The Functional Morphology and Ecology
THE FUNCTIONAL MORPHOLOGY AND ECOLOGY
OF THE SPATANGOID GENUS BRISASTER GRAY
by
PETER EDWIN GIBBS
B.Sc, University of Leicester, 1961
A THESIS SUBMITTED IN PARTIAL FULFILMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF ARTS
in the Department
of
Zoology
We accept this thesis as conforming to the
required standard
THE UNIVERSITY OF BRITISH COLUMBIA
April, 1963 In presenting this thesis in partial fulfilment of the requirements for an .advanced degree at the University of
British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that per• mission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives., It is understood that copying, or publi• cation of this thesis for financial gain shall not be allowed without my written permission.
Department of Zoology
The University of British Columbia, Vancouver 8, Canada.
Date April 1963 ABSTRACT
The functional morphology, taxonomy and ecology of the Spatangoid genus Brisaster Gray (Family Schizasteridae) from Howe Sound, British
Columbia, have been investigated.
Brisaster predominantly inhabits a mud substratum and burrows to a depth of 1 cm., constructing both a respiratory funnel and a double sanitary apparatus. Burrowing and feeding activities resemble those of Spatangids. The absence of a sub-anal fasciole in Bri saster correlates with its shallow burrowing habit. Despite the lack of a sub-anal fasciole, the ciliary current pattern of the test is similar to that of the Spatangids which possess this fasciole. This suggests a common ancestral form (perhaps for all Spatangoids) in which the basic ciliary pattern had evolved; thus the different types of fascioles appear to have evolved as superimpositions on the basic ciliary pattern rather than the reverse.
The zoogeography and taxonomy of the genus is briefly reviewed and the need"for qualification of certain taxonomic criteria stressed.
The latero-anal fasciole, for example, is an unreliable taxonomic character since it disappears with growth to varying degrees in diff• erent species. This possibly reflects changes in niche since the isolation and speciation of the genus took place.
Biometrical analyses of the lengths of the ambulacral petals and the test height indicate that only a single species of Bri saster is present along the west coast of North America. Formerly two species had been described. Taxonomic priority is given to the species i i i
Brisaster latifrons; B. townsendi thus becomes a synonym. Synonymy is also suggested for the Japanese species B_. owstoni, in view of paleo- geographic evidence.
The gonads of Brisaster do not develop until the second year and, as the ova appear to require two years to develop to maturity, the females probably first spawn in their third year. On this basis, the first, second and third (and older) year classes were identified in the populations occurring in Howe Sound. These populations showed marked differences in their size-frequency distribution but a similar age class composition. The size differences of individuals appear to correlate with differences in the population density, larger individuals being found in less crowded areas. It is suggested that these density differences are a result of the irregular settlement of a restricted pelagic larval stage. The differences in the size of individuals can therefore be related to differences in the individual growth rate.
Passive interference, both inter- and intraspecific may be responsible and two possible mechanisms have been suggested. i v
TABLE OF CONTENTS
Page
INTRODUCTION ...... 1
CONDITIONS IN HOWE SOUND 3
MATERIALS AND METHODS 4
RESULTS 9
IA Morphology of Brisaster .. 9
IB Observations on the mode of life of
Brisaster .. 12
1. Ci1iary currents 12
2. Burrowing activity 14
3. Feeding activity 15
II Biometrical analysis .. .. 16
III Ecological investigations 20
DISCUSSION .. .. 27
Morphology and mode of°life 27
Taxonomy and zoogeography 30
Ecological studies 36
SUMMARY .. .-. 43
LITERATURE CITED 46
APPENDIX 50 V
LIST OF TABLES
Table Page
1 Approximate numbers of the various types of tube-feet
•n Brisaster, at two test lengths 11A
2 Analysis of covariance for mean lengths of anterior (II
and IV) and posterior (I and V) ambulacral petals of
individuals from three populations (Stations 1A, 3 and
6) used in the biometrical analysis. Data presented in
Fi gure 6. .. 17B
3 Length (mean and range) of individuals, and slopes of
regression lines calculated for samples of three popula•
tions (Stations 1A, 3 and 6) used in the biometrical
analysis .. •. 18A
k Analysis of covariance for height and length of indiv•
iduals from three populations (Stations 1A, 3 and 6)
used in the biometrical analysis 19A
5 Analysis of covariance for area of peripetalous fasciole
and size number (length x width) of individuals from two
populations (Stations 3 and 6) used in the biometrical
analysis. Data presented in Figure 8 208
6 Details of samples collected from stations at two
depth strata - 135 and 65-75 fathoms 22A
7 Analysis of variance for length of individuals from
three populations (Stations 1, 2 and 3) 23A vi
LIST OF TABLES, cont'd.
Table Page
8 Statistical comparison of lengths of individuals in
three populations (Stations 1, 2 and 3) 23B vi i
LIST OF FIGURES
Figure Page
1 Chart of Howe Sound, British Columbia, showing dredging
stations .. S'
2 Positions of measurements taken on the test and used in
biometrical analysis. 6A
3 Main features of the test of Brisaster 9A
k Course of the main ciliary currents on the test of
Brisaster 12A
5 Burrowing of Bri saster in (A) lateral and (B) oral
view. . 13A
6 Relationship between mean lengths of anterior (II & IV)
and posterior (I & V) ambulacral petals in Individuals
from three populations (Stations 1A, 3 and 6) 17A
7 Relationship between the mean lengths of the anterior
(II & IV) and posterior (I & V) ambulacral petals and
size number (length x width). 18B
8 Comparison of the area of the peripetalous fasciole
against size number (length x width) in individuals
from two populations (Stations 3 and 6). 20A
9 Length-frequency of all samples collected in Howe
Sound on July 12, September 10, and October k and 5,
1962. 21A vi i i
LIST OF FIGURES, cont'd.
Figure Page
10 Ovarian structures in Brisaster 24A
11 Percentages of size groups for ova diameters in second/
and third year (and older) females 25A
12 Length-frequency of year classes of females, as indicated
by ova diameters, in three populations (Stations 1, 2
and 3). 26A
I LIST OF ABBREVIATIONS USED IN FIGURE 3.
a.tf. anterior sensory tube-feet
al.tf. anterior lateral sensory tube-feet
an. anus
an.fasc. anal part of latero-anal fasciole
f.gr. feeding grill of small spines
fb.tf. funnel-building tube-feet
g.p. gonopore
lat.fasc. — lateral part of latero-anal fasciole
mad. madreporite
mo. mouth o.tf. -- oral feeding tube-feet
peri.fasc. — peripetalous fasciole
plas. plastron
pl.tf. posterior lateral sensory tube-feet
pp.tf. -- peri-plastronal sensory tube-feet
pr.p. periproctal plates
prot.s. -- protective arch of spines covering ambulac prot.s.an. .-- protective spines covering anus ps.p. peristomial plates
r.tf. respiratory tube-feet san.tf. sanitary tube-fuilding tube-feet san.t.b.s. -- sanitary tube-building tuft of spines sp.s.p. spatulate spines of plastron X
ACKNOWLEDGMENTS
The author is indebted to Dr. Paul A. Dehnel for his advice and assistance in the planning and execution of this study. Gratitude is also expressed to Dr. Dennis H. Chitty, Dr. Cyril V. Finnegan and
Dr. William S. Hoar, for their critical reading of the manuscript.
The author is also indebted to Capt. Dale-Johnson and the officers and crew of the C.H.S. "Ekholi" for their skill and patience in operating the dredge, sometimes under difficult conditions. The generous and able assistance of many people, especially Mr. Thomas H.
Carefoot, in the preliminary sorting of the catch on board ship is gratefully acknowledged.
This study was carried out while the author Was in tenure of a
N.A.T.O. Studentship given by the Department of Scientific and
Industrial Research, London, England and the research was aided by grants from the National Science Foundation of the United States and the National Research Council of Canada. 1.
INTRODUCTION
Recently, Nichols (1959a, b), in an extensive investigation of the morphology and mode of life of the irregular echinoids, particularly
the Order Spatangoida, has demonstrated the importance of a functional
interpretation of those morphological characters used as taxonomic criteria. Similarity in the conspicuous features of the text, such as the fascioles, need not indicate phylogenetic affinity: the possibility of convergence in different stocks must be appreciated. Nichols1 study on the Spatangoida was confined to members of the Family Spatangidae, both living and fossil. Clearly, further investigation of other groups
is necessary before the results can be extended to an elucidation of the phylogeny of the Order Spatangoida. Part of the present study, therefore, is an attempt to interpret the morphology of the genus Brisaster, a typical member of the Family Schizasteridae, in terms of its mode of life, relating also the observed morphological changes occurring during its ontogeny.
Those Spatangoida grouped in the Family Schizasteridae all possess both a peripetalous and a latero-anal fasciole. Little is known about their biology, possibly because they are, typically, deep-sea inhabitants and many genera are represented only in Antarctic waters. The only representative of the Family Schizasteridae along the west coast of
North America, north of the Gulf of California, is the genus Brisaster
(formerly named Schizaster by Agassiz, 1898) which has a wide latitudinal distribution, extending from the Gulf of Panama (type locality) to
Alaska and Japan, at depths varying from 6 to 995 fathoms. The number 2.
of species represented in this geographical area is uncertain.
Both Clark (1917) and Mortensen (1951) have attempted to recog•
nize two species of Brisaster on the west coast of North America —
B. latifrons and B. townsend1 — using the characters on which Agassiz
(1898) originally divided the type material. These characters are
highly variable and the difficulties of identification have been explained
on the grounds of Intergradation and hybridisation between the two species.
In an attempt to resolve this problem, a biometrical analysis has been
carried out and the result considered from ecological, zoogeographical
and evolutionary viewpoints.
During preliminary investigations, it was observed that the
populations of Brisaster in different areas of Howe Sound, British
Columbia, exhibit marked differences in abundance, distribution and the
size of individuals and therefore offered an opportunity to obtain cer•
tain ecological data. The paucity of previous research concerning the
ecology of Spatangoids as a whole made a study of this type highly
desirable.
The investigation was, therefore, divided into three interrelated
areas of study, (I) an investigation of the morphology and mode of life of Brisaster. (ii) a biometrical analysis to establish the taxonomic
status of the specimens under consideration, and (iii) a preliminary
investigation of the ecology of Bri saster.
P 3.
CONDITIONS IN HOWE SOUND
Howe Sound is a fjord with steep-sloping sides and irregular bottom (Fig. 1). The main deposit is chiefly thick mud of glacial origin. At the mouth of the inlet above 50 fathoms muddy-gravel and stone sediments are found. The maximum depth of the Sound Is about 135 fathoms.
The prevailing hydrographic conditions are remarkedly constant below a depth of 10 fathoms and, at the depths from which Brisaster were dredged, 1 ittle variation in salinity and temperature occur. The data available (in the Data Reports of the institute of Oceanography,
University of British Columbia and also from data kindly supplied by
Dr. H. Waldichuk of the Pacific Biological Station, Nanaimo, B.C.) indicate that over the year, below 50 fathoms the temperature varies between 7.5* and 9.5°C and the salinity ranges from 30.6 to 31.3°/oo.
These conditions appear constant over the whole study area. MATERIAL AND METHODS
Collection of samples
All samples were collected from Howe Sound, British Columbia at various stations (Fig. I) with a depth range from 48-135 fathoms.
Samples were taken with a heavy naturalist-type dredge with a rectangular mouth frame 48 inches long and 18 inches high. Whichever way the dredge landed on the bottom, samples were taken, as both of the long sides of the frame were equipped with digging edges. The net was double, with an outer net of i inch bar, and an inner lining of £ inch bobbinetting.
Both of the lower ends were open and tied with a cod-end knot. Consider• able washing of the sample occurs at deeper stations and to prevent loss of the smallest animals the fine-mesh lining was necessary (as recommended by Holme, 1961).
All samples were passed through a series of sieves down to 2 mm. opening and the urchins removed. When the volume was very large it was convenient to reduce the amount of deposit in the net by washing the sample before bringing on board. At the three main collection areas
(Station 1, 2 and 3) the complete fauna was collected (for details see
Appendix). Animals were preserved in 5% formalin.
It was found impossible to standardize dredging technique because of the great depths involved in the deepest hauls, and in some cases, difficult weather conditions. The sample volume varied considerably and a qualitative evaluation only was possible. Quantitative sampling was not convenient as large numbers of Brisaster were required for analysis and, as the density of Brisaster is low in some areas (between 65-75 fathoms for example), collecting by dredging was far more profitable in the limited V 4A.
Figure I. Chart of Howe Sound, British Columbia, showing dredging stations. Bottom sediments and depths (in fath• oms) are indicated. 30'N
STATION I 135 f. MUD ^/;'"[)PASSAGE
-r——•—^ 49°20'N
\ i j STATION I2A STATION IA A,f MUD.ST0WS| STATION 1 1 125 f. MUD 6RAVEL 0 12 3 75f. MUD NAUTICAL MILES STATION I2B| i MUD.STONES 50 FATHOM CONTOUR I72'- GRAVEL 100 FATHOM CONTOUR I23°20'W 5.
time available. With respect to the size-frequency distribution between
successive samples taken from one area, the sampling technique proved
remarkedly consistent.
Positions and depths were read from the Canadian Hydrographic
Service Chart 3586; slight discrepancies in depth were adjusted using
the echo-sounder and read to the nearest fathom.
Aquarium observations
Behaviour was observed in an aquarium with circulating sea water
(30°/oo salinity and 8°C temperature - simulating field conditions) and
a 3-inch bottom layer of mud from Station 3.
The urchins were dredged from 135 fathoms (Station 3) in October, when vertically isothermal conditions prevail in the Sound. (Two previous attempts to maintain the urchins, in May and July, failed, probably
because they were subjected to a sharp rise in temperature on being
brought up through the thermocline in the surface layers of the water.)
The urchins were transported to the laboratory in thermos flasks con•
taining water with a salinity of 30°/oo and a temperature of 8°C.
Abrasion of the specimens was prevented by cushioning with pieces of netting. In this manner, the urchins were obtained in good condition, and, without exception, all urchins introduced into the aquarium began burrowing actively.
For observations on the ciliary currents of the test, the urchins were placed in a shallow dish of sea water and observed under a low power binocular microscope. The currents were followed using fine carmine particles. Illumination was kept as low as possible in order to maintain a low temperature (about 8°C). 6
Biometrical analysis
The following measurements (Fig* 2) were made on dried and cleaned
tests of 2**2 specimens taken from three stations (1A, 3 and 6) in July,
using vernier calipers, reading to an accuracy of ±0.1 mm.:- (a) length
(b) width (c) height and (d, e) the length of the petaloid regions of
the ambulacra I, II, IV and V (using Loven's scheme, 187*0* The length of the test was measured from the anterior groove to the posterior end;
the asymmetry of the test makes this measurement, rather than the total
length, easier to perform and reproduce. The height was taken from the mid-ventral line of the plastron to the apical system and the lengths of the petaloid regions from the ocular pore to the oral end of the petal.
The area of the peripetalous fasciole was also measured by running a planimeter along the centre of the fasciole to establish its length (f) and two measurements were taken of its maximum (g) and minimum (h) width, at the end of the petaloid area (Amb. I in this case) and in the inter- ambulacra I region (between Ambs. I and II) respectively. The mean of
the latter two measurements multiplied by the length was used as a value for the area of the fasciole. The product of length and width was used as a measure of the size of the urchins.
Statistical methods
Linear regression lines were calculated, using the least squares method, for each of the mensural characters and for each of the three populations investigated in the biometrical study. Using analysis of covariance, the slopes of the regression lines for each character were tested for significant differences between populations. Where the resultant variance ratio (F) was insignificant, the adjusted means test 6A.
Figure 2. Positions of measurements taken on the test and used in the biometrical analysis. The ambulacra are numbered according to Loven's (187*0 scheme in (A). (B) LATERAL VIEW 7. was applied (as given in Steel and Torrie, I960). The correlation coefficient (r) is indicated in the results where applicable.
In comparing samples from the three main populations studied, an analysis of variance was used to test for significant differences in the average size of individuals. Student's "t" test was used to compare samples between and within the two depth strata investigated.
In all tests, significance was taken at the 0.05 level, the 0.01 level was considered highly significant.
Histological technique
The ovaries were removed and fixed with 5% formalin in sea water.
To reduce the brittleness of the yolky ova in sectioning, the material was transferred from 95% alcohol to methyl benzoate during dehydration
(Pantin, 19**8) and then embedded In paraffin wax (M.P. 56°C). Serial sections were cut at 10/VJ and stained with Erhlich's baematoxyl in, using eosin as a counters tain.
The ova were measured using an ocular micrometer, reading to the nearest 0.005 mm. Ova are highly irregular in shape and to obtain an estimate of their diameter, maximum and minimum diameters were measured and a mean taken. In the ovaries with large mature ova only a few of the latter are sectioned through the centre in any one section and, as a measurement of the maximum diameter was desired, only those mature ova sectioned through the germinal vesicle were measured. This method is not completely accurate as the germinal vesicle is usually eccentrically placed in echinoids (Harvey, 1956). Quantitatively, this means that the larger size groups are underestimated in the percentages given below
(Fig. II), which are based on random counts of a number of sections. 8.
However, as the size range of the ova is the primary interest, this
underestimation of the mature ova numbers need not affect the main points in discussion. RESULTS
I A. Morphology of Brisaster
Main features of the test: the test outline of Brisaster is heart-shaped with an anterior groove formed by Ambulacrum III which is deeply sunken with angular sides on the aboral surface (Fig. 3A)• The other four ambulacra are paired and aborally are sunken into petatoid shaped areas (petals). An important characteristic of Brisaster is the shortness of the posterior pair of petals (Amb. I and V) as compared to the anterior pair (Amb. II and IV).
Two types of fascloles are present. These are areas of small spines, termed clavulae, that are heavily ciliated and augment the ciIiation of the test surface. The peripetalous fasciole circumscribes the petaloid regions of the ambulacra on the aboral surface and has
re-entrant angles in the interambulacral areas (Fig. 3A). The latero- anal fasciole is found completely developed in only very young specimens generally less than 10 mm. in length (Fig. 36)• The lateral part of this fasciole soon disappears with growth, leaving only the anal part
(Fig. 3C) which, usually also disappears In very large specimens (above
55 mm. in length).
Tube-feet: in contrast to the regular echlnoids, the tube-feet of Spatangoids show a marked division of labour in different parts of the ambulacra and five types can be identified:- aboral funnel-bui1 ding sub-anal sanitary tube-building, oral feeding, respiratory and sensory.
Each of these types possesses a different histological structure which reflects their function (Nichols, 1959c), although there may be super- 9A.
Figure 3. Main features of the test of Bri saster.
A-C: Positions of the specialized tube-feet in different
parts of the ambulacra, and the form of the fasci•
cles. Drawn with spines removed.
D-F: Shape and arrangement of spines. A representative
number is shown on one side.
G: Small specimen of Bri saster showing latero-anal
fasciole completely developed. Specimen size =
9.5 mm. in length and 8.5 mm. in width.
For list of abbreviations see page »x ANAL VIEW LATERAL VIEW 10
ficial similarity, as, for example, between the aboral and sub-anal
burrow building types.
In Brisaster, the aboral part of the anterior ambulacrum contains
numerous funnel-building tube-feet (Fig. 3A), which are penicillate and
very extensile. Each of these tube-feet has a terminal disk bearing a
fringe of papillae. The respiratory tube-feet are located in the sunken
petaloid regions of the paired ambulacra, the posterior pair of petals
containing about one-half the number of respiratory tube-feet as the
anterior petals. The entire aboral regions of the ambulacra containing
the funnel-buiIding and the respiratory types of tube-feet are enclosed
by the peripetalous fasciole.
The oral feeding tube-feet, placed in phyllodes surrounding the
peristome (Fig. 3B) are penicillate and bear numerous papillae over the
surface of the terminal disk. At the posterior end of the test, there
are usually four pairs of sub-anal sanitary tube-building tube-feet
(Fig. 3C). In external appearance, these are similar to the funnel-
building tube-feet.
In the ambulacra between the regions bearing the above four types of tube-feet, the tube-feet are small and inconspicuous. The histo•
logical structure and activity of these tube-feet suggest a sensory
function (Nichols, 1959b, c). A noticeable feature of the sensory
tube-feet in Brisaster is that they are considerably larger in the
anterior half of the test.
Judging from the external appearance, certain of the tube-feet
vary in their function, especially those placed on the periphery of a
'functional1 area. Thus, in the anterior lateral and posterior positions of the oral phyllodes, certain tube-feet may be either feeding or sensory. Likewise, the most anterior pair of the sub-anal tube-feet on the oral surface may be sensory (i.e. lacking papillae on the terminal disk). In all cases where this variation has been observed, opposite pairs of tube-feet always have the same morphology and there• fore, presumably, function.
Each of the ambulacral plates bears a tube-foot and, as the plates increase in number by addition from the apical system and the peripetalous fasciole remains fairly constant in its position, tt follows that only the funnel-building and respiratory tube-feet increase in number with the growth of the urchin. The number of the other types of tube-feet remain fairly constant. In Table 1, two arbitrary test lengths have been selected to illustrate the numbers of the types of tube-feet observed in each case. In the apical region of the anterior ambulacrum, there are normally four pairs of undeveloped, non-functional (lacking terminal papillae) funnel-buiIding tube-feet and these are not included in Table 1.
Spines: there are no large spines above the ambitus in Brisaster
(Figs. 3D, F). The sunken petaloid regions of the paired ambulacra and the deeply grooved anterior ambulacrum are protected by arches of small, flattened spines (Fig. 3D) and small spines similarly protect the anus.
Orally, the peristome is surrounded by minute spines that form the feed• ing grill. .The plastron bears the large, spatulate spines (Fig. 3E) which are the main 1ocomotory organs (Nichols, 1959b). Posteriorly, surround• ing the sub-anal tube-feet, are two conspicuous tufts of long, slender spines. 11A.
Table 1. Approximate numbers of the various types of tube-feet
In Bri saster, at two test lengths. j
Functional Posi tion Length of Test Type on Test 13 mm, 46 mm.
Sub-anal 6-8 8-10 Burrow- Building Ambulacrum III 20 73
Ambulacra 11 & IV 88 ) 160 ) Respi ratory > 128 \zkk Ambulacra 1 & V 84 j 40 1 Feedi ng Oral 29- 31
Ambulacrum 111 8- 10
Sensory Lateral 43-V
Peri-plastronal 14- 18 12.
I B. Observations on the Mode of Life of Brisaster
(1) Ci1iary currents
The ciliary currents of Brisaster conform to the general pattern
found in all Spatangoids. The cilia;' are found on the spine bases and
the epithelium between them (Gislen, 1924). On the clavulae (the small
spines composing the fascicles) the cilia are arranged in two rows on either side of the stems, diametrically opposite each other (Nichols,
1959c). The direction of the current on the test is mainly centrifugal on the aboral surface (Fig. kA). As the currents pass down each of the petaloid regions of the ambulacra, small streams are directed by the
respiratory tube-feet into the interambulacral areas. The peripetalous
fasciole, which has re-entrant angles in the interambulacra, is in close proximity to the petaloid areas and is mainly responsible for drawing
the respi ratory current down the respi ratory funnel. The arrangement of
this fasciole and the large surface area of the flattened respiratory
tube-feet combine to effect an efficient respiratory exchange - necessary feature for an animal living in a habitat where the accumulation of hydrogen sulphide may be expected. In the anterior ambulacrum an apical eddy is present in the form of a small current that runs centripetally back over the apical disk and then down the anterior petaloid regions.
According to Nichols (1959b) this eddy is important in preventing the passage of gametes to the mouth during spawning.
On the oral surface, the current is mainly towards the posterior end (Fig. k&), passing down the peri-plastronal areas of the Ambulacra
I and V and across the plastron. In the perist/omial region there is a reversal of the direction of the current where the current flows towards 12A.
Figure k. Course of the main ciliary currents on the test of Bri saster. (C) ANAL VIEW 13.
the mouth on the labrum.
The currents described so far are very similar in pattern to
other Spatangoids investigated by Nichols (1959b). Peculiar to the
Family Schizasteridae is the possession of a latero-anal fasciole but
no sub-anal fasciole. The lateral part of the latero-anal fasciole was
not present in the six specimens investigated, but probably in the very
small specimens, when present, it assists the peripetalous fasciole in
creating the respiratory current. The anal part was present and was
shown to cause currents flowing away from the perlproctal regions (Fig.
4E)• Under natural conditions this probably effects the removal of
faecal material down the sanitary tubes.
As out1ined above, the focal point for the ciliary currents of
the test, in all of the Spatangoids investigated by Nichols, is the
sub-anal fasciole. The centripetal current within this fasciole results
in a current that flows away from the urchin down the constructed
sanitary tube or tubes, in a form such as Brisaster, despite the absence
of the sub-anal fasciole the currents converge at a point similar in its
position to that of the sub-anal fasciole of other Spatangoids, i.e. at
the base of the plastron. The resultant current presumably in flowing
away from the test diverges to flow down the two sanitary tubes (Fig. 5).
While the currents of the test surface flow towards the base of the
plastron, the large bases of the spines (that form two conspicuous
tufts at the posterior end) also produce a current that flows directly
down the longitudinal axes of the spines. This current may be straight or in a spiral motion. Thus, the sanitary current is caused by two
ciliary mechanisms and the evolutionary implications of this are discussed 13A.
Figure 5. Burrowing of Bri saster in (A) lateral and (B) oral view. Various types of tube-feet are shown in both diagrams, and the course of ciliary currents at the poster• ior end of the test is shown in (B). (A) LATERAL VIEW
(B) ORAL VIEW 14. below.
(2) Burrowing activity
As Nichols (1959b) has pointed out, "observation of moribund sea-urchins, or those denuded of some of their important spines, is worse than useless when they are burrowing," although in past litera• ture it has been repeatedly described. Because of the soft nature of the substrate, the majority of the specimens collected in this study were in very good condition and, despite a change in pressure during collection, the twenty-two specimens introduced into the aquarium com• menced burrowing immediately and were completely submerged within thirty minutes. AH continued to burrow actively for three days, during which time the observations were completed, but after this period the majority began to come to the surface and cease active burrowing. (This tendency is attributed to a reduction in the salinity of the water in the aquarium, caused by a leak in the cooling system.) However, a number of small specimens (below 15 mm. in length) continued to burrow for two weeks.
In the initial stages of burrowing there was little forward move• ment, and the complete covering of the test largely resulted from the action of the large spatulate spines of the plastron. At the posterior end of the urchin, the spines forming the two conspicuous tufts moved in a circular direction thus excavating the sanitary tubes which were slightly less than test width apart (Fig. 5)* These are maintained by the sanitary tube-buiiding tube-feet which are closely associated with the spinal tufts, but observations on their activities proved impossible because of the extremely fine particles of the substratum. However, these extensile tube-feet were observed to extend up to 3 cm., which was probably the 15. distance to which the sanitary tubes were maintained.
The depth of burrowing was approximately 1 cm. and funnels were excavated to the surface, down which the respiratory current was drawn.
These funnels were renewed and maintained by the funnel-bui1 ding tube-feet situated in the anterior ambulacrum. A new funnel was produced by the most anterior of these tube-feet. This occurred when the old funnel had
become situated over the apical region of the test (Fig. 5A) as a result of forward movement. The disused funnel soon became blocked by infilling.
The respiratory funnel and sanitary tubes are temporarily prevented from collapsing by the use of mucus as a binding material (secreted by special muscle-operated glands in the epithelium of the tube-feet) which is plastered on the walls by the tube-feet or via the spines (Nichols, 1959b).
The activity of the burrowing urchin could easily be traced by the track of disturbed material produced at the surface. Because of this, observa• tions on the rate of movement could be made and several of the larger specimens were noted to move about 20 cms. in less than three days.
Judging from the rates recorded for other urchins (Nichols, 1959b), this appears to be rather sluggish and perhaps should be treated with caution.
(3) Feeding activity
Feeding activity was observed when several specimens burrowed to the edge of the substratum in the aquarium. Observations confirm those already recorded by Nichols (1959b). The oral feeding tube-feet, were observed to excavate, together with the anterior spines of the test, a space below the mouth. The tube-feet were extended one after another in continuous succession, each picking up a cluster of particles on the sticky papilose terminal disk, and then retracting and conveying the 16.
the material to the mouth. Here the adhering material was wiped off by
the feeding grill of small spines that arch over the mouth.
I I Biometrical Analysis
A number of characters have been used by previous workers
(Agassiz, 1898, 1904; Clark, 1917; Mortensen, 1950 to separate the two
species Bri saster lati frons and B. townsendi. The main distinguishing
character used is the relative lengths of the petaloid regions of anterior
and posterior pairs of ambulacra. Other characters include the position of the greatest width of the test, the width of the petaloid regions
compared to the width of the anterior ambulacrum and whether or not the
test shows aboral flattening. Examination reveals that none of the above
characters clearly separate the two species, even though certain forms closely resemble specimens illustrated by Agassiz, Clark and Mortensen.
The material was collected from three stations (1A, 3 and 6) covering a depth range from 68-135 fathoms. The length-frequency distribution of the samples is shown in Figure 9 (Group C). It was neces• sary to collect the specimens from different localities for two reasons.
Firstly, the size range of the individuals varies widely in different areas, and in collecting from several localities a more complete size
range was obtained, although no specimens approaching the largest
recorded by Mortensen (1950 — 77 mm. in length — were taken. Secondly,
it was possible that the two species were separated with respect to their depth range, as in the Atlantic genus Spatangus; S.. purpureus being found from low tide to 30 fathoms and j>. raschi below 100 fathoms. This could occur despite the fact that the two species were described on specimens 17. dredged from similar depths.
Biometrical analyses were carried out on three morphological characters; (i) mean lengths of the anterior and posterior pairs of petals, (ii) height of the test, and (iii) area of the peripetalous fasciole. (i) and (ii) were selected as the more important of the characters used by previous workers, and (iii) was analysed for the
reasons given below.
(i) According to Agassiz (1898, 1904) and later Clark (1917) and Mortensen (1951) the length of the posterior petals (I and V) is one-half the length of the anterior petals (II and IV) in Q. townsendi, whereas in B. latifrons, the length of the posterior petals is only one-third of the anterior petals. Figure 6 plots the mean lengths of petals I and V against those of petals II and IV for the three popula• tions. Using this character as a criterion, the absence of any separa• tion of the points demonstrates that a single species is present, at all
three localities and over the depth range investigated. As expected, the absolute variation between the petal lengths increases with an in• crease in the size of the individuals and if only extreme examples were available then possibly two species would be considered present, but the large numbers in these samples disprove this.
To investigate any differences between the three populations with respect to the lengths of the petals, analysis of covariance was carried out and the statistical data obtained are shown in Table 2. No signific• ant difference is shown between the slopes of the regression lines of the three populations (p = > 0.05) but a highly significant variance ratio is obtained (p =<0.01) using the adjusted means test. To explain this 17A.
Figure 6. Relationship between mean^lengths of anterior
(II & IV) and posterior (I & V) ambulacral petals in
individuals from three populations (Stations IA, 3 and 6).
Total regression line, calculated by least squares method,
is drawn in. Not all points are shown. For statistical data see Table 2.
17B.
-Table 2. Analysis of covariance for mean lengths of anterior (II and IV)
and posterior (I and V) ambulacral petals of individuals from three popu•
lations (Stations 1A, 3 and 6) used in the biometrical analysis. Data
presented in Figure 6.
Sou rce Equation of ( s(b) s(y.x) Deviations from Regression Regression ! d.f. Sums of Mean F Squares Square Value
1A Y=-0.305+0.540x 0.0176 0.471 118 26.206
3 Y=-0.579+0.545x 0.0156 0.570 87 28.239
6 Y=-0.863+0.572x 0.0398 0.846 30 21.461
Within 235 75.906 0.323 } 0.486
Reg. 2 0.314 0.157 J p=>0.1 Coeff.
Common 238 76.220 0.320 } 4.893
Adj. 2 3.12 1.566 J p=<0.01 Means
Total Y=-0.431+0.545x 0.00807 0.576 239 79.340 18.
result, the means and size ranges (lengths of specimens) of the samples and also the slopes of the regression lines must be considered (Table 3).
Inspection of these data indicates that the lengths of the posterior petals are increasing relative to the length of the anterior petals and allometric growth is apparent. The growth ratio decreases with
increasing length and could account for the highly significant difference between the samples, in that each sample represents a different part of
the allometric growth curve.
Allometric growth of the petals would explain the observation made by Mortensen (1951) that the younger specimens typified lati frons while the older specimens typified j}. townsendi (see Discussion). The total regression line for the whole sample has a slope (b) of 0.5^5.
Thus the observed ratio of the petal lengths conforms better to original description given to B,. townsendi.
A further factor that may account for the taxonomic -confusion in the past is shown in Figure 7, which is a plot of the mean lengths of the petals against total size (length X width) of the individuals. The increase in the variability of the length of the posterior petals (r = O.965) is greater, relative to that of the length of the anterior petals (r = 0.985) and this effect is accentuated when proportionate length is considered.
Thus, allometric growth and variability in the lengths of the petals in the single species present appear primarily responsible for Identification of two species.
(ii) Mortensen (1951) uses the aboral flattening of the test in
B. townsendi as a further distinguishing character. An analysis of co- variance of the height and length of individuals was made and the statist- Table 3. Length (mean and range) of individuals, and slopes of
regression lines calculated for samples of three populations
(Stations 1A, 3 and 6) used in the biometrical analysis.
, Length (mm.) 1 i Slope of i Mean Range Regression
Station 1A 20.92 9.4-27.3 0.5403
Station 3 26.45 11.6-36.3 0.5455
Station 6 34.52 22.5-46.8 0.5723 18B.
Figure 7. Relationship between the mean lengths of the anterior (II & IV) and posterior (I & V) ambulacral petals and size number (length x width). Three populations
(Stations IA, 3 and 6) are indicated. Not all points are shown. Details of the regression lines calculated for individuals with a size number above 200 are as follows:-
Equation of regression line s(b) s(y.x)
Anterior petals Y=6.*t5 0.0121x 0.000139 0.722
Posterior petals Y=3.07 0.00663x 0.000120 0.622 01 1 i i i i i i i i i • 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 SIZE NUMBER - LENGTH [MM) X WIDTH (MMJ 19.
Ica1 data obtained are presented in Table k. No significant differences
are shown between calculated regression lines for each population and a
graphical plot (not presented) shows no separation of the points. Thus,
the analysis of this character confirms the conclusion that a single
species is present.
(iii) Measurements were also made on the area of the peripetalous
fasciole of individuals collected at Stations 3 and 6. As Nichols (1959b)
has pointed out, the deeper the urchins burrow the stronger the fasciole
that will be required because of the greater eddying effect in the
current being drawn down the funnel. If the two species of Brisaster
could have been separated using biometrical methods and both co-existed
in the same habitat, it may have been possible to demonstrate that they were ecologically separated in that they burrowed to different depths:
this difference being expressed morphologically by a difference in the
area of the peripetalous fasciole. However, as shown above, only a single
species appears present, but it is pertinent to present these data as
they surprisingly demonstrate a highly significant difference between
the two populations with respect to the area of this fasciole.
The data concerning the area of the fasciole of individuals in
two populations (Station 3 and 6) are summarised in Figure 8 and Table 5.
No difference is found statistically between the slopes of the regression
lines but a highly significant (p = < 0.01) variance ratio (F) is obtained when the adjusted means test Is applied. Accounting for this difference
in the area of the fascioles of individuals at Station 3 and 6 is difficult, since it is possibly the combined effect of two factors - allometric growth and phenotypic variation. Inspection of the data indicates a 19A.
Table 4. Analysis of covariance for height and length of individuals from three populations (Stations 1A,~3 and 6) used in the biometrical analysis.
Source Equation of s(b) s(y.x) Deviations from Regression Regression d.f. Sums of Mean F Squares Square Value
1A Y=0.605+0.592x 0.0144 0.561 111 34.887
3 Y=0.706+0.588x 0.0127 0.696 85 41.287
6 Y=0.224+0.535x 0.0356 0.988 28 27.319
Wi thin 224 103.493 0.4561 2.152
Reg. 2 1.963 0.982 J p= 0.1 Coeff.
Common 226 105.456 0.466 1 1.237
Adj. 2 1.155 0.576 J p= 0.1 Means
Total Y=0.978+0.575x 0.00682 0.684 228 106.661 20. (
positive allometric ratio between the area of the fasciole and the total
size of the individual. This is to be expected since the total test
surface area decreases proportionately to an increase in size (i.e.
volume), and, therefore, a stronger respiratory current is required by
a larger animal. Also, as the peripetalous fasciole circumscribes the
petaloid regions of the ambulacra, and the posterior petals increase in
length relative to the anterior petals, a relatively greater increase in
length and, therefore, probably area would be found in larger sized
animals. Considering the differences in the size range of the two
populations then positive allometry may explain, at least in part, this
difference.
The difference between the individuals of these two populations,
however, appears too great to be purely a cause of allometric growth.
It may also be a result of a difference in habitat conditions (i.e.
substratum) that is sufficiently great to cause phenotypic differences
between the populations. The fascioles of the Spatangoida have been
demonstrated to be very sensitive to differences in the substratum
(Nichols, 1962) and it is possible that a similar sensitivity is a partial
cause of the difference recorded here, even though no observable differ•
ence in the substratum could be detected.
111 Ecological Investigations
During the collection of specimens for an investigation of the
morphology and mode of life of Brisaster, marked differences in the
size-frequency distribution of samples taken from different localities
were noted. In widely separated localities such as Trincomali Channel, 20A.
Figure 8. Comparison of the area of the peripetalous fasciole against size number (length x width) in individ• uals from two populations (Stations 3 and 6). Population and total regression lines, calculated by least squares method, are indicated. For statistical data see Table 5. 80
200 400 600 800 1000 1200 1400 1600 \B00 200~0 2200 SIZE NUMBER - LENGTH (MM.) X WIDTH (MM.) 20B.
Table 5. Analysis of covariance for area of peripetalous fasciole and
size number (length x width) of individuals from two populations (Stations
3 and 6) used in the biometrical analysis. Data presented in Figure 8.
Source Equation of s(b) s(y.x) Deviations from Regression Regression d.f. Sums of Mean F Squares Square Value
3 Y=17.38+0.06l3x 0.00324 9.465 29 7346.168
6 Y=51.66+0.0508x 0.00801 15.926 82 7355.745
Wi thin 111 I47OI.9I3 132.450 1 2.26
Reg. 1 300.020 300.020 ) p=>0.1 Coeff.
Common 112 15001.993 133.946 1 59.36
Adj. 1 7950.628 7950.628 ) p=«0.01 Means
Total Y=13.73+0.0723x 0.00339 14.380 111 22952.621 21.
near Nanaimo, British Columbia and in the Queen Charlotte Channel, Howe
Sound, British Columbia, ranging from 28 - 125 fathoms, enormous differ• ences in the size composition of the populations were observed. Similar differences were recorded during the collection of specimens for the
biometrical analysis (Fig. 9 - Group C). Clearly, these differences
required investigation and the apparent inverse relation between size and depth was a primary consideration in establishing the stations to be occupied in Howe Sound. Two main depths were chosen for practical
reasons (chiefly topographical) and these were 65-75 fathoms and 135 fathoms (the main channel depth). At 135 fathoms three main areas were selected - Stations 1, 2 and 3 (Fig. 1). The Bri saster individuals collected at these three areas were studied in detail. The stations at
65-75 fathoms were widely scattered over the study area (Stations k, 5,
6, 9 and 11) in order to gain a more complete evaluation of the distri• bution of Bri saster and also because Bri saster occurs in far less numbers at this depth. Two other stations (7 and 8) at shallower depths (48-55 fathoms) were occupied but not included in the analysis as only single specimens were taken (Fig. 9 - Group D). At three Stations - 10, 12a,
12b - no specimens of Bri saster were dredged, probably because of the coarse sediments present in these areas. The analyses are only qualita• tive, but relative density between stations could be estimated by com• paring the volume of deposit in the sample with the number of individuals taken. The highest density was found at Stations 1 and 2 and the density at Station 3 was less. The density of individuals at the 65-75 fathom zone was found to be invariably less than in the deeper zone.
The length-frequency distribution of all samples collected in 21A.
Figure 9. Length-frequency of ali samples collected in
Howe Sound on July 12, September 10, and October k and 5,
1962. A 1 mm. grouping is used. Number of individuals in each sample is indicated.
Group A - Stations 1, 2, 3. (Depth stratum 135 fathoms)
Group B - Stations k, 5, 9, II. (Depth stratum 65-75
fathoms)
Group C - Stations IA, 3, 6. (Samples used in biometrical
analysis)
Group D - Stations 7, 8. (Samples not included in an•
alysis. GROUP A GROUP C
25- STATION IA.(I2I) July 20
20 - STATION 2.(109)
Sept. 15 - STATION 3.(89) July 10 -
5 -
0 -
i 10 - l0 < T STATION 6.(32) July Q 5 - > 0 -
10 "STATION 3.(14)
5 _ Sept. GROUP D STATION 8.(1) ol 1 1 July tr UJ 20 25 m1 0 STATION 3.(93) STATION 7.(1) Oct. Sept.
0 -4- —I—1 —I— 10 15 20 25 30 35 40 50 55
10 STATION 9. (28) GROUP B STATION 4.(2) Oct. 5 Oct.
0 1- 35 40 45 50 10 - STATION 11.(19) STATION 5.(4) Oct. 5 - Oct.
0 -4- 0 10 15 20 25 30 35 40 45 25 30 35 40 LENGTH (MM.) 22.
July, September and October 1962 are shown in Figure 9. Length- frequency distributions of the two sets of samples taken at each of the
Stations 1 - 3 are similar (Fig. 9 - Group A), although there is varia• tion between the populations in these three areas. The length-frequency for samples taken between 65-75 fathoms (Fig. 9 - Group B) show a much greater variation, but inspection reveals that the mean length of the individuals from this depth zone is much greater than the mean length of the individuals taken at 135 fathoms. A Student's Mt" test was performed between all the samples (statistical details are given in Table 6) taken from these two depth strata. (The effect of growth in the period between sampling was assumed to have a negligible effect on the results.) The details are as follows:-
"t" = d/Sd = 9.28/0.9 = 10.31
The resulting "tM value is highly significant (p = < 0.01), indicating that the urchins are much larger in the shallower zone than in the deeper zone. Before attributing this size difference to the difference in depth, variation between populations at the same depths must be considered.
Using the large samples obtained from Stations 1, 2 and 3 an analysis of variance was performed (Table 7) and the resulting variance ratio (F value) was highly significant (p = <0.01). To investigate furtherthe differ• ences, a Student's "t" test was performed between the three populations involved and the details are given in Table 8. The greatest difference is found between the two farthest separated stations while between adjacent stations smaller differences are found. The above analyses show that, besides a size difference in the populations at different depths there is a size difference between populations at the same depth. 22A.
Table 6. Details of samples collected from stations at two depth
strata — 135 and 65-75 fathoms.
.1 1 t 65-75 Fathoms Depth Stratum 135 Fathoms Month Station Number of Length (mm.) Length (mm.) Specimens Mean Range Mean Range
July 3 89 26.45 11.6-36.3
6 32 34.52 22.5-46.8
Sept. 1 68 17.8 6.0-24.9
2 109 21.33 9.0-27.9
3 14 28.02 I7.3-34.5
Oct. 1 76 18.49 8.9-27.4
2 73 20.57 10.8-28.1
3 93 23.06 9.3-34.2
4 2 44.45 40.0-48.9
5 4 34.12 28.9-39.1
9 28 27.49 8.7-41.9
11 19 26.51 9.2-34.5
Total 607 21.71 6.0-36.3 30,99 8.7-48.9
522 Specimens 85 Specimens 23.
Thus, the responsible factor does not appear to be a function of depth
(although this may have an indirect effect) but rather one of differ• ences between areas. Several causes may be responsible for differences of this type, i.e. differences in growth rate of longevity of individuals, differences in larval settlement in successive years, or even migration.
Before any of these possibilities can be excluded it is necessary to obtain some indication of the relative age class composition in the various populations.
A number of techniques have been used by previous workers to determine year classes in populations of urchins. Moore (1935) was able to demonstrate annual growth rings in the plates of the test in
Echinus esculentus (Durham, 1954, has since demonstrated their presence in a number of irregular echinoid species) and thereby determine the age of individuals and year classes of the populations involved. This technique was used on the test plates of Bri saster but no growth rings could be demonstrated. Another method of determining year classes taking the modes of the size-frequency distribution was used by Swan (1958).
This method, however, requires very large samples and also assumes spat- falls in successive years. The modal distribution of the samples used in this study is very indistinct and is not a good basis for distinguish• ing year classes. One of the outcomes of this study, however, is a method of distinguishing the first three year classes of female Brisaster, using ova sizes.
An inspection of the gonads was carried out on each of the individ• uals taken in September and October from Stations 1, 2 and 3. The gonads and gonopores develop when the urchin reaches a length of between 11 and Table 7. Analysis of variance for lengths of individuals from
three populations (Stations 1, 2 and 3).
1 1 Source 1 d.f. 1 Sums of 1 Mean F Value Squares Squares
Total 521 31537.0
Means 2 17343.0 8617.0 317
individuals 519 14194.0 27.35 p=« 0.01 23B.
Table 8. Statistical comparison of lengths of individuals in three populations (Stations 1, 2 and 3).
Populations d.f. Probabi 1 i ty
Stations 1 and 2 22.3 ' 324 «0.01
Stations 2 and 3 18.8 376 «0.01
Stations 1 and 3 34.5 338 «0.01 24.
15 mm. and probably urchins below this size represent the first year
class. The spawning period is unknown in Bri saster. No differences in
the external appearance of the testes could be detected but consistent
differences in the ovaries with respect to the size of females were
observed. Up to a certain length, which differed in the female urchins
from the different areas, the ova were small, whitish in colour, and
appeared closely attached to the ovarian wall. In females above this
critical length, the same small whitish ova were present (in smaller
numbers) but also moderate numbers of large yolky ova were present and
these appeared detached from the wall of the ovary.
These differences were investigated histologically. Five females
from each of the two size groups (17-19 and 22-24 mm. length) were
selected randomly from the specimens collected from Station 2 in October.
The ovarian structure of these two groups i-.seshown in the photomicrographs
in Figure 10. The immature ova of the 17-19 mm. group female take up
haematoxylin very readily, while the mature ova of the 22-24 mm. size
group females are more eosinophilic in the cortical regions. This
histological difference probably reflects the accumulation of yolky
material in the cortical regions of the mature ova.
With the method described above measurements were made on large
numbers of the ova present in these two groups of ovaries. The results
are shown in Figure 11 and the difference in the size range and frequency of the ova underlines the histological differences. These differences
can only be interpreted on the basis that two year classes of females
are represented, the females with immature ova being the 2nd year class
and the females with mature ova being three years old and above. Thus, 24A.
Figure 10. Ovarian structure in Bri saster. Sections stained with haematoxylin and eosin. Approx. x37.
(A) Section of ovary of second year female showing im•
mature ova.
(B) Section of ovary of third year (or older) female
showing both immature and mature ova. (B) 25. the ova require two years to develoD^maturity and the female urchins do not spawn until they are in their third year. In Echinocardi um cordatum (apparently the only other Spatangoid thus investigated), the ova require one year to mature and the females spawn in their second year (Moore, 1936). This variation possibly reflects the difference in the depth range of the two species, Echinocardium being an intertidal- sublittoral species while Bri saster is essentially an archibenthic form.
The results of this investigation can now be interpreted in terms of the samples taken from the three populations under study.
Figure 12 illustrates the distributions of the two age groups of females in the samples, the total distribution also being indicated.
As can be seen, no females below 15 mm. in length were identified as such and it is possible that some of the smaller sized second year females were mi sidentified because of the extreme smallness of the ova present in the developing ovary. Generally, the second year class females and males show a normal distribution in each sample, therefore there is no reason to believe there is any difference in the growth rate of the two sexes. Those urchins with an age of 3 years and above also show a normal distribution, the individual year classes being
indistinguishable. This is to be expected since the absolute growth
rate will decrease as the animal approaches the maximum size of individuals in the populations and the year classes will lose their modal identity in the distribution.
The critical lengths between second and third year (and older) animals for the three populations are shown in Figure 12. The critical lengths are:- Station 1, 18.5 mm.; Station 2, 20.0 mm.; Station 3, 25A.
Figure 11. Percentages of size groups for ova diameters
in second and third year (and older) females. 50
NUMBER OF OVA MEASURED 40 2ND YEAR - 443 3RD YEAR 8 OLDER-472 tu 2ND YEAR o 30 < ui o * 20 LJ 0.
3RD YEAR 8 OLDER 10 /
0 . 02 .04 .06 08 .10 .12 .14 .16 .18 .20 .22 .24 .26 .28 .30 OVA DIAMETER (MM.) 26.
25.5 mm. Again the greatest difference is found between the farthest horizontally separated populations, adjacent populations showing inter• mediate differences. The critical lengths also correspond to the maximum lengths reached by the urchins in each of the populations.
Thus, the primary cause of the noted differences in the size-
frequency of Bri saster populations in the deeper zone (135 fathoms) is clearly due to differences in the growth rate and these would also determine the maximum size attained by the individuals. In each case,
the age composition of the populations appears similar and, judging
from the frequency distributions, each has a continuous larval recruit• ment in successive years.
The samples from the shallower zone (65-75 fathoms) were not
large enough to permit a similar comparison, but examination of the
gonads revealed a similar correlation between the apparent critical
length and the maximum size attained. At these depths, however, the
frequency distributions indicate an irregular and limited larval
recrui tment.
Other contributing factors and a possible explanation for the
growth rate differences are discussed below. 26A.
Figure 12. Length-frequency of year classes of females,
as indicated by ova diameters, in three populations
(Stations 1, 2 and 3). Second and third (and older)
year classes are shown, with critical lengths (cl)
dotted in. Total sample length-frequency is also indic•
ated. 10- SEPT. oil ro 5 - i—i 0 - _i i_ JSS^ &2L OCT. 5- r--i ' i r- i * • t < t-J i i 0 - *7777T"| 10 15 20 25 35 LENGTH (MM.) 10- SEPT. cil ro 5 -
r--i i i 0 - 1 OCT. 5- r--\ r-T < i 1 i i ( ' l r— CO 0 •777*1 10 15 20 25 35 LENGTH (MM.) 27.
DISCUSSION
Morphology and mode of life
The importance of a functional interpretation of the morphology of Spatangoida has been stressed by Kermack (195*0 and Nichols (1959a,
b, 1962). Both of these workers have demonstrated that the conspicuous
features, of the test, such as the fascioles, cannot be used as rigid
taxonomic criteria, as they express adaptation to a mode of life rather
than phylogenetic affinities. In an extensive investigation of the morphology and mode of life in British irregular echinoids, Nichols
found a close correlation between the particle size of the substratum and the modes of burrowing, sanitation and feeding exhibited by various
species of Spatangidae. Certain morphological features, such as the position and degree of development of the fascioles and the specialisa•
tion of the tube-feet to perform different functions can also be cor•
related with the mode of life of the particular urchin involved.
Certain substrata have physical properties that are amenable to
burrow-building, shell-gravel and mud for example (in the former the
large, irregular particles interlock and in the latter there is a flocculation of the colloidal particles), while other substrata, such as silt and sand, have no similar tractable properties. Since the cross-sectional area of the sanitary tube or tubes determines the amount of waste that will pass down it, those urchins living in the less tractable substrata will require all of the available tube-feet to plaster the walls of one tube only and thus their maximum size will be limited (e.g. Echi nocardi um cor da turn) . Those urchins living in more 28.
tractable substrata (e.g. Spatangus purpureus) will not have this size
limitation since two sanitary tubes can be built by the same number of
tube-feet (Nichols, 1959b).
Bri saster builds both a respiratory funnel and a double sanitary
•drain1, confirming the observation that the physical properties of a mud substratum are amenable to burrow-building. The number of sub-anal
burrow-building tube-feet present in Brisaster is higher (8-10) compared
to the British ecological counterpart, Brissopsis lyrifera, having only
six which, Nichols (1959b) notes, is the only Spatangoid in this area
to build both a respiratory funnel and a double sanitary apparatus.
Bri ssopsi s possesses the peripetalous fasciole but differs from Bri saster
in that it possesses a sub-anal fasciole and burrows to a greater depth (6-8 cm.).
Despite the absence of the sub-anal fasciole, the ciliary cur•
rents of the test in Bri saster do not differ essentially from the pattern shown by Spatangoids which possess this fasciole, in that the focal point of the currents is at the base of the plastron. This basic similarity in the ciliary currents of the test in these two divergent
groups indicates that the ciliary pattern had evolved in the ancestral stock and that this has been retained, despite morphological modifica•
tion and diversification in the habits of the descendents. The sub-anal fasciole appears to have evolved to improve the efficiency of sanitation in several groups (the Spatangidae and Brissidae for example), enabling these forms to burrow deeper in the substrata. The reverse of this situation is well documented in the fossil record of the Hicraster group; Nichols (1959b) has correlated the disappearance of the sub-anal 29.
fasciole in Micraster (Isomi craster) senonensi s with a change in the
niche of this urchin from a burrowing to a surface-ploughing habit, the
latter habit having less rigorous requirements, with respect tb sanita•
tion and maintenance of a respiratory current.
Without the development of a sub-anal fasciole it follows that
the urchin is limited to shallow burrowing only, as is found in Brisaster.
A compensating mechanism for effecting sanitation must be present and
it is possible that the latero-anal fasciole temporarily performs this
function. In being developed on the posterior half of the test, this
fasciole strengthens and directs the respiratory current towards the
posterior end, thus causing a strong current to flow down the sanitary
tubes. However, as the fasciole atrophies with growth, this cannot be
the major mechanism involved.
Increasing the cross-sectional area of the 'drain' by increasing the diameter of each of the sanitary tubes would result in a more efficient sanitation. The cross-sectional area of the 'drain' can be correlated with the physical properties of the substratum and the number of tube-feet available for plastering the mucus onto the walls of the tubes. Both Brissopsi s and Bri saster are mud-burrowers, and the floc- culation of the colloid particles composing this substratum make it amenable to burrow-building, thus, both species have a double sanitary arrangement. It was noted above that Brisaster possesses more sanitary tube-building tube-feet than Brissopsis and this suggests that Brisaster compensates for the lack of a sub-anal fasciole by constructing sanitary tubes of larger diameter, thus improving the water flow down the 'drain'
(the importance of a strong respiratory current in a substratum where 30.
hydrogen sulphide may be present needs to be stressed) but this mechan•
ism is probably not as efficient as the fasciole arrangement. This
would account for the differences in burrowing depths of Bri saster and
Brissopsis.
Clark (1948) has recorded that both Brisaster townsendi (= lati-
frons) and Brissopsis pacifica were taken at stations in the vicinity of
the Channel Islands, off the coast of California. Assuming similarity in
the burrowing activities of Brissopsis pacifica and B. lyri fera it would
appear that this distributional overlap is only possible because of an
ecological separation between the two species with respect to the depth
of burrowing. Possibly a similar ecological separation occurs in the
Eastern Atlantic between Brissopsis lyrifera and Bri saster f ragilis.
The presence of fascioles in only very young stages, followed by
the atrophy of the fasciole with growth, appears to be correlated with a
change in niche of the urchin. An example of this is found in the
evolution of the fossil Micraster group (the fossil record of which is
remarkably complete). Micraster (Isomicraster) senonensis, a surface
ploughing form, evolved from a burrowing stock and retained the sub-anal
fasciole in its very early stages only. The atrophy of the lateral part of the latero-anal fasciole in some species of Bri saster, while being
retained in others can be explained on these grounds, and it is instructive
to approach this problem from a zoogeographical viewpoint.
Taxonomy and zoogeography
Mortensen's (1950 classification of the Spatangoids is based
largely on the types of fasciole present in the various groups, although, 31. as Nichols (1959a) has pointed out, the fascioles do not necessarily make useful taxonomic criteria (the possibility of convergence of different stocks can be visualized). In his key to the species of the
genus Brisaster, Mortensen (1951) uses the presence of the lateral part of the latero-anal fasciole (in most large specimens) to separate the
Atlantic species -- J3. f ragi 1 is and its closely related form J3. capensis — from the Pacific and sub-antarctic species ~ _B. lati frons (= townsendi),
J3. owstoni, J3. kerguelensi s, and B^. moseleyi. The presence of the
lateral part of the latero-anal fasciole in _B. latifrons has been shown above and thus Mortensen's key requires further qualification.. However,
this important morphological link between the Pacific and Atlantic does
illustrate the close affinities of these two separated stocks.
Easton (I960) points out that the echinoids on both sides of the
Atlantic evolved from different stocks. During Eocene times the
American waters were dominated by the Clypeastroids and the European seas were dominated by the Spatangoid stock. The faunas became mixed during the Miocene and Pliocene, on both sides of the Central Atlantic.
The exact mechanism of this reciprocal migration is still unexplained.
The Central American land-barrier was not closed until the end of the
Miocene (Olsson, cited in Durham and Allison, I960) and the subsequent
isolation of the genus Bri saster can thus be synchronised with the presentday knowledge of paleogeography (although there is a slight discrepancy with respect to the identification of Brisaster (as j3. maximus) from Oligocene rocks of Oregon by Clark (1937) and Weaver (19^2):
Grant and Hertlein (1938) record jS. townsendi from the Pliocene of
California). 32.
The separation of the species of Bri saster has resulted in a
slight morphological divergence in that the lateral part of the latero-
anal fasciole is atrophied in the early stages of the life history of
j3. latif rons whi le it is usually retained in B. f ragi lis. From this it
would appear that jJ. f ragi 1 is represents a form closer to the ancestral
genus type and that it probably burrows deeper than B. lati frons. The
loss of the lateral part of the latero-anal fasciole in _B. latif rons sug•
gests that this species has undergone a slight change in niche, to a
shallower burrowing habit, since its isolation from the main Atlantic
stock took place. The lateral fasciole has thus become functionally
unnecessary and therefore atrophies as there is no selection pressure to
retain it.
Since the closure of the migration route between the American
continents at the end of the Miocene, the range of j3. fragi 1 is appears
to have retreated north as it is no longer found in the Caribbean
region. On the Pacific Coast of North America Bri saster appears to have
migrated extensively so as to include the whole of the west coast of
North America and Japanese waters. There is no evidence to suggest that
two separate invasions of the Pacific Coast took place or that sympatric
speciation has occurred and, therefore, it is logical to assume that only
a single species of Bri saster is represented in this area. An apparent discontinuity appears in the north west Pacific region (although Baranova,
1955, reports J3. latif rons present in the Bering Sea) so that the speci• mens of Bri saster, designated B. owstoni by Mortensen (on rather dubious evidence of the number of pedieellariae present) found in Japanese waters may represent an incipient species. The need for more invest!ga- 33.
tiomof the area where Bri saster is unrecorded in the north west Pacific
must be filled before any conclusions can be reached.
To digress at this point, a number of other aspects of the zoo•
geography of the genus Bri saster are of further interest. Ekman (1953)
has discussed the related phenomena of wide latitudinal distributions,
equatorial submergence and the concept of 'bipolarity1 (s.l.). The wide
latitudinal distribution of Brisaster latifrons in the eastern Pacific
has already been noted. A survey of the records indicating the depths
and positions where Brisaster latifrons has been taken (Agassiz, 1898;
Clark, 1917, 1948) reveals that in the lower latitudes this species
exhibits 'equatorial submergence'. Also, the discontinuities in the
distributions of the Bri saster species in the Pacific and Atlantic
Oceans are remarkably similar to those shown by many other species that
exhibit 'bipolarity'. Thus, Brisaster appears to be another genus to add to the extensive lists of genera* exhibiting the above related
phenomena; the various theories that have been postulated to explain
these phenomena are reviewed by Ekman (1953) and Hedgpeth (1957).
From the above discussion on the zoogeography of the genus Bri sas•
ter the evidence suggests the presence of one species along the west coast of America. This supports the conclusions drawn from the bio• metrical analysis given above, and if Gause's principle is applied,
from ecological reasoning also. As the author cannot be considered first
reviser, the rule of priority, as stated in the International Code of
Nomenclature (1961) (Article 24(a)), must be applied and the name of
Brisaster latifrons retained (as the description of this species appears before that of J3. townsendi in Agassiz's 1898 paper), thus the j3. town- 34. sendi becomes a synonym, despite the fact that the original description given to the latter is more appropriate. Before closing the discussion of this important taxonomic point, a review of the literature is neces• sary.
The type specimens of the two species were not recognized by
Agassiz (1898) as belonging to the genus Bri saster Gray (1855) and were treated under the synonym genus Schizaster. Mortensen (1907), in dis• cussing the genus Schizaster, recognizes four genera — Paraster,
Schizaster (s.s.), Tripylaster and Bri saster. He separates Bri saster from other Schizasterids on the number of genital pores (3), the low test with posterior vertex and the peculiar globiferous pedicellariae.
Clark (1917) accepts Mortensen's classification and includes six Recent species in the genus. Thus, the two species originally described from material dredged in the Gulf of Panama — Schizaster latifrons and S_. townsendi become included under the genus Bri saster.
Agassiz (1898) characterizes Schizaster (= Bri saster) lati frons in its possession of short posterior ambulacral petals, the great development of the 'anterior extremity' (?) and the breadth of the anterior ambulacrum, while S_. (= Bri saster) townsendi has a flatter test, wide lateral ambulacra, small anal system and close primary tuberculation of the plastron. Agassiz (1904) further separates the two species on the character of the plates adjacent to the labrum but this is shown to be incorrect by Mortensen (1951).
Clark (1917) retains both species and further separates them on the position of the maximum width of the test. He points out, however, that there is a tendency for intergradation between the two species, 35. 9
possibly because of hybridisation. In referring to B. latifrons, Clark draws attention to the fact that the original material was scarce and,
in fact, the type specimen characters are, in part, due to immaturity and that the species B. . lati frons is only appropriate when applied to young specimens. From these points he, rather illogically, concludes
that the species is probably valid. In his 1948 paper, he omits the species B_. latifrons, presumably because he no longer recognizes it as distinct from 8. townsendi but refrains from comment on the matter.
Mortensen (1951) retains both species and agrees with Clark on
the tendency for intergradation and possible hybridisation.; He believes
it impossible to distinguish between the young specimens of, the two species and, of the specimens he collected in the Straits of Georgia, near Nanaimo, British Columbia, he observes that "larger specimens are typical Townsendi, while the young specimens might rather be regarded as latifrons;". As pointed out above, this observation may be explained on the basis of allometric growth in the ambulacral petals. Mortensen continues, "it would have no sense that all the larger specimens should be one species, all the smaller specimens from the same locality another species ... I think my doubt as to the validity of the latifrons as really specifically distinct from Townsendi justified;". Thus, Mortensen, while recognising the synonymy of the two species, did not confirm his observations with mensural data and inconclusively left the problem.
The biometrical data presented in this study document the rejection of
B_. townsendi as a separate species.
From the ecological viewpoint it is difficult to suggest how two species, being so closely related, could exist in the same habitat. 36.
The morphological basis of separating the two species cannot be used to infer differences in niche. Certain differences between populations have been demonstrated, and while these differences cannot be considered significant at the species or even sub-species level, various features, such as the peripetalous fasciole, show a degree of variability to an extent that cannot be explained on the basis of allometric growth and the differences in the growth rate of individuals in populations in different localities. It seems probable that subtle differences in the physical nature of the substratum are reflected as phenotypic differences in the individuals of the population.
Ecological studies
Size differences between populations of Bri saster in different areas of the deep channels of Howe Sound have been shown to be due to a difference in growth rate in these populations, while the age composition appears to be very similar. At lesser depths, the mean size of the urchins is significantly larger than those at greater depths. Similar differences have been noted by other workers in a number of marine invertebrates.
Segerstrale (1960-62), in an intensive investigation of the bivalve
Macoma balti ca, found a predominance of full grown individuals in the deeper water populations, which contrasted with those in shallower water, where younger individuals were well represented. At first, he assumed that the deeper water populations were not native and were maintained by migration from shallow water populations. Subsequent sampling, however, demonstrated that this view was erroneous and that the phenomenon was 37.
caused by a failure of the spat fall in successive years in the deeper
zones (evidently because of predation by the amphipod Pontoporeia). He
was also able to demonstrate that individuals living in the shallower
zone had a more rapid growth rate but short life span (a probable effect
of the higher temperature regime) and that they also grew to a larger
size (possibly because of the higher nutritive content of the substratum).
Ford (1925), Stephen (1928-29), Kreger (19^0) and Kristensen (1950)
have recorded differences in the average size of different populations
in various lamel1ibranchs and have correlated this effect with the
population density, the average size being greater when numbers are
fewer.
Observations on Spatangoid populations are less well documented.
The ecology of the Atlantic species Echinocardium cordatum, which occurs
intertidally, is perhaps best known of all. Moore (1936) showed that
an active migration into the intertidal zone takes place in the interval
from four to twelve months after settlement of the young below low water,
thus, explaining the difference between littoral and sublittoral popula•
tions. Ursin (I960), in an extensive study of the echinoderm fauna of
the North Sea, found that the size-frequency of populations of Echinocar-
dium in different areas differed greatly in that the size of the main
stock varied. He found evidence to suggest that this was a consequence
of differences in the growth rate and correlated with density— much
larger specimens occurred where the density was low. Ursin points out
that the dominating groups in samples may not represent one year class
each. By accident, heavy spatfalls may have occurred in successive years and owing to slow growth, such successive spatfalls may later 38. produce a single mode in the size-frequency distribution.
The only other record of differences in populations of Spatangoids is given by Brattstrbm (1948) who recorded that specimens of Brissopsis lyrifera in the deeper parts (below 100 metres) of Gullmar Fjord were much larger than those found at shallow depths, the reverse of the situation found in Bri saster. He suggests that this difference is poss• ibly due to different ecological conditions, or perhaps a difference in the intensity of competition with other animals in the two communities found in the shallow and deep zones of the Fjord.
The problem of extremely large specimens in some localities has been correlated with the senility of the individuals by Mortensen (1920) in Brissopsis lyrifera and by Thorson (1946) for Asterias rubens. It is presumed that surplus nourishment is used for vegetative growth rather than for reproduction, because in these large specimens the gonads appear undeveloped and are typically thin and black-coloured. None of the larger specimens of Brisaster examined in this study possessed undeveloped gonads and this explanation for the occurrence of large-sized specimens can be rejected.
Summarising the findings of the workers outlined above, three main causes have been found to explain differences in the size of individuals in populations of marine invertebrates and these are
(1) larval settlement distribution, (2) migration, and (3) differences in growth rate, either physically or biologically induced.
Nothing is known about the larval stages of Brisaster latifrons.
The fact that larvae have never been recorded is suggestive of a very restricted planktonic phase in the life history, and this is also 39.
indicated by the large yolky eggs found in this species. Such eggs
usually give rise to a 1ecithotrophic larva which characteristically has a short pelagic existence (Thorson, 1950), limiting the distribution of the larval stages.
Inspection of the length-frequency diagrams for the two depths strata sampled in Howe Sound, reveals the fact that the populations of
Bri saster in the deep channels have a fairly continuous larval recruit- men ty but, in the shallower zone, the larval settlement appears more
irregular in its distribution. Certain size groups are well represented
in this zone while others are absent (as found by Ursin). This observa•
tion might be explained on the basis of a restricted larval phase and the spasmodic transport of larvae to the shallower zones by currents in certain years.
While the possibility of active migration of Bri saster individuals from the deep channels to the shallower areas (similar to that- exhibited by Echinocardium, only at a deeper level) cannot be excluded, there is little evidence to support it. This explanation involves the assumption that no autochthonous populations exist at shallower zones, which does not fit the observed facts and could only be proved by a marking-recapture method (which, in this case, would be highly impractical, if not imposs• ible).
The differences in the growth rate of the populations in the deep channels of Howe Sound cannot be referred to differences in hydrographic conditions (as found by Segerstrale, in Macoma), as these are remarkably constant, both in salinity and temperature. Similarly, no differences were observable in the physical nature of the substratum between the ko. localities investigated although possibly present. The penetration of
Fraser River water into Howe Sound occurs and quantities of fluvial material must be deposited, especially in the Queen Charlotte Channel.
This material may influence the physical properties of the substratum, but in the absence of evidence to the contrary its effects are assumed to be negligible. Thus, rejecting environmental differences to explain the differences in growth rate, the biological aspects must be investigated.
The correlation between density of populations and growth rate of individuals, as found by other workers cited above, appears applicable to the Bri saster populations investigated since the average size of the individuals is greater where numbers are least. The sparse numbers in the 65-75 fathom depth zone, a result of irregular spatfall, probably enables the individuals present in this zone to reach a larger size.
In explaining any correlations between density and growth rate, the concept of competition is constantly applied. Intraspecific com• petition for food and space in sedentary marine invertebrates has been reviewed by Knight-Jones and Moyse (1961), but apparently no one has postulated a mechanism whereby competition may operate in populations of errant organisms, particularly burrowing detrital feeders. In an active burrower, such as Bri saster, under conditions of high population density, movement may be restricted to such an extent that the quantity of food that can be obtained is greatly decreased and thus a decrease in the growth rate of individuals is to be expected. Should other members of the community be present in high numbers also, this effect would be accentuated, perhaps to different degrees in different com• munities (possibly this competition mechanism explains the differences 41.
in the size of Brissopsis lyrifera in different communities as reported
by Brattstrbm (1948)).
Evidence for this hypothesis is to be found in a comparison of
the faunal composition of the communities at Stations 1, 2 and 3 (see
Appendix). At Station 1 and 2 a high density of Bri saster can be cor•
related with the small size range of the populations. The fauna at
these two stations is fairly dominated by the tubicolous polychaete
species, chiefly Maldane glebifex, Pista fasciata and to a lesser extent
Pectinaria belgica, and many discarded tubes are present in the sub• stratum. At Station 1, the number of species, particularly molluscs, is notably higher than at the other stations.
The presence of large numbers of other organisms in the community, expecially the tubicolous polychaetes (and also their discarded tubes) could restrict the burrowing activity of Brisaster, which is dependent on this activity .to obtain its food supply in the form of organic material adhering to the substrate particles.
The maintenance of the respiratory funnel and sanitary tubes wouil;d be more difficult under crowded conditions and thus the urchin would be forced to expend more energy on these activities, than it would under less crowded conditions. This limitation would not apply to those errant infaunal animals that either extend a siphon into the substratum surface, e.g. lamel1ibranchs, or construct a permanent tube to the surface, e.g. Recti naria, down which a respiratory current is drawn; in these animals no energy is expended in excavating and maintaining temporary constructions that are liable to become blocked through the activities of other animals. 42.
Summarising, the causa] factors responsible for the size- limitation in populations of Bri saster is thus envisaged as both intra- and interspecific competition, probably better regarded as passive
'interference1 within the community for space which (i) because of restriction of feeding activity, results in a reduction of the growth rate of individuals to varying degrees in different populations, or
(ii) the forced expenditure of more energy in the construction and maintenance of the respiratory funnel and sanitary tubes under crowded conditions, which reduces the growth rate and therefore maximum size of the urchins involved. Quite possibly both mechanisms may be operat• ing. 43.
SUMMARY
(1) The morphology of the Spatango id genus Bri saster Gray (Family
Schizasteridae) has been described and interpreted in terms of its observed mode of life. A biometrical analysis of the specimens occur•
ring along the west coast of North America has been carried out in order to clarify certain taxonomic problems and the result of this analysis considered from ecological and zoogeographical viewpoints.
Certain aspects of the ecology of Bri saster in Howe Sound, British
Columbia, have also been investigated.
(2) The burrowing and feeding activities conform to those already described for Spatangids. Bri saster burrows to a depth of about 1 cm. in mud and constructs both a respiratory funnel and a double sanitary apparatus. The similarity of the ciliary current pattern of the test in Spatangoids, regardless of the presence or absence of a sub-ana] fasciole, is a feature that suggests descent from a common ancestral stock. On this interpretation, the different types of fascioles found in Spatangoids have evolved as superimpositions on the basic ciliary pattern rather than the converse.
(3) The mode of life in Bri saster is compared to the ecological equivalent Brissopsis, which possesses a sub-anal fasciole and burrows to a greater depth. Comparing the number of sanitary tube-building tube-feet in these two Spatangoids, it would appear that Bri saster compensates for the lack of a sub-anal fasciole by building sanitary tubes of a larger diameter. kk.
{k) A biometrical analysis of those characters used as criteria for
distinguishing two species of Bri saster along the west coast of North
America, indicates the presence of a single species and that the
taxonomic confusion in the past is probably due to allometric growth
and variability, particularly in the lengths of the ambulacral petals.
Taxonomic priority must be given to Brisaster latifrons; Bri saster
townsendi is thus a synonym, as may be the Japanese species Bri saster
owstoni. These conclusions are supported by paleogeographic considera•
tions.
(5) The need for qualification of certain taxonomic criteria, in
view of the paleogeography of the genus, is stressed; certain criteria
are not distinct and, in the latero-anal fasciole, for example, the
specific differences involving the extent of its development, probably
reflect siight diversification in the niches of the several species
since their isolation.
(6) Investigations in Howe Sound, British Columbia, have revealed
marked differences in the size-frequency distribution of samples taken
from populations of Bri saster occurring at two depths (65-75 and 135
fathoms) and between populations at the same depth (135 fathoms).
Individuals at the lesser depth were significantly larger than individ•
uals at the greater depth, and this appeared to be correlated with the
population density; larger individuals being found in less dense con•
ditions. A short pelagic larval phase is suggested to account for the observed irregular density pattern, particularly in the shallower zone,
and for the differences in the larval recruitment of the populations at 45. the two depths.
(7) Three age classes can be distinguished from ovarian structure.
In both males and females, the gonads do not develop until after the first year; in second year females only small, immature ova are present, while in third year and older females, both immature and mature ova are present. Thus, spawning in females does not appear to take place until they are in their third year, and the ova require two years to develop to maturity.
(8) The age composition of the populations in all areas of Howe
Sound appears similar, and thus the size differences of individuals can be attributed to differences in growth rate which, in turn, depends on thepopulation density.
(9) Possible factors responsible for a differential growth rate are reviewed, and it is concluded that inter- and intraspecific competi• tion are probably operating. Two mechanisms involving passive inter• ference are postulated whereby, under crowded conditions, either (i) burrowing and, therefore, feeding activity of the urchins is restricted, or (ii) the construction and maintenance of temporary constructions, such as the respiratory funnels and sanitary tubes, require the expendi• ture of more energy than under less crowded conditions.
(10) The predominant and community present in Howe Sound is best described as a Bri saster latif rons--Ma1dane glebifex community. 46.
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Ursin, E., I960. A quantitative investigations of the echinoderm fauna of the central North Sea. Medd. Danm. Fisk. Havunders., N.S., 2:1-204.
Weaver, C.E., 1942. Paleontology of the marine Tertiary formations of Oregon and Washington. Univ. Wash. Publ. Geol., 5:1-789. 50. APPEND IX Fauna list for Stations 1, 2 and 3
Stations 1 2 3 Anthozoa: Pachycerianthus SD. + Polychaeta: Nephthvs punctata + + Goniada annulata + + Glvcera sp. + Aphrodi te parva + Lumbrinereis inflata + Podarke ouqettensis + Syllis pulchra + Eunice sp. + Harmothbe imbricata + + HarmothOe lunulata + Pectinaria belqica + + Sternaspis fossor + + + Maldane qlebifex + + + Pista fasciata + + Travisia pupa + Notomastus lineatus + Melinna cristata + Lvssippe labiata + Spiophanes cirrata + + Scionella japonica + Si punculi da: Goldfinqia sp. + Asteroidea: Leptychaster pacificus + Ctenodiscus crispatus + + + Ophi uroi dea: Ophiura lutkeni + + + Echi no idea: Brisaster latifrons + + + Holothuroi dea: Chiridota albatrossi + + + Mo1 padia intermedia + + Gastropoda: Exiloidea rectirostris + Admete couthouyi + + Polinices pal 1ida + + Odostomia SP. + Acteocina culcitella + Scaphopoda: Den ta Hum pretiosum + + + Lamel1i branchia: Nucula tenuis + + + Pseudomusium vancouverensis + Macoma auadrana + Pandora bi1i rata + Cardiomya californicus + + Thvasira qouldii + Yoldia thraeciformis + + + Venericardia ventricosa + + + Venericardia aleutica + Crustacea: Ampelisea SP. +
Total number of species 33 21 19 Footnote to Appendix
The composition of the predominant community in Howe Sound does not conform to any of those described by Thorson (1957). Following
Petersen's (1914) classical basis for naming communities on the basis of the most numerous, most conspicuous and for the area most "charact• eristic" species of animals to designate this community, it is best regarded as a Bri saster lati frons--Maldane glebi fex community.
The polynoids Harmothtie imbricata and H. lunulata occurred as commensals on Bri saster and were found in the aboral regions of Ambul• acra II, III and IV.