BULLETIN OF MARINE SCIENCE, 30(1): 90-101, 1980

STUDIES ON THE BIOLOGY OF ALATUS GMELIN (: ) WITH NOTES ON ITS POTENTIAL AS A COMMERCIAL SPECIES

Avril M. Siung

ABSTRACT There is potential for development of lsognomon ala/us as a commercial species. Its ability to live in bottom sediments and on suspended collectors, intertidally, and subtidally, in widely fluctuating conditions of salinity and suspended matter, allows it to grow in a variety of environments. The primary prodissoconch larva was identified by the "indirect method" and described. A high percentage of ripe gonads was present in the population throughout the year. Peak spawning periods occurred after the onset of the rainy season when salinity decreased. The presence of eggs and sperms in the water induced other indi- viduals in the vicinity to spawn. Spatfall prediction is possible by monitoring salinity changes and larval concentrations in the water.

Isognomon alatus is widely distributed throughout the Caribbean from Florida to Tobago. It is commonly found in mangrove swamps attached by byssus to the prop roots of the red mangrove tree ( L.), attached to jetty pilings, or growing in bottom sediments in shallow water. Despite its abundance, very little is known about this species or genus. The only publication on I. alatus (Trueman and Lowe, 1970) deals with the effects of temperature and littoral exposure on heart rate. The only other available paper on the genus lsognomon (Yonge, 1968) is a comparative study between several species of Malleus and 1sognomon. From Yonge's descriptions 1. ephippium seems most nearly to resemble I. alatus. In Jamaica I. alatus is eaten in small quantities although it is not as much in demand as the Caribbean mangrove oyster, Crassostrea rhizophorae Guilding. Due to overexploitation and to the destruction of many mangrove areas C. rhi- zophorae numbers have been greatly reduced. Now I. aLatus is being eaten as an alternative to C. rhizophorae and must be protected or cultured to prevent de- pletion of its natural stocks. This preliminary study was conducted in Jamaica and encompasses aspects of the biology of I. alatus including identification of the planktonic larvae, deter- mination of the breeding seasons, growth and mortality rates. Physical parameters affecting the distribution of the species were also investigated.

MATERIALS AND METHODS

This work was carried out at the University of the West Indies Marine Laboratory situated at Port Royal near an expanse of mangrove swamp. Field observations were also made at various locations around the island. Physical parameters measured were temperature, salinity, and suspended matter in the water. Surface water temperatures were obtained with a mercury thermometer, and salinity readings by silver nitrate titrations. Total suspended matter was measured by vacuum filtering I litre of sea water through Whatman's Quantitative filter paper. The filter paper was washed in distilled water, then kept in a dessicator for 3 to 4 days and weighed until a constant reading was obtained. The dry-weight difference of the filter paper before and after filtering was used as a measure of the amount of suspended matter present in the water. tows were taken with a No. 10 Clarke-Bumpus net. Two 5-minute tows, one at the surface and one just off the bottom, were made at weekly intervals. Water depths in the working area were

90 SlUNG: BIOLOGY OF 91

Figure I. I. a/atlls shells: a, the normal circular form; b, the posterior extensions of the shell margins. only I to 2 m. Plankton was preserved in 4% buffered Formalin and later sorted under a dissecting microscope. Mortality and growth experiments were carried out on spat collected on panels hung between the mangrove prop roots. Panels were made in the shape of a picture frame made of perspex (Ieucite) into which eight microscope slides could be placed contiguously. The slides were held in place by a single sheet of black perspex bolted to the back of the frame. In this way only one side of each slide was exposed as a settlement surface and the eight slides together formed a settlement area of ap- proximately 150 cm2, It has been shown that many sessile marine invertebrates including bivalves preferentially settle on shaded undersurfaces (Hopkins, 1935; Cole and Knight-Jones, 1939; Pomerat and Reiner, 1942; Crisp, 1967; Ritchie and Menzel, 1969), and accordingly the panels were hung in a horizontal position with the collecting surfaces facing down. Additional methods and materials used in each aspect of this study are described in the following sections.

Gross Anatomy The model shell height of I. alatus is between 4 and 5 cm, but specimens as large as 9 cm have been found. The shell is roughly circular in shape with flat 92 BULLETIN OF MARINE SCIENCE. VOL. 30. NO. I, 1980

h

I f n b

ht v

em pbr a qa ea I] v - m mf

p / / Figure 2. lsognomon a/atus, left shell valve and mantle removed (arrows indicate the regions of inhalent and exhalent water flow); a, anus; b, byssus threads; ca, catch or smooth adductor muscle; cm, cut mantle; f, foot with smaIl sucker; g, gills; ht, heart in pericardium; h, hinge; I, labial palps; m, mantle cavity; rnf, mantle folds bearing sensory tentacles; n, nacreous layer of shell; p, periostra- cum; posterior byssal retractor muscle; qa, quick or striated adductor muscle; r, rectum; v, velum; vm, visceral mass.

valves (Fig. la). The hinge line is usually straight and elongated. The ligament is subdivided into a series of inner ligament layers with intervening outer layers. The number of inner ligament layers in J. alatus ranges from 8 to 12, depending on the length of the hinge line. Yonge (1968) described the formation and structure of this multivincular type of ligament in J. isognomon and J. ephippium. The shell color varies from reddish-brown, especially in young specimens, through yellow and brown to almost black in some of the older individuals. Growth lines are present on the surface of the shell. The interior is composed of a pearly nacreous layer that extends as far as the pallial line. Apart from the normal circular shape, J. alatus is often found with elongated posterior margins formed mainly from prismatic shell growth (Fig. lb). Individ- uals of this growth form are found especially in areas where they are crowded or attached to the sea floor where sedimentation is taking place. Elongation of the posterior regions of the valves may assist in raising the above other fouling organisms or the sediments on the sea floor and so allow free flow of water through the valves. This growth form is similar to the shape of the hammer oy- sters (Malleus sp.) and J. isognomon (Yonge, 1968), whose elongated bodies are adapted to living partially buried in soft substrates. The soft body (Fig. 2) is roughly oval in shape and is covered by an opaque SlUNG: BIOLOGY OF ISOGNOMON ALA TUS 93

GOO ,u

Figure 3. The definitive prodissoconch larvae of I. ala/us. Scale = 500 ILffi.

mantle. The mantle margins, including the velum, are pigmented and well devel- oped, bearing numerous sensory tentacles. The visceral mass may be cream to bright orange in color due to the presence of gonad tissue. The heart is easily visible within the large transparent pericardium lying between the adductor mus- cles and a recess on the posterior side of the visceral mass. The foot is reduced and is located at the anterior end of the animal extending from the visceral mass just below the mouth (Fig. 2). Byssus threads are produced at the base of the foot, while the tip acts as an exploratory organ especially in young individuals. The foot can extend from the body as much as 1 em, where it may attach to the substratum by means of a sucker-like structure at the tip; subsequent contraction of the foot enables the body to be pulled along behind. 1. alalus is therefore able to explore a substrate before byssus threads secure the animal permanently. The posterior byssus retractor muscles are well developed and attach to the shell above the adductor muscles.

Larval Identification Larvae of the genus lsognomon have not previously been described. The planktonic larvae of 1. alalus were identified by the "indirect method" (Menzel, 1955) since attempts to rear the larvae from fertilized eggs failed. Plankton tows taken at weekly intervals yielded 13 different types of prodissoconch bivalve 94 BULLETIN OF MARINE SCIENCE, VOL. 30, NO. I, 1980 a. Right Valve

t Left Valve

~." ... .. • 0° • • • : • • • .' •••• °0 • • •••• :...... • • b. • ,0 '0' '0 •

...... • • 0° . . '...... Right Valve

Figure 4. A diagram of the (a) inside and (b) dorsal views of the larval hinge of [, ala/lis. larvae and many undifferentiated straight-hinge stages. The various definitive prodissoconch larvae were each placed in separate small culture dishes with sea water and a clean empty shell (any species). The shell served as a settlement substrate and the sea water was changed every 24 h. To prevent temperature fluctuations of the water, the dishes were partially immersed in a continuous flow of sea water. After the first or second day, some of the larvae attached to the shells. The position of these settled larvae on the shells were noted and the shells were placed in tanks of running sea water, These larvae were reared in the lab- oratory until they became identifiable by their adult characteristics. Four of the larvae including those of I. alatus were successfully identified. It is possible that the other 9 larval types were infaunal bivalves requiring sand or mud for settle- ment. The prodissoconch of I. alatus taken from plankton tows were relatively con- sistent in size varying from 210 J.Lm to 237 J.Lm in height, and 262 J.Lm to 280 J.Lm in length. This is the size reached prior to settlement. The shape of the larvae is distinctive, being oval with slightly projecting but rounded umbones (Fig. 3). The umbo of the left valve is more pronounced than the right and the anterior shell margin is rounded while the posterior is more pointed. The structure of the hinge was studied after soaking the larvae in a solution of sodium hypochlorite diluted 1:1 with sea water, which dissolved the organic tissue of the hinge and separated the valves (Forbes, 1967). The hinge of the prodis- soconch (Fig. 4) has a simple arrangement of teeth similar to that described by Rees (1950) for the primary prodissoconch of the superfamilies pteriacea (to SlUNG: BIOLOGY OF ISOGNOMON ALATUS 95

500)A

Figure 5. Various sizes of I. alalus settled larvae. Scale = 500 Mm. which Isognomon belongs) and Ostreacea. However, the small taxodont teeth present in the central portion of the primary prodissoconch hinge are absent from that of the prodissoconch of I. aLatus. Instead there are four rectangular teeth with clear gaps between them, posterior to the central portion of the hinge, while anteriorly there are five more teeth with spaces between them. After settlement and byssal attachment of the larva, growth occurs by extension of the shell around the margins of the original prodissoconch. There is also a change in direction of the growth axis which brings about the more rapid exten- sion of the posterior margins of the shells. Continued growth along this new axis eventually results in the adult form of I. aLatus (Fig. 5). Development of the larval hinge into the adult form was not investigated, but it is probably by formation of new areas of inner ligament which appear at intervals posterior to the original larval ligament (Yonge, 1968). The planktonic larvae of I. aLatus collected at Port Royal were consistently brown in color from primary prodissoconch to settlement. The newly settled spat maintained this coloration, but as they increased in size to about 5 mm in height reddish tinges appeared.

Breeding Season Carter (1930) and Day (1967) suggested that among coastal marine invertebrates in the tropics rainfall is the ecological factor controlling breeding patterns. In the absence of thermal fluctuations, changes between the rainy and dry seasons may probably be the controlling factor in stimulating spawning. Van Someren (1960), Hunter (1969), Nikolie and Melendez (1968), and Bacon (1971), all obtained cor- relation of the breeding seasons of various oysters with changes in water salinity. At Port Royal simultaneous recordings of salinity and temperature were taken together with observations on the breeding patterns of I. aLatus. 96 BULLETIN OF MARINE SCIENCE, VOL. 30, NO.1, 1980

Gonad condition, availability of planktonic larvae, and actual spatfall counts, are by themselves not adequate to give a true picture of breeding patterns. The presence of ripe gonads does not necessarily indicate an imminent spawning period, neither does the absence of larvae in the plankton indicate no spawning. There is often poor correlation between larval concentrations and spawning in- tensity data (Prytherch, 1928; Galtsoff, 1930; Loosanoff and Engle, 1940), and spatfall may be poor for reasons other than reduced spawning activity. Hence, gonad condition, as well as plankton samples and spatfall were examined together over a I-year period. Spatfall data were recorded from collecting panels. Two were placed in the mangroves, one intertidally and the other subtidally, 1.5 m below mean low water. At 2-week intervals the panels were removed for spatfall examination and re- placed with clean slides. At monthly intervals, 30 adult gonads were examined from I. alatus collected from the Port Royal mangroves. The condition of the gonads was estimated on a scale from 0-4; zero denoting individuals in a watery condition with no gonad development and 4 indicating gonads well developed and ready to spawn. Gonads which scored 3 and 4 were considered ripe while those below 3 were rated unripe. The gonad examinations showed, that at each sampling period, well over 50% of the individuals collected had ripe gonads all year round (Fig. 6a). A reduction in the percentage of ripe I. alatus was recorded in August, October, and Decem- ber 1973. The numbers of planktonic larvae collected increased, and heavy spat- fall occurred in September and November, 1973, showing good correlation be- tween all the recorded parameters. Temperature recorded during the same period (Fig. 6b) indicated cooling of the water from November onwards through the winter months, reaching a low of 26°C in January the following year before rising again to a maximum of 30°C in August. Commencing in August salinity declined from a high summer value of 36%0 at the start of the rainy season (Fig. 6c) to a salinity minimum of 14%0 in October. This was caused by hurricane Gilda, however salinity rose to 33%0 by December. The decrease in salinity beginning in August coincided with a high percentage of individuals with "watery" gonads, followed by a large number of larvae in the plankton and spatfall in September (Fig. 6a). Very low salinities in October coincided with a reduction in planktonic larvae and spatfall which in- creased again in November when salinities returned to 33%0. Summer water tem- peratures of 29-30°C showed little change until November, at a time when peak breeding had already begun. Thus, although the peak breeding seasons may seem to occur during the cooler months of the year (i.e. at the end of the summer), a temperature drop is unlikely to be the trigger mechanism for mass spawning. Thermal fluctuations over a 24-h period in the tropics are often greater than the seasonal changes. It is therefore probable that the decrease in salinity at the beginning of the rainy season triggers spawning among I. alatus and perhaps an optimum salinity range is needed for successful larval development and spatfall. The very reduced salinity that occurred in October during hurricane Gilda was probably the cause of the reduction of plankton and spat when the organisms should have been actively reproducing. It is widely known that in addition to certain physical conditions! the presence of male and female gametes in the water can stimulate ripe adults to spawn. Using 30 adult specimens of I. alatus, a suspension of eggs and sperms was added to the water in small culture dishes. Between 30 sec and].5 h after the suspension was added, 12 males and] female spawned. This supports the observation that adult males are more responsive to this kind of sexual stimulation than females (Galtoff, ]964). SlUNG: BIOLOGY OF ISOGNOMON ALATUS 97

a.

100

500 %of 75 No. of adults larvae: with ripe planktonic - ~ gonad 50 300 settled _ • 0 25 100

b.

30

Temp. 28 0('

26

c. 35 Salinity 0/ 30 /00

2S

20

15

:\1 J .J A S () :\ D .J F :\1 A 1973 1974

Figure 6. Histograms: a, the percentage of ripe gonads, planktonic and settled larvae in the I. a/alUS population; b, the sea temperature °C; c, the salinity %0.

The non-spawning individuals later examined were found to be mostly females or spent adults. Control individuals kept in ordinary sea water did not spawn. At Port Royal, once reduced salinities bring about increased spawning, the presence of gametes in the water can help stimulate more individuals to spawn. Recovery of spent gonads after periods of increased spawning seems to be rapid for I. alatus. This is suggested in the consistently high percentage of ripe adults present only short periods after heavy spawning (Fig. 6a). 98 BULLETIN OF MARINE SCIENCE, VOL. 30, NO.1, 1980

50

40

..n LJ.J 0< 30 l- I LJ.J ~ -~ f -~ J'J" :E ,>1/ 20 /;' z ,II, '1' 1 / I- ,Il'f I 10 /'. (,) /, III LJ.J /1' I '1' / / / ,/ II~ ,I' il/ 0 S D J M A M 1973 1974

TIME IN MONTHS

SUBTI DA l & ------INTERTI DAl GROWTH

Figure 7. Graph showing the growth rates of [, alarus individuals in the intertidal and subtidal zones.

Growth and Survival Growth and survival rates of I, alalus were recorded on individuals settled on collecting panels, Bimonthly observations were made on these panels which were hung in the Port Royal mangroves, one in the intertidal zone and one subtidally. The experiment ran from the end of September ]973 to May ]974. The growth of individuals was measured and the total number of I. alalus present at each examination was also recorded to determine their survival. The results showed little difference between the intertidal and subtidal growth rates (Fig. 7). Growth was fairly rapid achieving a height of 50 mm within 6 months. Survival rates as a percentage of the total number of individuals on each panel were 33% survival intertidally and 17% subtidally. However, the greater mortality SlUNG: BIOLOGY OF /SOGNOMON ALATUS 99

Caribbean Sea l17'w t

10 ,Miles,

~M8jor mangrove areas

Figure 8. Map of Jamaica showing the main mangrove areas and the locations (1-8) of I. ala/us populations. in the subtidal zone is not significant because of the small sample size. I. alatus is able to survive both in the intertidal and subtidal zones despite the presence of fouling organisms, especially in the subtidal zone.

Distribution I. alatus has a wide distribution in Jamaica. It is found in all major mangrove areas that are situated primarily on the south coast of the island (Fig. 8). The unifying parameters of all these areas are euryhaline conditions and waters rich in organic and inorganic suspended materials. Nutrients from land runoff as well as from decaying mangrove material make these areas very productive. Salinity and total suspended matter were recorded at all locations where I. alatus was found (Table 1). A salinity range from 10.9 to 40%0 shows that I. alatus is well adapted to living in these extreme conditions. The range in total suspended material was also great-5.0 to 49.48 mgtl of sea water. Its ability to withstand these turbid waters allows I. alatus to live in areas unsuitable for other filter feeders. The average mean tidal range in Jamaica is only 23.50 cm. However, exami- nation of mangrove prop roots show that 15% of the I. alatus population at Port Royal live intertidally while 75% are found subtidally, even in the presence of many fouling organisms.

DISCUSSION AND SUMMARY From the above account I. alatus appears to be well adapted to the conditions which exist in mangrove swamps. The hardiness of the species has probably been the reason for its wide and abundant distribution in the Caribbean. Although f. alatus appears to breed in most months of the year, peak periods of spawning occur during times of changing salinities, from the onset of the rainy seasons in September and October through January. Monthly gonadal examination of f. ala- tus showed a surprisingly high percentage of with ripe gonads throughout 100 BULLETIN OF MARINE SCIENCE, VOL. 30, NO. I. 1980

Table 1. Locations of I. a/atus populations showing salinity and suspended matter conditions

SalinilY Susp~nded Maller Location %c mg/1

1. Bowden 29.~35.8 20.6 2. Drainage Channel, St. Thomas 35.2 10.5 3. Falmouth 33.5 49.48 4. Bogue Islands 32.5 14.80 5. Black River 33.0 37.76 6. Old Harbour Bay 32.~35.0 10.96-14.8 7. Port Royal Mangroves 14.~37.0 5.~37.07 8. Fort Rupert Lagoon 10.9-40.0 12.40

the year. This was the case even after periods of heavy spawning, indicating a rapid recovery from the spent condition. The reduction in salinity at the beginning of the rainy season appears to be the trigger mechanism for the release of eggs and sperms from the ripe gonads, and this in itself can induce other nearby individuals to spawn. Simultaneous spawning ensures the presence of enough eggs and sperm to bring about fertilization. All mechanisms are therefore geared to ensure fertilization. The advantage of a breeding season, even in tropical coun- tries where conditions are favorable for year-round growth, is therefore evident for sessile marine organisms which depend on external fertilization.

Potential of I. alatus as a Commercial Species Factors to be considered to estimate the commercial potential for development of a species are: (1) availability of a source population, (2) suitable areas for culture, (3) reliable and abundant production of new individuals, (4) good survival, (5) good growth, (6) minimum labor costs, (7) minimum capital expenses, and (8) ready market for the end product. In Jamaica and throughout the Caribbean, I. alatus is commonly found in most mangrove areas. The species' ability to live in widely fluctuating conditions makes many coastal areas suitable for its cultivation. The abundant natural population is a good indication of its widespread success. Investigation into the breeding pattern shows a peak in spawning and larval settlement at the onset of the rainy season in Jamaica. By monitoring environmental conditions spatfall can be pre- dicted, thus allowing collection of an abundant supply of stock for commercial production. Preliminary studies of growth rate give favorable results, indicating that a harvestable size can probably be reached 6 months after settlement. Survival of I. alatus in the intertidal zone and subtidally even crowded among other fouling organisms appears to be good. Unlike Crassostrea rhizophorae, which:s now being cultured in several Caribbean countries, the problem of fouling does not arise for I. alatus. Cultivation of the latter is therefore not restricted to the intertidal zone or areas of low fouling. It may also be possible to grow this species on the sea bottom in shallow waters, thus eliminating the expense of constructing racks, rafts, or other types of collectors and growing out structures. Labor costs should be less than for the culture of C. rhizophorie since less effort will be required to control sessile organisms competing for space. In addition, no important predator of I. alatus has been identified. Attachment by byssus and not by cementation to the substratum also makes this species easy to handle and harvest. In Jamaica there is a market for I. alatus. At present it is sold fresh directly SlUNG: BIOLOGY OF ISOGNOMON ALATUS 101

to bars in the city. However, the freshly frozen product could be sold in super- markets and groceries and it should also be possible to market a processed prod- uct, cooked and shucked. Fresh I. alatus has a slight acid taste but this is lost when cooked.

Acknowledgments

Material in this paper was adapted from a thesis submitted for the Ph.D. degree in Zoology, University of the West Indies, Jamaica. It was supervised by Prof. l. Goodbody and Dr. B. Wade. I wish to thank Dr. R. Young for his critical review of the manuscript.

LITERATURE CITED

Bacon, P. R. 1971. Studies on the biology and cultivation of the mangrove oyster in Trinidad with notes on other shellfish resources. Tropical Sci. 12: 265-278. Carter, G. S. 1930. The fauna of the swamps of the Paraguayan Chaco in relation to its environment. 1. Physico-chemical nature of the environment. J. Linn. Soc. (Zool.) 37: 206-258. Cole, H. A., and E. W. Knight-Jones. ]939. Some observations and experiments on the setting behaviour of larvae of O. edlliis. J. Cons. Perm. lnt. Explor. Mer. 14: 86-105. Crisp, D. J. 1967. Chemical factors inducing settlement in Crassos/rea virginica (Gmelin). J. Anim. Ecol. 36: 329-335. Day, J. H. 1967. The biology of Knysna Estuary, South Africa: Estuaries. Pub. Amer. Ass. Ad. Sci. p. 397-407. Forbes, M. L. ]967. Genetic differences in prodissoconch of Gu]f of Mexico oysters. U.S. Bur. Comm. Fish No. 65: 338-347. Galtsoff, P. S. 1930. The fecundity of the oyster. Science 72: 97-98. ---. 1964. The American oyster Crassos/rea virginica Gmelin. Fish. Bull. Fish Wildl. Servo U.S. 64: 1-480. Hopkins, A. E. 1935. Attachment of larvae of the Olympia oyster as/rea lllrida to plain surfaces. Ecology 16: 82-87. Hunter, J. B. 1969. A survey of the oyster production of the Freetown Estuary, Sierra Leone, with notes on the eco]ogy, cultivation and possible utilization of mangrove oysters. Tropical Sci. I]: 276-285. Loosanoff, V. L., and J. B. Engle. 1940. Spawning and setting of oysters in Long Island Sound in 1937, and discussion of the method for predicting the intensity and time of oyster setting. Bull. Bur. Fish. No. 33: 217-255. Menzel, R. W. 1955. Some phases of the biology of Os/rea eques/ris Say and a comparison with Crassostrea virginica (Gmelin). Pub]s. Inst. Mar. Sci. Univ. Tex. 4: 69-153. Nikolie, M., and S. A. Melendez. 1968. EI ostion del mangle, Crassostrea rhizophorae Gui]ding, 1828 (experimentos inciales en el cultivo). Inst. Nacional de la Pesca Cuba. Nota sobre Inves- tigacion NO.7 del Centro de Invest. Pesq. 31 pp. Pomerat, C. M., and E. R. Reiner. 1942. The influence of surface angle and of light on the attachment of barnacles and other sedentary organisms. BioI. Bull. Mar. BioI. Lab., Woods Hole 82: 14-25. Prytherch, H. H. 1928. Investigation of the physical conditions controlling spawning of oysters and occurence, distribution and setting of oyster larvae in Milford Harbour, Conn. Bull. U.S. Fish. Comm. 44: 429-503. Rees, C. B. 1950. The identification of lamellibranch larvae. Hull Bull. Mar. Ecol. 3: 73-104. Ritchie, T. R. and R. W. Menzel. 1969. Influence of light on larval settlement of American oysters. Nat. Shellfish Ass. 59: 116-120. Trueman, E. R., and G. A. Lowe. 1970. The effects of temperature and littoral exposure on the heart rate of a bivalve mollusc, Isognomon a/alliS, in tropical conditions. Compo Biochem. Physiol. 38A: 555-564. Van Someren, V. D. 1960. The artificial culture of the edible East African oyster, Crassostrea cliclIliata (Born). East Afr. Agr. J. 25: 245-250. Yonge, C. M. 1968. Form and habit of species of Malleus (including the "Hammer Oysters") with comparative observations on Isognomon isognomon. BioI. Bull. Mar. BioI. Lab., Woods Hole. 135: 378-405.

DATE ACCEPTED: August 30, 1978.

ADDRESS: Zoology Department, University of the West Indies, Mona, Kingston 7, Jamaica, W.I.

PRESENTADDRESS: Institute of Marine Affairs, P.o. Bag /35. St James Post Office, St James, Trinidad. W.I.