<I>Isognomon Alatus</I>

BULLETIN OF MARINE SCIENCE, 30(1): 90-101, 1980 STUDIES ON THE BIOLOGY OF ISOGNOMON ALATUS GMELIN (BIVALVIA: ISOGNOMONIDAE) 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 individuals 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 (Rhizophora mangLe 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. rhizophorae 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. Plankton 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 ISOGNOMON ALATUS 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 animal 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 developed, 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 muscles 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 laboratory 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 settlement. The prodissoconch of I.

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