo cn to 4k Pacific *) I—* to to S CO to 4k Os 1 to h-1 o bo -
?2 Mediterranean 1 1 1 Cn 4^ bO 1 m • ■o I i “vl VO 1 1 •>3 •■o only 47 37 bo 4» 65 bo t— « vC C O 4^ Cn to Cn CO tO On C O 4-* b O O 4^ to o o vO Co CO NO H- TOTAL
1 s t column. 2nd column. ( ) = Bipolar sub-arctic and sub-antarctic. * Including Indo-Pacific Region (Siboga). 1 Approximate Numbers. 2 Including Atlantic-Mediterranean. 2 Mesoplankton — below 100 fathoms and mostly deeper. 3 Including Siboga results. ,! Below 200 fathoms, mostly not found in epiplankton at 4 Six in Red Sea only. night. Pacific and Mediterranean. 4 At least 300—500 m. wire out. 6 Including Indo-Pacific (Siboga). 1 Siboga results divided between Indian and Pacific accord ing to localities. — 9 —
NOTES TO TABLE I.
The following definitions have been used:— N E M E R T E A. The Pelagic nemerteans are after Bipolar-epiplanktonic. Those species which are Brinkmann (1917) and Wheeler (1934). found in surface layers in northern and southern cold POLYCHAETA. waters but when present are found only in deep layers Tomopteridae. The data have been supplied by Prof. in warm latitudes. P. F a u v e 1. Cosmopolitan. Those species which are found alike The cold water species are:— in surface warm and cold waters. Sub-arctic, ? Tomopteris helgolandica Greef. Deep Sea. Those species living below a depth of about Antarctic-sub-antarctic, Tomopteris carpenteri Quatref., 200 fathoms, that is below the limit of penetration of T. rosea Ehlers, ? T. opaca Treadwell, and light. The inhabitants of this region have been regarded ? T. tentaculata Treadwell. by many as likely to be found to be cosmopolitan. They T. septentrionalis Quatref. is regarded by Augener have not been divided among the oceans in Table I as (1929, p. 281) as a bipolar species. in many groups our knowledge of the true deep sea CHAETOGNATHA. The data have been supplied species is insufficient. by Dr. S. T. Burfield. Ritter-Zahony’s (1911) In those groups in which the division of species between mesoplankton species have been regarded as deep sea. arctic and sub-arctic, and antarctic and sub-antarctic, is The cold water species are as follows:— uncertain the numbers in the Table have been placed Antarctic-sub-antarctic, Heterokrohnia mirabilis Ritt.-Z. centrally between the two columns in each region. Below and Sagitta gazellae Ritt.-Z.; Bipolar, Sagitta maxima they have been written arctic-subarctic, and antarctic-sub- (Conant) (and deep sea), Eukrohnia hamata Mob. antarctic. S. gazellae and S. maxima are both very nearly related COELENTERATA to the warm water species S. lyra Krohn. If they can be The data for Medusae have been supplied by regarded as separate species, it is possible that S. gazellae Dr. P. L. Kramp. The Medusae have been divided has evolved in southern cold waters; and that S. maxima into the Mero-planktonic Antho-, Lepto-, and Scypho- has evolved in cold northern waters and become bipolar. medusae, and the Holo-planktonic Trachy-, and Narco 5. planctonis Steinhaus a deep water species appears in the medusae. Antarctic. SIPHONOPHORA. The data have been obtained To the number of warm water species should also be from Moser, 1925; Bigelow, 1911 and 1931; and added six species described by O ye (1918) from the Java Lens and van Riensdijk, 1908, with the Sea whose identity has not yet been confirmed, and assistance of Mr. A. K. T o 11 o n. S. euxina from the Black Sea. The deep-sea species are Sagitta decipiens Fowl., S. lyra Krohn, S. macrocephala The cold water species are:— Fowl., S. planctonis Steinhaus, Eukrohnia fowleri Ritt.-Z., Sub-arctic, Stephanomia cara (A. Agassiz). and Heterokrohnia mirabilis Ritt.-Z. Antarctic-sub-antarctic, Diphyes antarctica Moser; Cry- stallophyes amygdalina Moser (? deep sea), Pyroste- phos vanh'ojjeni Moser, and Stephanomia convoluia Crustacea Moser. CLADOCERA. The data have been taken from Bipolar, Dimophyes arctica Chun. Rammner (1933). There are no purely arctic or ant It is still uncertain whether Stephanomia cara should be arctic cladocera, Sub-arctic, Podon intermedius Lillj. ; regarded as a distinct species from the warm water Bipolar (sub-arctic and sub-antarctic) Podon leuckarti Stephanomia bijuga (della Chiaje). G. O. Sars, Podon polyphemoides Leuck., and Evadne The following are possibly deep-sea species, Chuniphyes nordmanni Lovén. multidentata Lens u. v. R., Chuniphyes problematica Moser, COPEPODA: all the data for copepods have been Clausophyes ovata (Kef. u. Ehl), Thalassophyes crystallina supplied by Mr. G. P. F a r r a n. Moser, Amphicaryon acaule Chun, Pterophysa (Bathyphysa) The following is the tentative list of cold water and studefri Lens u. v. R., and Bathyphysa sibogae Lens u. v. R. cosmopolitan species:— CTENOPHORA. The data have been supplied by Arctic and Boreal. Calanus cristatus, Calanus hyper- Dr. H. Mortensen, and the distribution in the boreus, Pseudocalanus elongatus, Microcalanus pusillus, oceans has been taken from Moser (1909). The cold Gaidius brevispinus (A), Pareuchaeta glacialis (A), water species are:— Pareuchaeta norvegica (B), Undinopsis similis, Undinella Arctic, Mertensia ovum Fabr.; Arctic and sub-arctic, oblonga (A), Scolecithricella minor (B), Scolecithricella Pleurobrachia crinita Moser and Bolina inju.ndibu.lum dentata, Temorites brevis, Heterorhabdus norvegicus (6 ), Martens; Sub-antarctic, Callianira antarctica Chun; Augaptilus glacialis (A), Mormonilla polaris (A), Antarctic, Callianira cristala Moser and Beroe Oncaea borealis (A), Acartia longiremis (A). compacta Moser. Antarctic. Calanus propinquus, Calanus acutus, Rhin- Beroe cucumis Fabr. and Pleurobrachia pileus Fabr. calanus gigas, Stephus antarcticus, Chiridius antarcticus, occur both in the arctic and antarctic, but also in surface Chiridius minor, Chiridius polaris, Gaetanus antarcticus, waters of warm latitudes and are cosmopolitan. Euchirella rostromagna, Undeuchaeta spectabilis, Pareu Moser (1909, p. 181) was of the opinion that the chaeta austrina, Pareuchaeta erebi, Pareuchaeta rasa, ctenophores in cold waters have undoubtedly evolved from Pareuchaeta antarctica, Pareuchaeta similis, Xantho- a centre of species formation in warm waters. calanus tenuiserratus, Onchocalanus frigidus, Cephalo- The deep sea species are Bathyctena chuni (Moser), phanes frigidus, Scolecithricella glacialis, Scaphocalanus Aulacoctena acuminata Mrtsn., and Beroe abyssicola Mrtsn. impar, Scaphocalanus sub-brevicornis, Amallothrix — 10 —
polaris, Amallothrix incisa, Racovitzanus antarcticus, species of the upper layers, as they are more nearly related Metridia gerlachei, Metridia curticauda, Heterorhabdus to upper layer warm water species than to deep sea species. austrinus, Heterorhabdus pustulifer, Haloptilus ocellatus, Paralabidocera hodgsoni, Oithona frigida, Oncaea PTEROPODA. Data taken from Meisenheimer curvata. (1905), Tesch (1904), B o nn e v i e (1913), Massy Sub-antarctic. Calanus simillimus, Eucalanus longi- (1920), and Pruvot-Fol (1924). ceps, Clausocalanus' laticeps, Pareuchaeta biloba, Can- Thecosomata. The cold water species are: — dacia cheirura. Antarctic-sub-antarctic, Limacina (Clio) australis Eydoux Bipolar. Microcalanus pygmaeus, Scaphocalanus magnus, and Souleyet; Sub-antarctic, Cleodora sulcata (Pfef Metridia lucens, Metridia longa (A), Heterorhabdus fer) (See Mackintosh, 1934, p. 86) (Limacina ant compactus, Candacia falcijera. arctica Woodward has not been included since Meisenheimer suggested that its separation lrorn Cosmopolitan. Calanus jinmarchicus, Ctenocalanus L. helicina was not certain). ranus, Gaidius tenuispinus, Scaphocalanus brevicornis, Bipolar, Limacina helicina Phipps. Pleuromamma robusla, Pleuromamma borealis, Hetero- Large numbers of L. helicina from the antarctic were stylites longicornis, Oithona similis, Oncaea notopus, examined by Massy (1920) and no reasons were found Oncaea conifera. for regarding the arctic and antarctic forms as different. A. With a tendency to be purely arctic. Limacina balea Möller may be bipolar in sub-arctic and B. With a tendency to be purely boreal. sub-antarctic waters. There are at least three deep sea species:— A. M P H I P O D A. The data have been supplied by Peraclis diversa Mont., Limacina helicoides Jeffreys, Dr. K. Stephen sen. and Clio jalcata Pfeffer. Hyperiidea. Gymnosomata. The cold water species are:— The cold water species are:— Arctic and sub-arctic, Clione limacina Phipps; Ant Arctic, Themisto libellula (Mandt ), 7 . abyssorum arctic and sub-antarctic, Clione antarctica E. A. Smith; (Boeck) (mainly): Antarctic (? subantarctic), Sub-antarctic, Spongiobranchaea australis d’Orbigny (See Mimonectes setosus Barnard, Vibilia edwardsi Bate, Mackintosh, 1934, p. 86). Cyllopus lucasi Bate, Janira macrocephala Dana, Hyperoche capucinus Barnard, ? Hyperia macromyx Clione limacina has been regarded as bipolar, but Massy Walker, Hyperiella dilatata Stsbb., Euprim.no ant- (1920) regards the antarctic form C. antarctica to “be artica Stebb., ? Brachyscetus antipodes Bate, entitled to specific rather than varietal rank”. (.Hyperoche lutkenoides Walker, and H. crypto- Heteropoda. The data have been taken from Tesch dactylus Stebb. uncertain species). (1906). There are no purely cold water species. Paralanceola anomala Barnard, and Hyperiella ant TUNICATA: The data have been supplied by Miss arctica Bovall. are deep sea antarctic species. A. B. Hastings. Cosmopolitan, Scina borealis G. O . Sars, Themisto gaudichaudi Guer. Appendicularia: (supplemented from Lohmann and Biickmann (1926). In addition to the 28 deep sea species given in Table I The cold water species are as follows:— there are a further 25 species which should probably be Arctic, Oikopleura vanhöffeni Lohm. and Oikopleura regarded as deep sea. The number of warm water species chamissonis Mertens; Arctic boreal, Oikopleura would thus be proportionately reduced. labradoriensis Lohm.; Antarctic-subantarctic, Oiko Gammaridea. pleura gaussica Lohm., O. valdiviae Lohm., O. dry- The cold water species are:— galski Lohm., O. weddelli Lohm., Fritillaria ant Arctic-subarctic, Cyclocuris guilelius Chevreux (deep arctica Lohm. and Fritillaria drygalski Lohm. ; sea), Pseudolibrotus glacialis (G. O. Sars), Cosmopolitan, Fritillaria borealis Lohm., with several P. nanseni (G. O. Sars), Orchomonella lobata Chevr. forms; Deep Sea, Fritillaria aberrans Lohm. Chuno- (deep sea), Apherusa glacialis (H. J. Hansen), Eusirus pleura mirogaster Lohm., and Bathycordaeus charon holmi H. J. Hansen, Cleonardo microdactylus Chun are possibly purely deep sea species (Loh K. Steph. (? deep sea,) Hyperiopsis voringi G. O. mann, 1931, p. 108). Sars, (deep sea). In his Weddell Sea report Lohmann (1928, p. 71) gives the distribution of certain species as still uncertain: E U P HAUSIACEA: the data have been supplied by these have been included here with the warm water Prof. W. M. Tattersall. species. The cold water species are as follows: — Lohmann (1905) has shown that the polar species Antarctic, Euphausia superba Dana and Euphausia of Oikopleura are the most recently developed. For their crystallorophias H. & T.; sub-antarctic, Euphausia origin he favoured Ortmann’s hypothesis that they are the frigida Hansen, E. vallentini Stebbing, E. triacantha remains of a universal warm water population left at the Holt & Tattersall, E. longirostris Hansen, E. hanseni poles when climatic changes set in, and which have since Zimmer, Thysanoessa macrura G. 0. Sars, and developed their own peculiar species. On the contrary he r . vicina Hansen. regarded Fritillaria borealis as the relic of the original Thysanoessa gregaria G. 0 . Sars is bipolar in sub polar stock which has given rise to warm water species. arctic and sub-antarctic regions of the north and south Whatever the origin of the Oikopleura species in polar Atlantic and north and south Pacific. waters it is at any rate certain that there are no bipolar species. The Oikopleuras are essentially inhabitants of the The genera Euphausia and Thysanoessa are cosmopolitan upper layers and are dependant on the nannoplankton for and upper layer genera on the whole. It seems likely that their food supply. It is thus unlikely that any would the cold water forms have evolved from the warm water survive sufficiently long in the deepest layers to have — I l —
6PO<7 ,,-.L 8 0 'S ü d 70' f a M W j M f i I 0 fUn MrontaKr Strçm * | * ------f * » " Struck,O t , ^ in cm fsec 1 “*_____< * - 5 < ...... 0 -1 • meridio/tater Richtung * 8odtnrehe*s Fig. 2. Circulation of the meridional component of the water movements in the Atlantic Ocean at 30 W. (after D e f a n t, 1927, p. 365, Fig. 53).
ANTARCTIC. WARM WATER. ARCTIC. < ^zzzzzzzzzzzzzzzzzzzzzzzzg > M. T 1000 DEEP SEA.
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- 3 0 0 0
Bl POLAR
T “I------T T T -I— - r - “ I— “ i— i — i— S 80° 70° 60° 50° 40° 30° 20° 10° 0° 10° 20 ° 30° 40° 50° 60° 70° N Fig. 3. Diagram showing how cold water species that have evolved from warm water species in the Northern hemisphere will tend to be. carried southwards in deep water to become bi poia r-epi plankton ic. Species which have evolved as ^ eeP fcea species and become adapted to low temperatures may also thus join the Antarctic community. The diagram is based on the circulation of water given in Figure 2. allowed transport from the north polar regions to the development of forms has occurred in the cold waters of antarctic. Lohmann (1931, p. 45) remarks however on the north and the south. the remarkable similarity between the northern O. labra- Doliolidae. There are no purely cold water species except doriensis and the southern species 0 . gaussica and 0 . val- possibly Doliolina resistibile (Neumann) in the Ant diviae and suggests that they may be regarded as a arctic. This species is however regarded by G a r - bipolar group of species in the genus Oikopleura. stang ( 1933, p. 213) as a variety of D. intermedium 0 . drygalski on the other hand is most nearly related to Neumann). the warm water species 0. albicans. Fritillaria borealis may on the other hand be a relic Salpidae. There is only one cold water, antarctic- of past ages left at both poles as Lohmann suggested. If sub-antarctic species Apsteinia magalhanica (Apstein) ; this be so it seems strange that they have not given rise to another species A. racovitzai (Berc. & Selys-Long- species peculiar to the poles. Another polar species champs) has been described on the basis of a single F. antartica Lohm., is confined to the Antarctic and has specimen as slightly different from A. magalhanica, but been shown by Lohmann to be very closely related to until more specimens are obtained its identity remains the warm water species F. fraudax Lohm. It seems more doubtful. A form of Salpa jusiformis Cuvier also occurs likely that Fritillaria borealis is a cosmopolitan species, in the Antarctic, viz. S. fusijormis form aspera which is known to show many forms, and that the parallel (Chamisso). - 12 —
regarded by some as Arctic. But, while obser The^great majority of cold water species belon vations made in the eastern Atlantic where the ging to “deep-sea” genera are found in the antarctic. transition zone is wide lead to this conclusion, the Further support is also given by the fact that there distribution on the western side, where the tran are also relatively more bipolar species found sition from Atlantic to Arctic water is abrupt, seems among “surface” than “deep-sea” genera. to indicate that this species may justly be regarded The actual number of bipolar epiplanktonic as an Arctic species. The difficulty in demarcating species is small compared with the total number of the Arctic zone is clearly shown by an examination species of plankton animals. It seems probable of the general oceanic circulation. In the north the that their bipolarity can be sufficiently explained main water movement in the deeper layers is by the conditions of the vertical circulation of the southwards; thus representatives of Arctic water ocean without the necessity for regarding them as are carried in the deeper levels to lower latitudes. hypothetical relics of a past homogeneous fauna. In Table I species regarded as purely arctic are It is worthy of mention that the majority of those whose distribution on the western side of the epiplanktonic bipolar species occur in sub-arctic north Atlantic indicates that their real home lies waters and are probably able to withstand a con in Arctic waters. siderable range of temperature. In seeking to understand the distribution of Space will not allow further discussion on species in the cold waters the vertical circulations problems raised by an examination of Table I. of the ocean must be considered as well as the It is only necessary to call attention to the fact horizontal current systems. In Figure 2 is re that our knowledge of the fauna of the Pacific produced a diagram showing the type of vertical and Indian oceans falls far short of that of the circulation in the Atlantic ocean (after D e f a n t, Atlantic ocean. The differences and phylogenetic 1927). It is evident that in the deeper layers the relations between those species which are peculiar main body of water is moving in a southerly to the Atlantic and those peculiar to the Pacific direction. Thus species evolving outwards into cold also appear worthy of study. And certain groups water from a warm surface species producing of soft-bodied animals, such as the Ctenophores, centre will form cold water northern and southern Siphonophores, and Gymnosomatous Pteropods, species (Fig. 3). But the ocean circulation is in which are difficult to preserve need special attention favour of carrying any northern species that can in the future. withstand and reproduce under deep water The actual contents of Table I appear slight conditions southwards to the Antarctic, so producing in proportion to the labour involved in its produc bipolar species. Thus it is likely that any species tion, but it is surely a noteworthy tribute to the in the cold northern waters, whether a relic from labours of workers in the past that it has been past ages or one more recently evolved, may possible to produce such a table at all. become bipolar, but not so antarctic species. In As regards future work Steuer (1933) has this way the number of pure cold water species is suggested that in the planning for further expedi further reduced in the north. In this respect it tions attention should especially be paid to the is interesting that two of the groups with no bipolar distribution zones in coastal regions and also to species, Tunicata and Ctenophora, are essentially the Pacific ocean. There seems also to be some composed of surface living species. essential difference between the fauna of the western Cold water species may also be evolved as deep and eastern halves of the Atlantic worthy of closer sea species from the warm surface layers. Again study; and the examination of the deep-water faunas taking the oceanic circulation into consideration the of the different ocean basins needs special attention. odds are in favour of polar species evolving from For the study of geographical distribution, deep water species to the south rather than to the whether over large or over small areas, attention north for in the south existing species will be must also be drawn to the valuable results likely continually carried into antarctic waters. An to be obtained by the use of the continuous plankton examination of the data on Copepods kindly sup recorder invented by Hardy (1926). plied by Mr. Farran rather bears this out. If It has been remarked that the distribution of the genera be divided into surface genera, (in which the coastal forms needs further study. This leads less than 50 °/0 of the species in a genus can be us to examine the results of the plankton collections regarded as deep sea species), and deep sea genera by the International Council. In no part of the (in which 50 °/0 or over of the species in a genus world have the coastal waters received greater can be regarded as deep sea species) we find the following result. attention than in the area which comes under the aegis of this Council. The whole area bordering Total 7>'°f .N°.of No of our coasts was examined for a period of years No No. of Arct'C A"tarcc''c Bipolar species and. bo- and Sub- "pp from 1902 to 1908. As a result it was possible r real spp. ant. spp. to publish the Plankton Resumés in which detailed Surface genera 76 525 13 19 5 pictures of the distribution of most of our species Deep Sea genera 36 228 3 18 1 have been reproduced. In the final summary' of — 13 - results Ostenfeld (1931) subdivided the plank C 1 e v e made could not hold good, because the ton populations of the area covered into a number composition of the plankton was modified as the of convenient groups according to their distribution. conditions in the moving water mass altered, and While it is possible to pick out species that are because it changed with time of year. He also truly representative of the different divisions there insisted that full use of plankton organisms as is naturally considerable overlapping among the indicators could not be made until their life majority of species. Examination of the distribution histories and habits were better known. In this charts in the Plankton Resumés shows that there latter respect Kramp (1927 and 1930) has ably is a complete range of animals from those that shown how a thorough knowledge of the biology are purely brackish water species to purely oceanic of hydroids and medusae may be of practical species. This range may be exemplified as follows importance in the study of water movements. (Fig. 4) : — There is no doubt that with our increasing Brackish water e. g. Eurytemora affinis knowledge much more use of plankton animals as Littoral and brackish water indicators under certain conditions could be made e. g. Acartia bijilosa at the present time. It is therefore worth examining Neritic, low salinity the conclusions and observations of previous e. g. Oithona nana workers. Gran (1902, p. 104) has said “the Neritic, characteristic forms of the regions can be used as e. g. Centropages hamatus indicators, in so far as they are dominant; there Atlantic oceanic, existing all the year round under will however always be fairly wide boundary neritic conditions. regions, where the characteristic forms of both e. g. Centropages typicus current regions are mixed”. “Species may be found Atlantic oceanic, existing only part of the year in outside their regions. When they occur as secondary neritic regions, constituents of the plankton together with indi e. g. Aglantha digitalis genous species we can conclude that water from These types also show the major geographical the distribution region of the visitors must occur distributions outlined above, and overlap in range as a mixture.” is likewise shown here. But whereas many species The results of the plankton collections of the occur over rather a wide area, the boundaries for Belgica expedition to the Greenland Sea were certain species coincide with the limits for certain critically examined by Damas and K o e f o e d water masses. (1907) from the point of view of the use of At the beginning of the International Council plankton animals as indicators. They conclude programme it was emphasized that “The study of that “the use of pelagic organisms as indicators the floating organisms had particular worth for the of currents is a local science, based on the solution of hydrographical problems”. While it knowledge of the special habits of each form, in has always been fully realised that it should be the region considered”. “The same species can in eventually possible to make use of plankton orga fact be an excellent indicator for the polar current nisms as an aid to the study of water movements, in the north and for the Atlantic current in the the International survey did not appear to come south”. fully up to expectation in this respect. There are They give two examples of circumstances under various reasons to account for this. The identifica which the plankton can be employed with certainty tion of some species was not always certain. The as an indicator of currents. collections were not always sufficiently regularly 1. An organism which, being totally absent in taken. Large nets were not used and therefore the an ocean basin appears there suddenly and larger animals were not caught in sufficient periodically. In the Greenland Sea this case is numbers. The results for each group were given realised by the species, of southern origin, which to specialists to work up and the plankton has the Gulf Stream brings from temperate and even perhaps not been sufficiently studied as a whole. tropical regions. Examination of the report shows that only one or 2. The occurrence of meroplanktonic organisms two species in any group are likely to be suitable off shore. Cyanea capillata and the medusae born as indicators; and it is often the association of on the continental plateau of the Norwegian coast these different species that gives the necessary are good examples. They can be regarded as indication. Owing to their very widespread distri certain indicators for coastal water; this was also bution the majority of species are of little value maintained by Gran (1902, p. 105). in this respect unless examined in great detail. Gran (1902, p. 106) has also pointed out that An indicator must be a practical indicator. while plankton organisms may be reliable as We owe the first idea of the use of plankton indicators of surface waters, it is more difficult to indicators chiefly to the work of C 1 e v e and make use of them for studying the water move Aurivillius (1898). Gran (1902, pp. 98— ments in the deeper layers owing to their habits of 106) showed that such sweeping generalisations as vertical migration. — 14 —
Do-
3 5 7 //
\W 4 W/A 6 8 T77777777 10 x x x x
Fig. 4. (as to legend: see the following page). — 15 —
Plankton animals must therefore be used with racial investigation so exemplified by the work of discrimination if they are to be employed as the late Prof. Schmidt, and by the studies by indicators, and they must be taken in their context. Steuer on copepods. Thus we must be on our guard at times against The occurrence of immigrants into an endemic drawing conclusions from the occurrence of a Atlantic plankton community has been dealt with single indicator, owing to the possibility that an in detail in the Gulf of Maine by Bigelow organism having been brought into the region some (1926, pp. 51—69). In Figure 5 is reproduced a time previously finds conditions suitable for the chart from his report showing the chief routes establishment of a local race. But the appearance followed by planktonic immigrants entering the of a number of indicators together, with different Gulf at different levels. This chart shows the habits, must be taken as an indication of influx of course of circulation of the water masses as clearly water (See also Kramp, 1927, p. 231). as do hydrographic data. Bigelow says for Furthermore because doubt has been cast upon instance that “The lines of dispersal followed, the use of an animal as an indicator in one respectively, by Sagitta serratodentata, Eukrohnia, locality it does not necessarily imply that the same and S. maxima within the gulf correspond closely animal may not be useful elsewhere. with the dominant drift of water at as many levels Thus, as Gran (1902, p. 105) has pointed out, — that is, surface, mid, and deepest — as made Calanus hyperboreus must be used with caution an evident by the physical data afforded by tempera an indicator of Arctic water in Norwegian waters, ture and salinity and drift bottles” (1926, p. 64). since it is indigenous in some Norwegian fjords, Bigelow stresses the value of knowing to yet in other regions it may well be of value. what extent the immigrants are capable of living Similarly Clione limacina, a boreal-arctic species, and reproducing under the new conditions (see cannot be used in name as an indicator of Arctic Table II). He says (1927, p. 923) “Clearest water because it lives successfully in temperate evidence of the drift within the Gulf is afforded, Atlantic waters also. Yet in regions where there of course, by such species as are comparatively is no possibility of Arctic influence such as the short lived there and can not reproduce in its low mouth of the English Channel the species may be (or high) temperature”. Such knowledge would used as an indicator of Atlantic water. It is be of great value in judging how recent an influx extremely likely however that with a thorough had been and the age of the water masses. knowledge of its growth and size at maturity in It is especially valuable to watch for any Arctic and Atlantic waters it should be possible correlations between the presence or absence of to discriminate between Clione of Arctic regions certain plankton animals with that of fish and and those from Atlantic waters. This is a type of larger swimming animals. Frost, Lindsay and Thompson (1934) have for instance shown Fig. 4. a correlation between the abundance of Squid in Distribution regions of coastal and oceanic plankton Newfoundland waters and that of Sagitta serrato animals in north eastern Atlantic and adjacent waters. dentata. This is of clear value for the cod fisheries. 1. Brackish water, Eurytemora affinis; region of greatest In the North Sea Meek (1928) showed that the abundance in August and November (after Scott, two species of Sagitta, S. elegans and S. setosa, were plate XVIII). apparently representative of different water masses. 2. Littoral and brackish water, Acartia bifilosa; region The study of the Sagitta populations has been of greatest abundance (after Scott, 1911, Pl. XXII). carried a stage further at Plymouth, (Russell, 3. Neritic — low salinity, Oithona nana; region of greatest abundance (after Farran, 1911, Pl. XV.). 1935 a), and it has been found that S. elegans is 4. Neritic, Centropages hamalus; region of greatest an indicator of Atlantic water and S. setosa of abundance (after Scott, 1911, Pl. XVIII.). Channel water. In association with S. elegans we 5. Atlantic species, Acartia clausi, area of distribution find the typical Atlantic community consisting of (after Jespersen, 1934, p. 123.). Aglantha, Cosmetira, Stephanomia, Clione, Mega- 6. Arctic — boreal species, Calanus finmarchicus, showing nyctiphanes, and Luidia larvae. These or closely southerly limit of abundant occurrence (after F a r allied species are the indicators that K ü n n e ran, 1911, p. 83). (1934) has given for the south west Dogger Bank 7. Arctic species e. g. Limacina helicina. 8. Area in which the Arctic species Calanus hyperboreus swirl. It is thus possible that the two species of was recorded as abundant; this is in the East Iceland Sagitta may prove of great importance as aids to Polar Current (after Farran, 1911, p. 89 and Pl. XI, the study of the distribution and movements of and Damas, 1905, PL I.). water masses in the North Sea. In the Channel the 9. Limit of area within which the warm water species two Sagitta populations appear to sway to and fro Euchaeta hebes occurred on at least 20 °/o of the past Plymouth according to the strength of flow of occasions on which observations were made (after Atlantic water into the North Sea from the North. Farran, 1911, Pl. XIV.). This is in agreement with Carruthers’ theory 10. Southern limit in North Sea and Eastern ^ limit in Channel of Arctic-boreal species Clione limacina (after that the Straits of Dover act in buffer relationship Paulsen, 1910, p. 53). between the waters of the North Sea and the 16 —
Table II. List of Plankton Animal Indicators. Arctic water. 1. A high temperature is fatal and they soon perish. ( I ) 1) Ctenophore Mertensia ovum. Gulf of Maine ( 1 ) Medusae Ptychogena lactea 55 55 55 ( 1 ) Sarsia princeps Labrador Current (20) Pteropod Limacina helicina Gulf of Maine ( 1 ) Barents Sea and North of Iceland (3) Tunicate Oikopleura vanhöffeni Gulf of Maine (1) Newfoundland (2) 2. Able to survive for a considerable period and even reproduce to some extent. (1) Copepods Calanus hyperboreus East Iceland Polar Current (4, 5, 18) Gulf of Maine (1) Metridia longa East Iceland Polar Current (4) Gulf of Maine (1) Pteropod Clione limacina 55 55 55 ( I )
Mixed arctic and atlantic water. Species unable to breed in high (or low) temperatures (1) 1. Surface layers. Chaetognath Sagitta serratodentata Gulf of Maine (1. 19) Euphausian Nematoscelis megalops (1) 2. Intermediate layers (say below 50 m.) Chaetognath Eukrohnia hamata (1, 19) Siphonophore Diphyes arctica (1) 3. Deepest layers (150 m. or deeper) Chaetognath Sagitta maxima (1- 19) Entering north western North Sea Euphausian Thysanoessa longicaudata North Sea (6) In Newfoundland waters (2) a relation has been shown between the abundance of Sagitta serratoden tata and that of the squid Illex illecebrosus during the years studied, 1931, 1932 and 1933. Atlantic water. Water entering north western North Sea round north coast of Scotland. Euphausians Meganyctyphanes norvegica North Sea (6) Thysanoessa inermis (in winter only) (6) In regions away from influence of Arctic water. Medusae Aglantha digitalis var. rosea S. w. Dogger swirl (7) Channel (8) Cosmetira megalota S. W. Dogger swirl (7) Cosmetira pilosella (in spring and sum mer only) Channel (8) Siphonophore Agalma elegans S.U. Dogger swirl (7) Stephanomia bijuga Channel (8) Chaetognath Sagitta elegans (should probably also Channel (8) prove of value in southern N. Sea) Euphausians Meganyctiphanes norvegica Channel (8) Thysanoessa inermis Channel (8) Nyctiphanes couchi S.W. Dogger swirl (7) Pteropod Clione limacina Channel (8) S.W. Dogger swirl (7) Echinoderm Luidia sarsi young Channel (8) S.W. Dogger swirl (7) Tunicate Oikopleura labradoriensis S. W. Dogger swirl (7) Newfoundland (typical of mixed tem perate water) (2) •*) Numbers in brackets are those of the references at the foot of the table. — 17 —
Warm atlantic^water. Tunicates Salpa fusiformis North Sea (9, 10, 11), Channel (8), Gulf of Maine (1) Salpa democratica Ditto Cyclosalpa bakeri North Sea Doliolum nationalis North Sea, Channel (8) Doliolum gegenbauri North Sea (12), Channel (8) N.B. There are no boreal or arctic salps or doliolids ; the presence of any species in these groups may therefore be regarded as an indication of water from warmer latitudes. Medusae Liriope exigua Channel (8) Siphonophore Muggiaea atlantica Channel (13) Muggiaea kochi Channel (8) Physophora borealis North Sea and Norwegian coast (17, 18)
Cupulita sarsi 55 55 55 55 55 (17, 18) Actinia Arachnactis larvae 117) Cirripedia Lepas fascicularis (17) Copepoda Anomalocera patersoni Norwegian Sea (18) Coastal water. Medusae Cyanea capillata Norwegian Ocean (14, 17) Cyanea lamarcki North Sea (17) Sarsia princeps Spitzbergen (17) Sarsia flammea Catablema eurystoma „ Bougainvillea superciliaris „ Any meroplanktonic species with a fixed bottom stage in its life history is indicative of coastal water, and its distance off shore is dependant on its length of life in the planktonic stage (14, 18). Chaetognath Sagitta setosa Channel (8) (probably also North Sea) The coastal water of the Channel (8) and southern North Sea (7) is also distinguishable from At lantic water by its poverty in animal plankton. Certain species are given by Kramp (15) as likely to be carried into the southern North Sea via the Channel. Medusae Turritopsis nutricula, Amphinema dinema, Slabberia halterata, Gossea corynetes, Octorchis gegenbauri, Cosmetira pilo- sella. Euphausian Nyctiphanes couchi. Of these Cosmetira and perhaps Nyctiphanes will have been carried in Atlantic water. Eastern and Southern Kattegat. Kramp (16) gives as visitors introduced from the north:— Medusae Bougainvillea britannica, Laodicea undu- lata, Staurophora mertensii, Melicertum octocostatum, Mitrocoma polydiademata, and Cosmetira pilosella.
References. 1, Bigelow (1926); 2, Frost, Lindsay and Thompson (1934); 3, Paulsen (1910) ; 4, Farran (1911); 5. Damas (1905); 6, Kramp (1913); 7, Künne (1934); 8, Russell (1935); 9, Schmidt (1909); 10, Bow man (1923); 11, Apstein (1911); 12, Lucas (1933); 13, Gough (1905); 14, Damas and Koefoed (1907); 15, Kramp (1930); 16, Kramp (1927); 17, Damas (1909); 18, Gran (1902); 19, Huntsman (1919); 20, Kramp (1926). Fig. 5. Chief routes followed by planktonic immigrants entering the Gulf of Maine at different levels, immigrants at surface; ||||, immigrants at intermediate levels; =, immigrants at the deepest level. (After Bigelow 1926, p. 65, Fig. 33).
Channel. Thus a correlation appears to be shown tentative but it should be the aim of future research between the occurrence of S. setosa off Plymouth to produce a similar table in greater detail. and the presence of concentrations of Rhizosolenia Especial attention should be paid to a study in the southern North Sea studied by S a v a g e and of the life histories of some of the larger arctic- Hardy (1935) (R u s s e 1 1, 1935). When the At boreal species such as Sagitta elegans and Clione lantic flow from the North is strong Rhizosolenia limacina. It is characteristic of species with this styliformis and phosphate rich water is carried into wide type of distribution that they show quite the southern North Sea and S. elegans is pushed considerable differences in the size to which they westward in the Channel so that S. setosa appears grow and at which they mature both from place off Plymouth. to place and season to season. Our knowledge of From the knowledge now available I have the life histories of such species should then be drawn up in Table II a list of certain plankton brought together and comparisons made between animals and the water masses of which they can be different water masses. It will probably be regarded as indicators in certain regions. In this possible then to distinguish between different types list I have included only those animals that are of growth and maturity for the different waters. sufficiently large and easily identified to be A species such as Calanus jinmarchicus is also well practical indicators. At this stage the list is known to show similar characteristic variations ; — 19 —
Fig. 6. The Lofoten area with the surface temperatures for March to April 1922. The darkened areas show the centres where rich hauls have been made during winter time for both species investigated, and to which the spawning of Calanus hyper boreus is limited. The area shaded is the cold area into which most numbers of Calanus hyperboreus, females and larval stages, are carried after spawning time,and where the main spawning of Calanus finmarchicus takes place (after Sømme, 1934, p. 9. Fig. 1). but if we are to fix upon indicators they must be been stressed by Gran and Kramp it is first practical indicators and therefore species that do essential that the life-histories and habits of the not require microscopic examination for identifica animals should be thoroughly known. tion or measurement. Owing to the use of small The different stages in the life histories of many silk nets in the International Investigations the of the commoner plankton animals of north Eu larger species were not caught in sufficient numbers ropean waters have now been described from the to show their utility. morphological point of view. For the life histories From the point of view of fishery investigations of the copepods we are indebted especially to such it must be realised that plankton indicators are of workers as Oberg, Grobben and W i t h, extreme importance. If correlations can be shown while L e b o u r has gone a long way towards to hold consistently for a number of years it should describing many of the planktonic larval stages of be possible to predict certain conditions in the bottom animals. But we still know very little of the fisheries. Large plankton indicators can be seen at habits and biology of the different developmental a glance and the delay necessary for the detailed stages in the sea. It is a remarkable fact that the analysis of water samples is avoided. actual yearly sequence in the succession of Broods It would seem that these larger plankton of our commonest plankton animal, Calanus fin animals should prove of much greater value as aids marchicus, was only demonstrated in detail for the to hydrography than the nannoplankton organisms. first time in 1933 by N i c h o 11 s at Millport. The In charting the distribution of water masses it is early work of Damas and Koefoed on the the boundaries along which the masses mix that copepods in northern waters has now been carried we wish to have demarcated. With their rapid rate considerably further by the work of Ruud, of reproduction nannoplankton organisms will Jespersen, and Sømme. The life story of quickly increase in numbers in water which has Calanus hyperboreus in Norwegian waters has been only been seeded with individuals carried either in made especially clear by Sømme. The length of wind borne surface layers or in small eddies at life and seasonal occurrence of many Hydromedusae intermediate depths. A false impression would thus have been worked out by Kramp in Danish be produced; whereas by the change in numbers waters. The growth and breeding of Sagitta elegans of the larger animals it should be possible to gain and Sagitta setosa are now known at Plymouth some idea of the quantities of water oT different (Russell, 1932—33). These are a few examples origin that have been intermingled. But as has (see Fig. 9) ; but still the number of species that
2* — 2 0 — have received such detailed attention remains sur used in the past. Hensen, when he introduced prisingly few. This is an aspect of zooplankton quantitative methods into plankton research, devised research that needs to be especially developed, for a net on lines which he calculated would give advance in other directions is still likely to be greatest efficiency. In his examination of the prevented by lack of this necessary knowledge. An fishing capacities of many different types of understanding of the biology of individual species plankton nets K ü n n e (1929 and 1933) has shown may throw quite unexpected light on obscure that Hense n’s type of net is still the most problems. For instance Sømme (1934) has efficient. With nets of coarse mesh for the capture shown how the differences between the horizontal of the larger plankton animals the filtration errors distribution of Calanus finmarchicus and Calanus are likely to be small if the plankton is not very hyperboreus in offshore Norwegian waters are the abundant. But with the finest silk required to retain results of slightly different habits of spawning and the small copepod nauplii the errors due to fil in the time of their vertical migration after the tration will be large. The filtration coefficient of winter months. After migrating from the deep the net varies widely according to the amount of layers the species are carried away in the surface plankton present and may be anything up to as waters “from the winter localities out over the high as 6 or over (K o k u b o and Tamara, coastal banks” (Fig. 6). 1931). But if the biology of plankton animals is to be We have therefore always hesitated to regard fully understood, it is necessary also to know their catches with fine meshed plankton nets as worthy relations with their environment. We must know of exact treatment. But recently a net has been their food supply, their enemies, and the nature of designed by Harvey (1934) which gives an the balance that exists in the community in which actual measure of the volume of water that has they live; and also their relations with the physical passed through the net. The meter can be used with and chemical factors of their environment. It is silk of any required mesh, and with simultaneous only by gaining this knowledge that we can study hauls it should be possible to use it for the the question of production of life in the sea, which calibration of large coarse meshed nets. As a result may be intimately bound up in one of the greatest of the introduction of this net it is now possible problems of marine biology, namely the causes of to examine the relations between the phytoplankton the fluctuations in the populations of our food crop retained by the net and the animals grazing fishes. When we consider the relations amongst the upon it. But the work can be carried a stage animals themselves we find that we have at our further. The effects produced by the ravages of command a considerable knowledge that should be predatory plankton animals on the plant feeding put to greater use in the future. Largely through population should be observed. Such animals as the researches of L e b o u r (1918— 1923) we have medusae, ctenophores, and chaetognaths, are notably a very detailed knowledge of the food of most of voracious, and unusual abundance of any of these our commoner plankton animals and young fish. will lead to a considerable decrease in the food The work especially of B u 1 1 e n (1908 and 1912), available for post-larval fishes as well as to the Jespersen (1928), Savage (1931), and destruction of the fish themselves, (Russell, Hardy (1924) has shown to what extent the 1935 b.). The balance of life would be soon upset different plankton animals are preyed upon by by a sudden increase in the numbers of these fishes. But the interactions of the plankton predators and the increasing demand for food as animals upon one another have been little they grew in size. explored. Lohmann (1908) by his great work As Hardy (1924, p. 33) has said “It is only in Kiel Bay gave a picture oi the total plankton by having some concrete representation of the production throughout a year under the conditions normal marine community that one can attempt to there existing. Since then the work has not predict the effect which an abnormal increase of been repeated in other regions. This is perhaps one or other particular fo rm —• may have largely because we have been awaiting the develop in the balance of the whole”. He produced an in ment of method. It is now possible to measure structive diagram reproduced here (Fig. 7) which plant production much more rapidly by chemical gives a good illustration of the lines on which this methods, and possibly we shall soon be able to say aspect of research should develop. the same for the measurement of total zooplankton When we see a sample of plankton caught from (see Cooper, 1934). But the necessity for a a given body of water at a given time we must study of the interactions between species still realise that this does not represent a static con remains, and this was not dealt with by Loh dition. We are looking at a cross section at one mann because life histories were insufficiently moment in one place of a continuous phenomenon known. We have also been held back by the lack of moving both in time and in space. The sample is methods for comparing the numbers of plants and the result of past history and possesses the animals from known volumes of water. There has potentiality for future history. It will consist of a been great want of uniformity in the types of nets population of producer plants, of plant consumers, f i g n Preliminary 'diagrammatic representation o f the relation of the Hernng to the Plankton Community as a whole. Links in the nutritive chains taken from the results, of other researches shown m dotted line.
VOUNG 1 2 -4 2 m m J HERRING ( 42-130 «M. ADULT HERRING
PLEdRO TOMOPTERf SAG1TJ X MEDUSAE BRACHIA
CAPOD CUMACEAE B A L A N U S MYSIOAE LARVA LARVAE.
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THALASSI ICUIA MELOS»« CENTRUM OSIRA
Fig. 7. Preliminary representation of the relation of the herring to the plankton community as a whole (after Hardy, 1924, Fig. 11). and of predatory carnivores. The population will that dissolved substances are taken up in significant be made up of the balance of plants composed of amounts. Indeed it is likely that all metazoan the original stock plus the sum of the increase by plankton animals depend mainly for their food reproduction and the decrease by death and con supply on particulate living or detrital matter and sumption; and similarly the balance of consumers on colloidal material (see G e 11 i s and Clarke, resulting from their interactions with the carnivores. 1935). Recent observations by Marshall, Orr, Bigelow (1931, p. 131) has rightly pointed out and Nicholls (1935) on the respiration of that “As yet we know little of the interrelationships Calanus finmarchicus have confirmed the earlier of different species or groups of animals in the observation by Ostenfeld and Knudsen sea beyond the obvious fact that some prey on (1913, p. 424) on Calanus hyperboreus which others, but we may be certain” he says “that in indicated that the figures put forward by P ü 11 e r many cases interrelationships of less obvious sorts were too high. P ii 11 e r estimated that in general are vital links in animal economy”. Calanus required as food 39 °/0 of its body weight But although we know something of the type of daily in summer. Marshall, Nicholls and food devoured by most plankton animals, it is Orr found that for stage V Calanus this figure lay difficult to visualise the course of events between between 6.2 and 7.6 °/0 in summer. In a recent successive samples because little is known about the survey of the phytoplankton and zooplankton col actual food requirements of the animals. The lected with H a r v e y’s measuring net at Plymouth theory of P ü 11 e r that plankton animals must it was concluded that the summer zooplankton per draw a large proportion of their food supply from cubic metre required daily 120,000 diatoms of dissolved organic substances in the water has re average size. The daily production of this quantity cently been critically reviewed by Krogh (1931). of vegetation could reasonably be expected. It is He concludes that “there is no convincing evidence” a quarter of the average diatom population caught — 22 —
in the measuring net during this period, a net which is most necessary to explain some of the types of undoubtedly lets many very small plant organisms behaviour that we now know to be shown by pass through it. This is in agreement with Loh- animals in the sea. Perhaps in no way does the ma n n ’s conclusions for Kiel Bay that during the plankton animal give indications of changes in spring and summer months the producer plankton physiological state better than in its habits of was in excess. The problem of the food require changing vertical distribution. The migration ments of plankton animals is open to attack in two towards the surface at night is our first indication ways. Direct observation may be made in the of direct reaction to some external or internal laboratory on the amount and type of organisms stimulus. We now know that not only are there that any species will eat. Work of this nature has these nightly wanderings, but that the animal also already been started at Woods Hole by Clarke exhibits changes throughout its life history accord and Gel lis (1935) and also in Professor A. C. ing to its age. At times also the animals become H a r d y’s laboratory at Hull. Such work indeed as apparently irresponsive to external changes; at the notable research on the rearing of planktonic different times of year we find the same species crustacean and molluscan larvae by L e b o u r behaving in a different manner, pointing to (1924—34) and polychaete larvae by Wilson physiological differences between different broods. (1928—33) at Plymouth was originally made The external factors affecting the behaviour of the possible by the work by Allen and Nelson animals as shown by vertical distribution, namely (1910) on the culture of marine diatoms, which salinity, temperature, oxygen content, and hydrogen are now used so largely as food in rearing ion concentration have been investigated in the past. experiments. On the other hand observations may It is only recently, however, largely through the be made on the rate of respiration under controlled pioneer work of Poole and Atkins, that wTe conditions in the laboratory. In order that this have obtained the means of determining the second method can prove applicable to conditions variation in intensity and composition of that most in the sea it will be necessary to know the weights important factor, light. In the vertical distribution and chemical composition of the different plankton of the same species in regions many latitudes apart organisms. Such work is already started and was there is no doubt that the main controlling factor foreshadowed many years ago by B r a n d t (1898). is that of temperature. If an animal is adapted Observations on the weight and chemical com to live within a certain range of temperatures the position of a number of plankton crustacea have lethal effects of temperatures above this range and already been made, notably by Orr, W i m - the temperatures limiting successful reproduction penny, and Klem (1929). The vitamin content must naturally overweigh the possible disadvantages of plankton oils is also being investigated (Drum that may arise owing to abnormal light conditions. mond and Gunther, 1934). Studies on the But the changes shown from day to day and season weights of copepods by Bogorov (1932— 34) to season in any one place are undoubtedly see the inauguration of an extensive programme on manifestations of physiological change which af the part of Russian biologists to assess the total fects the animal in its response to light conditions. quantity of life in the sea at any time from quanti Until the post war period our knowledge of the tative counts on the basis of a knowledge of the vertical distribution of plankton animals was weights and composition of the different species. largely confined to the gross differences shown by At first sight it would appear that the task of the results of collecting from widely spaced depths weighing and analysing all the plankton animals is in ocean waters, and the conditions in coastal likely to be excessive. Indeed it is not so great as waters had not been systematically studied. Yet it appears, the number of species are limited and the general facts of daily and seasonal migrations once the necessary data have been obtained for any were fully recognised. In recent years the study of one species the results are final and need but to be this problem has been carried out on a finer scale tabulated for future reference. These results could in shallow waters (Russell 1925— 1934), and no doubt be further simplified by classifying the similar work in deep ocean waters is now being species into size categories somewhat after the style done at Woods Hole (Leavitt, 1935). As a of H e n t s c h c l’s (1934) recent suggestion. Ad result of the accumulated knowledge and ideas of mittedly the work is not exciting to the zoologist. past workers and of my own researches at Plymouth But the various attempts to obtain the necessary it became possible to draw hypothetical diagrams information such as the calculation of displacement showing the distribution to be expected at any by volume of models by Lohmann (1908), and time of day of animals requiring optimum light the recent suggestions by Gunther (1934) and intensity conditions and migrating under the sti Hentschel (1934) emphasize the need for mulus of light (Russell, 1927). It can be agreement on some form of standard data. realized that by varying the scales of length of It will be suitable at this stage to draw attention day, strength of light, depth of optimum, trans to the need for more experimental work on the parency of the water, and swimming speeds of the physiology of plankton animals. Such knowledge animals any number of such diagrams can be drawn — 23 — to fit the different kinds of movements. Support in being carried further by Clarke (1933 and favour of this theory has come from field work. 1934 a) at Woods Hole with simultaneous The behaviour of female Calanus finmmcliicus as observations of distribution and light conditions shown by N i c h o 11 s (1933) fits the hypothetical (Fig. 8->. diagram to a remarkable degree. The study is now But it is no longer agreement with theory that
15 18 21 24 4 8 12______15
1 0 -
2 0 - 30- f T T T Y T t f .ÊltTTT16 19 22 1 t 4______ft7______10______13 ♦______t,1_6______JULY 10 SUNSET SUNRISE 15 4 5 10 15 C t 1000
16 19 22 4 7 10 13______16
4 0 -
8 0-
1 2 0
UÏ iituiFig. Vertical Distribution of Plankton Animals. in the Gulf of Maine, July, 1932. The changes in light intensity are indicated by the lines representing the 1. Hypothetical vertical distributions at different times in depth at which 1000, 100 microwatts etc. occurred (one the twenty-four hours to illustrate the behaviour of a microwatt may be taken as approximately equal to population of a species such as Calanus finmarchicus three metre-candles). The temperature curve at the in its diurnal movements, (after Russell, 1927, p. 237, right shows that migration right to the surface at night Fig. 4). was prevented by the thermocline (after Clarke, 1933, 2. Diagram showing vertical distribution of female Calanus p. 426, Fig. 5). finmarchicus at 3-hourly intervals during 24 hours on 4. Diagram showing vertical distribution of Stage \ January 25th—26th, 1932; sunset 4.27 p.m .: sunrise Calanus finmarchicus at 3-hourly intervals on Jan. 8.57 a.m . (after Nicholls, 1933, p. 150, Fig. 4). 25th—26th, 1932 (after Nicholls, 1933, p. 148, 3. Diurnal migration of adult females of Metridia lucens Fig. 3). — 24 -
we wish to observe. It is by seeking for disagree species survives the winter in these deep layers ments that advances are made. For it is where near the coast undergoing migrations towards the behaviour is contrary to this theory that we shall surface layers after the winter (see Fig. 8). find the clues to the differences in the physiological Sømme also remarks that near Tromsø these state of the animals themselves. species do not undertake extensive daily vertical It has been mentioned previously that at times migrations during the winter time. Beebe (1934) the animals appear to be irresponsive to light in his dives in the bathysphere below the photic changes. A very good example of this has been zone has remarked that in the regions of perpetual shown for the fifth copepodid stage of Calanus night the plankton and pelagic animals did not finmarchicus (Nicholls, 1933). The possible appear to show those phototropic responses to his significance of this state is worth consideration. search light so characteristic of animals of the sur The gonads of a copepod do not usually mature face fauna. Possibly these deep sea organisms are until the moult into the adult stage is completed. permanently irresponsive; but if this were so it The fifth copepodid stage is the final stage of would be difficult to understand the function of immaturity and immediately on developing into the the light emitting organs; it may have been that aduk stage the gonads start to ripen. After moult his search light was too powerful, or that the ing into the adult stage or possibly even im animals are only stimulated by rays from phosphor mediately before this moult the copepod becomes escent light. once more _ responsive to light conditions and The habit of some plankton animals to seek the exhibits typical migrational movements. After the deeper levels in winter is possibly quite compar successive production of the spring and summer able with that of hibernation of animals on land. broods, the offspring surviving from the final Indeed this habit may be largely responsible in brood have to pass the winter months. It is in this deciding to what degree an animal shall be oceanic. irresponsive fifth stage that Calanus exists for the Unless an animal can find the required depth in long period from August until the following the winter months it will not survive. Thus we can February, when the final moult into the adult takes draw the distinction between an animal that can in place. These animals thus have to survive a period certain coastal regions find sufficiently deep water when there may be a prolonged deficiency of their such as Calanus finmarchicus, which dies down to diatom food. O r r (1934; Marshall, a meagre band of survivors during winter in shal Nicholls, Orr, 1934) has shown that the low waters; one such as Calanus hyperboreus which Stage V individuals of Calanus finmarchicus are requires greater depths but may still be found in loaded with fat in the late summer. They must the deepest fjords; and an animal which is possibly therefore live to a certain extent on these reserves. even more oceanic such as Aglantha the last sum But they will obviously benefit if they can reduce mer brood of which may not be able to survive the their metabolic activity to a minimum. A daily winter in shallow water. Kramp (1927, p. 160) wandering up and down under the stimulus of in discussing the occurrence of Aglantha in the Belt changing light conditions will be unnecessary waste Sea says: “I think myself it is rather the slight of energy when no supply of food is being produced depth of this water which renders it less suitable in the surface layers. Light itself may have an as a permanent habitat for Aglantha”. Such oceanic effect upon the rate of metabolism, although the animals when brought once more into coastal results of Marshall, Nicholls and Orr waters after their vertical migration in the early (1935) seem to show that it is doubtful whether months of the year can reproduce and thrive under this effect will be felt much below a depth of five coastal conditions so long as the summer lasts; but metres. One of the most obvious ways for a during the hibernation period they must seek the plankton animal to reduce its expenditure of energy depths or perish. is thus by seeking the deeper layers and becoming It must not be overlooked also that at times irresponsive to the stimulus of changing light con certain plankton animals may live actually on or ditions. In this respect the observation made by in the bottom (Clarke, 1934, b ; Russell, Sømme (1934, p. 81) that copepods kept in the 1932—33). In Figure 9 are shown typical dark are dull and sluggish compared with those sequences of life histories. exposed to subdued light seems very significant. Enough has been said to show how a full know Evidence is not wanting that many plankton ledge of the biology of the different species of animals seek deeper layers in winter. It was long plankton animals is essential to explain the ago shown by Gran (1902) and Nordgaard differences in their abundance and distribution from that Calanus finmarchicus might be found in the time to time. In this respect it is not sufficient to winter in the deep water layers at the bottom of base conclusions on results obtained in another the Norwegian fjords. This habit has been locality. It has been found that the same species of examined more fully by S ø m m e for Calanus fin animal appears to show a different behaviour in marchicus and C. hyperboreus, and it has been one region from that which it shews in another. shown that a considerable stock of both these For instance in the waters off Plymouth the brood — 25 —
CALANUS FINMARCHICUS
STAGE V
BROOD WINTER SPENT IN DEEP WATER
CAIAKUS HYPERBOREUS
SPENT FEMALES
WIKTEE SPENT STAGE IV AND OVER IU BEEP WATER
SAGITTA ELEGAHS
AGLANTHA
WIHTER SPENT IU DEEP WATER
PHIAXIDIUM HEltl SPHERI COM
HIBERNATION OF HYDROID
HYDROID LIBERATING MEOTSAE
Fig. 9 (as to legend: see following page). — 26 —
of Calanus finmarchicus maturing in July lives very the delay in the development of the sexual organs much nearer the surface in the daytime than do the shown in the last summer brood of many plankton preceding broods. Whereas in the Clyde Sea area animals — that is the brood which survives the (Marshall, Nicholls and Orr, 1934) and winter. It seems almost that we must regard the in the North Sea (Gardiner, 1933) the brood successive annual production of broods as compar maturing in May appears to live nearest the sur able with the annual breeding cycle of an animal face. This seems to point to some essential that may live for a number of years. That is, after difference in the animals themselves. a prolonged period of reproduction a period of The seasonal behaviour and production of recuperation is necessary. In temperate waters this successive broods of plankton animals needs to recuperative period falls apparently for most be studied in polar and tropical regions, and also species during the winter months. May it not be in the deep waters of the ocean. The stimulus that some essential factor has been removed from of external temperature conditions has been largely the water during the summer which becomes avail regarded as responsible for deciding the time of able again only after a period of time or is perhaps onset of maturity and spawning. It is however lacking in the diminishing food supply? difficult to understand how this factor can decide
Fig. 9. Diagrammatic representation of the life histories of Calanus Minimum length. finmarchicus, Calanus hyperboreus, Sagitta elegans, Aglan Loch Striven 2.26 (28.viii.33) 0.135 tha and Phialidium hemisphaericum. These are examples Plymouth 2.461) (26.vii.26)3) of species in which several broods are produced during the 2.261) (16.X.30) 0.102) year, and of a species which produces only one brood a year. Sagitta elegans and Calanus finmarchicus are Arctic- The copepods grow larger in cold waters than in boreal species. The diagrams represent their life histories warm, e. g. Bogorov gives for Barents Sea : under boreal conditions. It is probable that in Arctic waters Females. Average total length. Temperature. they produce only one brood a year as the arctic species C. hyperboreus. In the diagrams the period of full maturity 69°30' N — 75° N. 3.22 mm. 4.7° C. is indicated in black and the state immediately preceding 75° N. — 77° N. 3.78 1.08° maturity is shown by shading. This latter period cor North of 77° N. 4.57 —0.50° responds to copepodid Stage V in the copepods and the Adler and Jespersen (1920) and Jespersen ripening of the male sexual organs in Sagitta. (1934) have given the following variations in length of The height of the diagram for each brood give an cephalo thorax indication of the size to which the adults grow, and above Eastern North Sea ...... ca. 1.9—2.7 mm. each brood is given a life size representation of the animal West Greenland Waters ...... 2.4—4.5 itself. The noteworthy feature is the prolonged existence in the final immature state during the winter and the 1) Originally given as total length, converted into retirement to deep water. cephalo-thorax length by factor 0.8 (see Clarke 1934 A. Succession of Broods of Calanus finmarchicus. This is p .0 0 ). based on the results of Marshall, N i c h o 11 s' and ) Formalin preserved material; there will have been some Orr (1933—35) at Loch Striven. There are three main slight loss in weight. breeding periods in the year covering the periods Fe 3) No observations later than Sept. 22nd. bruary—March, April — May — June, July — August. Spawning does not take place with the same intensity The percentage of fat is highest in Stage V: in 1933 throughout a whole period, but in any one year it is the autumn winter stock in September had 6 °/o more fat likely to be most intense in one month, e. g. February, than the corresponding individuals in the previous May and July. Individuals of the first brood, developing January to March. during the coldest period of the year grow to the largest Sømme has shown that in Norwegian waters the size and weight. Thereafter they decrease in size as the majority of C. finmarchicus spend the winter at depths temperature rises. The first born in any brood are greater than 200—300 metres. After the winter there is generally the largest for that brood. an annual vertical migration completed about the middle In 1933 the last brood which was hatched mainly of March ; development of the eggs takes place after this in July had established the autumn-winter stock of vertical migration. Stage V by August 14th. These persist mostly as Stage V and moult into adults in January and February. B. Seasonal cycle of Calanus hyperboreus. This is based The work of Farran (1927), Russell, and on S 0 m m e’s (1934) results for Norwegian waters. Bogorov (1934, a) shows that this sequence is There is only one brood in the year. Spawning essentially the same in the waters south of Ireland and takes place in February and March. The earliest in the western end of English Channel. developmental stages are passed through by April, after Maximum length. which speoimens earlier than copepodid Stage IV are not found. A few females start to ripen in November, Females. Median length of cephalo-thorax. Dry Weight. but the majority do not do so until mid-December and Loch Striven January. The males are found only between December (Marshall-Orr) 2.76 mm. (24.iv.33) 0.28 mg. and the beginning of May, mostly in January and Plymouth February. After spawning in February and March some (Russell) 2.821) (19.V.26) of the spent females may survive until the following (Bogorov) 2.891) (22.V.30) 0.2522) August. — 27 —
The winter is passed in deep layers mostly below The offspring arising from the September spawning 200—300 m. and deeper than C. finmarchicus. The do not start to mature until December when the male annual vertical migration after the winter months is sexual organs start to ripen. The female sexual organs completed by the middle of April; the development of do not start to ripen until January. There is an in the eggs takes place before this vertical migration. dication that there is a descent to deeper water during As there is only one brood there is no seasonal the winter. dimorphism in size. D. Probable succession of broods of Aglantha digitalis var. The copepods grow larger in cold waters than in rosea. This is based on results of Russell at Ply warm, e. g. Bogorov gives for the Barents Sea :— mouth (unpublished). Development is direct, without a hydroid generation. Large individuals 12 to 18 mm. in height, which have survived the winter living probably Females. AverageLen»th ^et Temperature. in deep water, spawn in February and March. The 69°30' N.— 75° N. 6.0 mm. 4.0 mg. 4.7° C. adults of this first brood grow to about 6 or 7 mm. in 750 N.—77° N. 7.1 8.6 1.08 height. During the summer months successive broods of North of 77° N. 8.0 13.4 —0.50 small individuals, 5 or 6 mm. in height, are produced. There is an indication at Plymouth that there are at least three such broods.. The offspring from the last A similar type of life history is shown by CalarMs spawning in August or September grow into the larger tonsus (Campbell, 1934) in the Strait of Georgia 011 sized individuals which survive the winter and spawn the Pacific Coast of North America. the following February. C. Succession of broods of Sagitta elegans. This is based E. Biology of the Medusa Phialidium hemisphaericum in on the results of Russell (1932—33) at Plymouth. the plankton. Based on results of Russell (un There are probably four main breeding periods published) and Orton (1920) at Plymouth. during the year, i. e. February, May, June—July, and The medusae are produced from the hydroid Clytia September. Individuals of the first brood developing at johnstoni. The hydroid starts producing medusae in the coldest time of the year grow into the largest adults. numbers in March. The medusae liberated early in the Thereafter they decrease in size in the successive broods year grow to a large size, individuals being found in as the temperature rises. May up to 19 mm. in diameter with as many as 32 tentacles. During the summer the successive adults are Approximate average length of adults:— smaller and smaller until in September they have a May (1930) ...... 19.5—20 mm. diameter of only 5 or 6 mm. with usually about 16 June—July ...... 13 — 14.5 tentacles. In November the hydroid polyps die down September ...... 10 —10.5 and the hydroid “hibernates” until the following March. February (1931) ...... 12 — 12.5 (Compare also, Kramp, 1927).
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