Reproductive patterns within sub-populations of Lytechinus variegatus (Lamarck) (Echinodermata: Echinoidea)

J. e,xp. mar. Biol. Emf., 1981, Vol. 55, pp.25-37 EIsev~er/North-Holland Biomedical Press

25

REPRODUCTIVE PATTERNS WITHIN SUB-POPULATIONS L Y~~C~~~ff~

VARZEGA

TUS

OF

(Lamarck) (~CHINODERMATA :

ECHINOIDEA)

ROBERT G. ERNEST’ ~epart~~~zt oftfurine

and BARMAN .J. BLAKE

Science, Universify of Sourh Florida. SI. Petersburg, FL 33701, U.S.A.

Abstract: The annual reproductive cycle of the sea urchin L,;rechinus variegurus (Lamar&f was examined in individuals collected from four habitats in the Anciote estuary near Tarpon Springs, Florida. Two periods of advanced gametogenic activity were apparent, but this bimodal pattern was not necessarily reflected by gonad growth curves. Differences in reproductive events among sub-populations were related primarily to patterns of gonad growth. Reproductive synchrony present during spring became less apparent as spawning and recovery within the sub-populations took place at differential rates during late-spring and summer. The final spawnout in late summer returned the sub-populations into gametogenie synchrony.

Intraspecific reproductive variability occurs among both geographically separated populations and proximal populations inhabiting heterogeneous environments. This variability may result from genetic divergence, phenotypic adaptation to dissimilar external stimuli, or both (Sastry, 1975). Locally, exogenous factors interact to produce climates which are compatible to various degrees with reproductive processes. Physical variables, such as temperature (Boolootian, 1966 ; Moore, 1966 ; Pearse, 1969, 1970; Moore & Lopez, 1972; Cochran & Engelmann, 1975), salinity (Moore, 1966; Moore & Lopez, 1972), water depth (Moore, 1934), and degree of exposure to turbulence (Ebert, 1968), have all been implicated as factors affecting the timing, intensity, and duration of reproductive events in various echinoids. Food availability, accessibility, and type also have been shown to be very important ~f~ounting for intraspecific reproductive variability, especially in patchy environments (Fuji, 1960a; McPherson, 1969; Dix, 1970; Gonor, 1973; Bernard, 1977). The amount of intraspecific plasticity illicited by environmental heterogeneity may be evaluated by comparing reproductive patterns of proximal sub-populations taken from different habitats, Echinoids are particuiarly suitable for this type of study because of their relatively limited mobility, large size, and annual reproductive cycles. In the Anclote estuary near Tarpon Springs, Florida, the regular sea urchin ’ Present address: Applied Biology, Inc., P.O. Box 974, Jensen Beach, FL 33457, U.S.A. ~22-0981/81/0~~~0/$02.50

Q 1981 ~tsev~er/North-Holland

Biomedical Press

‘6

ROBERTG.

I3RNESTANUNORMANJ.BLAKE.

,!_j~e~hinus vuriegatus (Lamarck) is perhaps the most conspicuous of the macroinvertebrates. It occurs in relatively large numbers and occupies a variety of habitats. Although L~techinus has been collected extensively for embryological material in the southeastern United States (Harvey, 1956; Brookbank, 1968), there have been relatively few studies of its reproduction. Gonad indices were used to follow its reproductive cycle at Bermuda and Miami (Moore ~‘f01.. I963 : Moore & Lopez, 1972), while elsewhere, only observations of spawning have been reported (Boolootian, 1966). Previous studies of reproduction in /~,t./eclzitzrl.shave not addressed the problem of environmental heterogeneity. Moore pt crl. (1963) were aware of this when they suggested that the depicted reproductive cycle of L~~~~~j~?~,~ at Miami may have been obscured by the failure to collect specimens at one permanent location. The present study, therefore, examined the reproductive response of Lytechinns to different habitats. Both gonad growth and gametogenesis were monitored, enabling evaluation of the effect of environmental heterogeneity on the interrelationship of these reproductive processes.

METHOIXAND

MATERIALS

SAMPLING

Four stations in the Anciote estuary (Fig. If were selected as samphng sites, each meeting the criteria of having different habitat characteristics and harboring sufticient numbers of urchins for repeated collections. Stations 1 and 2 were located near the mainland shore in w 1.5 m of water and were characterized by luxuriant beds of mixed seagrasses. Syringodium ,filiforme and Thalassin testudinum predominated, while Halodufe wrightii contributed a smaller percent of total cover. Drainage from numerous canals coupled with existing circulation patterns in the Anclote anchorage produced relatively uniform salinities at the two stations. Temperatures were often greatest at Station 2, since it frequently came under the influence of thermal effluents from a nearby power plant (Fig. 1). Station 3 was located in the center of the anchorage in m4.5 m of water. At this station, seagrasses were absent with the exception of an occasional shoot of H. wightii. Alternate food resources were present as drift algae which settled into depressions on the sea bottom. Station 4. located on the lee side of Anclote Key in RZ2 m of water, was characterized by alternating patches of Thnlassiu and bare shell-sand substratum. Hydrographic conditions at Stations 3 and 4 were similar and closely approximated Gulf of Mexico conditions. Urchins were collected monthly from November, 1974 to December, 1975. At each station, 35 individuals within a predetermined size range (55-65 mm test diameter) were collected by diving, transported to the laboratory in large aerated Styrofoam coolers, and processed immediately.

REPRODUCTIVE

PATTERNS OF ~YTE~HI~U~

VARIEGATUS

27

Fig. 1. Ancfote anchorage study site showing location of stations used for evaluating the effects of habitat on the reproductive cycle of ~~f[,c~i~us vuriegmts.

GONAD INDEX

Twenty-five urchins were dissected, the gonads from each removed, placed in pre-weighed aluminum pans, dried to constant weight at 50’32, and then weighed to the nearest milli~am. The tests (including spines) and viscera were treated in the same manner. Sex determinations were made whenever possible by examining gonads for extruded gametes. The gonad index (GI) employed during this study is defined as: GI =

dry wt of gonads x 100. total dry wt of organism

y

ROBERTG.ERNESI‘ANDNORMAN.I.BLAKE

Variation in the gonad index resulting from differences in size-frequency distributions among individuals from different stations (Moore cut t/l.. 1963) was minimized

by using urchins

of similar

test diameters.

HISTOLOGY

Gonads from the remaining 10 urchins per station were removed and fixed in Dietrich’s solution (Yevich & Barszcz, 1977). The tissues were later removed from the solution. processed on an Autotechnicon (Technicon Corp.) through a series of commercial dehydrating and clearing agents. and finally immersed and embedded in “Paraplast”. A series of 6-pm sagittal sections were stained with Hematoxylin and Eosin (Luna, 1960). ‘The gametogenic stage of development of each individual was ranked according to the method described by Fuji (1960b). These stages were used to compute a mean gonad maturity index (Green, 1978). Urchins closest to spawning condition (Stage IV) were assigned the highest rank (5). while those having spent gonads (Stage V) received the lowest (1). The mean for all individuals examined monthly at each station provided a relative estimate of reproductive ripeness. Additionally. each slide containing ovarian tissue was examined and a transect made through a representative portion of the slide. Along the transect, long-axis measurements of oocyte and ovum diameters were made of the first 50 gametes encountered in which the cross section included the nucleus. The relative quantities of nutrient material were estimated by measuring the thickness in cross section of gonadal tissue along the transect and calculating the percentage of total cross section occupied by nutritive phagocytes.

REX! LTS Mean monthly gonad index values indicated disparate trends in annual gonad development among the four stations studied (Fig. 2). Initially, the index at all stations increased from late November to early December. 1974 to high levels between January and March, 1975. Subsequently, a substantial decline occurred, suggesting spawning. Except at Station 1, where a bimodal pattern was observed, the gonad index continued to decline throughout spring and summer, reaching minimum values in October. By early December, 1975, gonads once again began to increase in size indicating the initiation of another cycle. In addition to the March peak, Station 1 data showed a second smaller peak in mid-summer (Fig. 2). This second peak was followed by a decline in the index to a minimum level in October. The marked decline in the gonad index at Station 3 between August and September, 1975, may also be indicative of a late-summer spawning, even though a bimodal annual pattern was not evident. KruskallWallis and STP non-parametric tests (Sokal & Rohlf, 1969) indicate

REPRODUCTIVE

PATTERNS

OF LYTECHZNUS

29

VARZEGATUS

that only during April were there no significant differences in gonad index values among stations (P < 0.05). During that month, gonads readily disgorged gametes if cut or agitated during extraction. This extremely ripe condition was common in

8-

Station ‘\

‘.

64-

1

‘\

1 80 80

40

’

20 100

Station

2

80

!! v)

60

” I-

40

Ill >

20

F: E

100

5

80

= l-

60 40 20

E 0 [r ul n

100 80 60 40 20

DJFMAMJJASON ) and nutrient accumulation (----) for Fig. 2. Relationship between patterns of gonad growth (urchins collected monthly from November, 1974 to December, 1975 at each of four stations: vertical lines represent 1 SD about the mean.

urchins from all stations and suggests that spawning was occurring throughout the anchorage. Based on gonad index curves, spawning appears to have begun in March at all but Station 4, where it began in February (Fig. 2). Subsequent to the spring spawning, mean monthly gonad index values showed considerable variation among stations, indicating a deterioration of reproductive synchrony.

ROBERT

30

Both timing and absolute differed January

G. ERNEST

AND

NORMAN

values of maximum

J. BLAKt

and minimum

gunad

among sub-populations (Table I). Maximum gonad at Station 2, in February at Station 4, and in March

development

size was reached in at Stations 1 and 3.

T!\HU I Results

of Kruskal~Wallis

(H) and STP ((I’,) non-parametric tests applied monthly gonad index values. Kurskal

Highest mean monthly gonad index

Station

I

Date

6.68 x.79 7.1 7.83 15.93*

7 1 3 11

to highest

and lowest mean

~Wallis test Lowest mean monthly gonad index

( 1975)

6 Mar. 3 Jan. 13Mar. 13 Feb.

Date (1975)

2.51 I .‘S 0.76 0.74 34.40*

9 I4 I4 9

Oct. Oct. Oct. Oct.

STP Highest I 2 490*

I3 325

* Significant

Urchins Stations

mean monthly

gonad

index

I4 2-3 24 443 460.5* 33X.5 Critical C, = 444.9

3 4 409

Lowest mean monthly h’ 75

11 794

13 372*

I4 360* C’ritul

gonad

Lli 24 345* 33l* I’, = 322.6

index 34 218

!V 21

at P < 0.05

at Station 2 achieved significantly greater gonad size than urchins at 1 and 3, while minimal size at Stations 3 and 4 was significantly lower

than at Stations 1 and 2 (P < 0.05). Observed differences in reproductive indices cannot be attributable to sexual variability in population structure, as no significant differences from a sex ratio of 1 were found at any station (x’_ P < 0.05). Although the timing of gametogenic activities differed slightly among stations, the progression of events was similar. Mean monthly ranks of gonad maturity indicate two periods of advanced gametogenic activity for all stations (Fig. 3). The first occurred in spring and, except for Station 2, closely corresponded to the time of highest mean monthly gonad index values. The second peak came in late summer and indicates a period of increased gametogenic activity not evident from gonad index curves at Stations 2, 3, and 4. Spring peaks were much more pronounced than late-summer peaks and again reflect sub-population synchrony of urchins during the early part of the year. As spawning and recovery took place at differential rates among individuals within each sub-population, mean monthly gonad maturity values declined. The presence of some mature (Stage 1V) and some spent (Stage V) urchins during most months suggests that spawning may be continuous over much

REPRODUCTIVE

PATTERNS

OF LYTECHINUS

VARIEGATUS

31

of the year with a certain percentage of urchins in each of the different stages of gonadal development.

.’

‘N..

I

6-

STATION

‘\

I 4-

I

3-

4

60

‘\

/’

i

2-

2

40

‘\

\

..’

/I

p\

$&_

- 30

‘\

/’

20

a E

-._ 6-

STATION

3 50

\

40

4-

30 20

STATION

4 50

40

30

20

A

S

0

N

Fig. 3. Mean rank of gonad maturity (---) and mean oocyte diameter (- .- .- .-.-) for urchins collected monthly at each of four stations: vertical lines represent 1 SD about the mean.

Curves of mean monthly oocyte diameters were similar to those for mean monthly gonad maturity ranks (Fig. 3). However, due to the similar sizes of mature ova (Stage IV) and advanced oocytes (Stage III), these curves are not as definitive in determining peak gametogenic development. Similarly, they are not as precise in detecting spawning commencement because unshed ova and advanced oocytes in recently spawned urchins (Stage V) are included in calculations of mean oocyte diameter. Declines in mean oocyte diameter curves do indicate that by May, the major spawning effort of the year had diminished.

311

ROBERT

G. ERNEST

AND NORMAN

J. BLAKE

Following the initial spring spawning, unshed ova of Stage V urchins were resorbed by nutritive phago&ytes refilling the vacated lumen. During this process, new oocytes were continually being produced along the periphery of the follicles. On no occasion during this period did oocyte production cease entirely. as it did following the final spawnout in late summer when a complete lysing of tissue within the follicles was observed. Relative quantities of nutritive tissue exhibited an inverse relationship with histological indexes. However, nutrient accumulation clearly accounted for early gonad growth in only two instances: during the winter at Station 2 and during the summer at Station 1 (Fig. 2). At Stations 1, 3, and 4, reiative quantities of nutritive tissue were at their highest winter levels prior to observed increases in gonad size. However, since increases in gonad size were obviously attributable to nutrient accumulation following the final spawnout, it is possible that some gonad growth had already occurred prior to the first collections in 1974. Bottom water temperatures at all stations gradually increased between December and May with relatively high levels being maintained between May and September.

2st / 26, 1 24i 221

=Jt Is,,;

‘41 L__.Lx

_L__Ly_T”-M”--+_Kll‘

Fig. 4. Bottom

water temperatures

_j.

for Stations

L--1

I (--

*TLC

)and2(

i .o-L

! for 1974 fY7.5.

Rapid decreases were observed between October and Novem~r, as the first winter cold fronts began passing thmnugh the area. Station 2, which came under the influence of thermal discharge from a near-by power plant (Fig. I], often experienced water temperatures 1-2 “C higher than those at Station 1 (Fig. 4). Differences between these stations, located in similar habitats and water depth, were most noticeable between May and August, when normal seasonal temperatures were highest.

REPRODUCTIVE

PATTERNS OF L YTECHZNCJS

VA

RZEGATUS

33

Results of gonad index measurements at Anclote suggest that substantial variability in reproductive patterns occurred among sub-populations of Lytechinus variegatus exposed to different environmental regimes. However, histological data indicate that within the overall population a rather uniform trend of gametogenic activity existed. Thus, if, as postulated for other marine invertebrates (Sastry, 1975), the annual reproductive cycle of L. variegates is endogenously controlled, it can be measurably modified by external factors. Intraspecific plasticity in reproduction is common among echinoids, with the intensity, timing, and duration of reproductive events varying among populations separated in space and time (Ebert, 1968; McPherson, 1969; Pearse, 1969; Dix, 1970; Moore & Lopez, 1972; Gonor, 1973). For species where both gonad index and histological data have been reported, maximum gonad size usually corresponds to periods when highest percentages of ripe individuals occur in collections (McPherson, 1965 ; Dix, 1970; Gonor, 1973; Niesen, 1977). During the present study, gametogenic cycles and gonad index curves were closely aligned during the initial gonad growth and spawning phase. However, at all but one location, advanced gametogenic activities occurring later in the year were not reflected by gonad growth curves. This is probably due to habitat-related intraspecific differences in patterns of nutrient accumulation. As Gonor (1973) states, “the process of nutrient accumulation and gametogenesis are interrelated, but their initiation and rates may be influenced independently by environmental factors”. In most urchins, the initial increase in the gonad index results from nutrient accumulation, and gamete production begins only after an adequate nutrient reserve has been established (Pearse, 1969; Gonor, 1973; Bernard, 1977). For urchins collected at Anclote, only two cases of gonad growth resulting from nutrient acc~ulation were apparent. The most obvious was at Station 2, where the gonad index reached its highest peak a month or two before spring peaks in gonad maturity and mean oocyte diameters. At other stations, nutrient accumulation and early gonad growth occurred either prior to the first collections or simultaneously with gamete proliferation and growth. During the summer, the gonad index curve at Station 1 also increased well in advance of corresponding rises in histolo~cal curves. Relative increases in gonad size were accompanied by resorption of remaining ova and advanced oocytes from the previous spawning and a concomitant accumulation of nutritive tissue. Resorption of unspawned gametes, a process common among echinoids (Bernard, 1977; Lane & Lawrence, 1979), also occurred at other stations even though no noticeable increases in gonad size were observed. It appears, therefore, that conditions at Station 1, in 1975, represented an optimal environment. Urchins during summer had sufficient energy in excess of metabolic demands to substantially augment nutrients generated from resorption of unspawned reproductive products.

34

ROBERT G. ERNEST AND

NORMAN

.I. BLAKE

Spawning commencement may be inferred from any of the indexes used to monitor gonad growth and gametogenesis. However. due to the small sample sizes and high variability among urchins used for histological examination, the alignment of gametogenic indexes with patterns of gonad growth was not exact. Furthermore, as evident from data collected during this study. reproductive indexes have the capacity for dramatic changes over a relatively short period of time. Thus, the precise time of spawning is difficult to determine from a monthly sampling program. It does appear, however, that within the Andote anchorage, a relatively synchronous period of gamete dispersal existed during spring. Spawning synchrony among local populations inhabiting heterogeneous environments has previously been reported for the echinoids Strongyiocersnotus nudus and S. intermedius (Fuji, 196OaL S. purpuratus (Gonor. 19’73) S. fianciscanus (Bernard, 1977), EclCnometra lucunts (McPherson, 1969), Evechinus chloroticus (Dix, 1970), and Dendrasfer rxcentricus (Niesen. 1977). Following the spring spawnjng. reproductive synchrony deteriorated. However, because the number of young primary oocytes relative to the number of advanced oocytes maturing into ova had increased substantially, histological indices declined to relatively low levels. These summer lulls were not comparable to the quiescent periods observed in other echinoids (Pearse, 1969). The continual proliferation of gametes and the presence of ripe or recently spawned animals in most collections suggests that similar to findings in other areas of Florida, ~~~~~~~~~svariegafus at Anclote has the potential to spawn over a relatively long portion of the year (Moore ef (II.. 1963; Brookbank. 1968). After the final late-summer spawnout, oocyte production did cease entirely, and a complete lysing of tissue within the follicle was observed in many specimens. it is uncertain if this process was universal. since it appeared as though resorption of remaining ova and oocytes had taken place in a few urchins. In any event, the final post-spawning phase represented a true period of gametogenic quiescence and appeared to ready the follicles for nutrient accumulation. Details of gametogenesis in L. V(zriegutzd\’ at Anclote were given by Ernest ( 1979). in generaI, ~metogenic processes proceeded in a manner closely agreeing with those described for other regular urchins (Fuji, 196Ob; Holland & Giese, 1965 ; Chatlynne, 1969; Pearse, 1969; Gonor, 1973; Bernard, 1977). Oocytes in various stages of development attained dimensions and assumed positions in the follicle typical of these echinoids. Initially, highly globulated nutritive phagocytes, characteristic of early deveIopmenta1 stages (Chatlynne, 1969), filled the follicles. Subsequently, oocytes produced along the periphery of the follicle began to displace nutritive tissue as they grew and migrated toward the center of the lumen. Eventually, mature gametes filled the entire lumen, nutritive tissue being confined to a thin peripheral band. Once spawning occurred, resorption of remaining ova resulted in a refilling of the follicle with nutritive phagocytes. At this point, gametogenic synchrony declined as continued gamete production. maturation and

REPRODUCTIVE

PATTERNS

OF ~~TEC~INU~

VARIEGATUS

35

spawning took place at differential rates within the overall population. Competitive interaction between phagocytic and gametogenic cycles has been demonstrated for numerous species of echinoids and appears to be a universal process (Holland & Giese, 1965 ; Chatlynne, 1969 ; Bernard, 1977 ; Lane & Lawrence, 1979). Thus, resorption of gametes both before and after spawning is probably always taking place. Deterioration of reproductive synchrony in Lytechinus following the initial spawning most likely resulted from variations in the rates of gamete production and the degree to which reproductive products were being resorbed. The mechanism which ultimately returned sub-populations of ~yrec~~~~~ into reproductive synchrony is unknown, but one possible explanation is advanced. Some factor may retard phagocytosis during the summer thus allowing for the final major build-up of gametes. Once spawning has occurred, complete lysing of tissue within the follicle would then initiate processes to commence the next cycle. Gonor (1973) similarly found for Strongylocentrotus purpuratus that the proliferation period of a new reproductive cycle did not begin until the delayed maturation and spawning of remaining large oocytes and ova of the previous cycle had been completed. If this pattern is also true for Lytechinus variegatus, then oocytes produced during one cycle would not be maintained until the next. Bernard (1977) and Gonor (1973) found this to be the case for species of St~~ngyZocent~o~us. However, evidence to the contrary exists for other species (Holland & Giese, 1965), and this phenomena may be subject to annual variation (Lane & Lawrence. 1979). Causal relationships between environmental factors and reproductive variability are difficult to infer from field studies, but temperature may be responsible for observed differences in gonad growth patterns between Stations 1 and 2. The most noticeable difference was the lack of a summer gonad growth phase at Station 2. Summer temperatures near the power plant discharge canal approached lethal limits for Lytechinus variegatus (Chesher, 1975), and numerous urchins either recently dead or in a deteriorated state were observed. Algal composition, which was normally quite similar at the two stations, was noticeably affected by the high summer temperatures at Station 2. Both types and quantities of algae present were reduced. Between July and August, seagrass cover at Station 2 also declined considerably, turbidity increased, and bluegreen algal mats began covering the area. The obvious stressful conditions imposed on urchins near the discharge probably affected the amount of energy available for reproduction by increasing metabolism (Moore & McPherson, 1965), reducing activity coefficients (Lawrence, 1975a), and lowering feeding rates (Moore & McPherson, 1965). Since a mixed diet of plant material appears to be of greater nutritional value to L. variegutus than a monospecific one (Lowe & Lawrence, 1976) temperature also may have indirectly affected reproduction by reducing available food choices. Furthermore, items available for consumption may not have been appropriate to the urchms’ mode of feeding (Lawrence, 1975b). The cumulative effect of these factors would be a reduction in the amount of energy available for reproduction, and nutrient reserves

36

ROBERT

G. ERNEST

AND

NORMAN

J. BLAKE

already in the gonads might even be withdrawn in response to other physiological demands (Ebert, 1968). The bimodal gonad index curve characteristic of urchins at Station 1 is unusual among echinoids. Gonad index curves of populations of Lytechinus at Miami have shown considerable annual variation, but distinctly bimodal patterns have not been reported (Moore & Lopez, 1972). When distinct winter and summer spawnings have been observed in other species (McPherson, 1965 ; Sastry, 1975), these patterns have been demonstrated to be phenotypic in nature (Sastry, 1966, 1975). It is suggested that the trends at Anclote are also phenotypic expressions of the particular environments acting on reproductive activities. Each of the stations sampled during the present study had unique combinations of food and physical regimes which resulted in different gonad growth patterns. Since prespawning size of gonads largely determines the amount of eggs produced (Gonor. 1973), reproductive outputs among sub-populations may also have differed. Although these data represent only a small portion of the spawning population and only one year of observation, they do serve to illustrate the independence of gametogenic activities and gonad growth in L. wriegatus.

ACKNOWLEDGEMENTS

The authors wish to thank J. Studt and J. Stevely for assistance in field work and Drs. J. M. Lawrence, A. N. Sastry, and R. Scheibling for their suggestions and critical review of the manuscript.

REFERENCES

BERN.ARD. F. R., 1977. Fishery and reproductive cycle of the Red Sea urchin. Stron~~~f~c,enrmtur ,fiunciscanus. in British Columbia. J. Fish. Res. Bd Can.. Vol. 34. pp. 604610. BOOLOOTIAN, R. A., 1966. Reproductive physiology. In. Ph,vsiology of’ Echinodermain, edited by R. A. Boolootian, Interscience Publ.. New York. pp. N-614. BROOKBANK, J. S., 1968. Spawning season and sex ratio of echinoids. Q. .I/ f’/cr .d<,crtl. Sci.. Vol. 30. pp. 177-183. CHA~LYNNE, L.G., 1969. A histological study of oogenesis in the sea urchin, S/~o~~~‘loc~ntrollrs purpuratus. Biot. Bull. mw. biot. Lab., Woodr Hole, Vol. 136, pp. 167--184. CHESHER. R. H.. 1975. Biological impact of a large-scale desalination plant at Key West. Florida. In, Tropiculmarinepollurion edited by E. J. F. Wood & R. E. 3ohannes. Elsevier Oceanogr. Ser. I?. Elsevier Sci. Pub]. Co., New York, pp. 99.-153. COCHRAN, R. C. & F. ENGELMANN, 1975. Environmental regulation of the annual reproductive season of Strongylocentrofus purpururus (Stimpson). Biot. Butt. mar. hiot. Lab.. U’oods Hole. Vol. 148, pp. 393-401. DIX, T. G., 1970. Biology of Evechinus chtororicus (Echinoidea; Echinometridar) from different localities. 3. Reproduction. N. Z. Jt mur. jieshw. Res., Vol. 4, pp. 385-405. EB~RT, T. A.. 1968. Growth rates of the sea urchin Srrongyk~centrotus prwpurrrrus related to food avaiIabiIity and spine abrasion. Ecology. Vol. 49, pp. 1075 -1091. ERNEST. R. G., 1979. Reproductive variability in LJtec’hinus variegarus (Echinodermata : Echinoidea)

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PATTERNS

OF LYTECHINUS

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