REPRODUCTION AND DEVELOPMENT IN THE ...

BULLETIN OF MARINE SCIENCE, 34(3): 424-434,

1984

REPRODUCTION AND DEVELOPMENT IN THE LUCINID CLAM CODAKIA ORBICULARIS (LINNE, 1758) Philip Alalalo, Carl 1. Berg, Jr. and Charles N. D'Asaro ABSTRACT The tiger lucine Codakia orbicularis is a large edible clam being investigated as a mariculture candidate in the Bahamas Islands. Gonad development and spawning seasons were assessed by monthly sampling of C. orbicularis from Grand Bahama Island, Bahamas and Key Biscayne, Florida. Histological examination of clams showed most of the populations sampled to be ripe between April and November. Natural spawning probably occurs May to October. Codakia orbicularis is dioecious, seldom responding to standard spawning techniques, including physical and chemical stimuli. Artificial fertilization by carefully stripping gonads produced 15-20% viable embryos. Eggs are 108-112 ILm in diameter and are singularly encased in a thick capsular membrane. Following fertilization, the gastrula, trochophore and early veliger stages develop within the capsular membrane. Upon hatching, planktonic veligers range from 150-174 ILm in shell length and develop to the pediveliger stage in approximately 12 days at 24°C. Metamorphosis occurs approximately 16 days after fertilization. Larval development within the superfamily Lucinacea is characterized by formation of a gelatinous capsule. The long planktonic development and facultative planktotrophy of C. orbicularis is unusual for lecithotrophic bivalve larvae. Larvae of C. orbicularis and other lucinids may also derive nutrition from chemosynthetic bacteria located within their tissues, as reported for adults.

The tiger lucine Codakia orbicularis (Linne, 1758) is a large (98 mm maximum shell length) edible clam ranging from Bermuda and Florida, throughout the Caribbean and southern Gulf of Mexico, to Brazil (Abbott, 1974). Codakia orbicularis was a staple food of the Arawak Indians, the first inhabitants of the Bahamas (Sears and Sullivan, 1978). It is harvested and eaten locally (Fischer, 1978) and is the focus of efforts considering the mariculture potential of bivalves indigenous to the Bahamas (Berg and Alatalo, 1981; 1982b). The raw meat of the clam is firm and sweet, but the gills are bitter when eaten raw. Often prepared in soups, the entire clam may be eaten after steaming. In the West Indies, C. orbicularis is found in patchy but dense assemblages, often in areas of high environmental stress (Jackson, 1972; 1973). Adult morphology and habitat of C. orbicularis have been reported (Allen, 1958), but most notable is their ability to live in areas of high hydrogen sulfide concentrations and to derive nutrition from chemosynthetic bacteria within their tissues (Berg and Alatalo, 1981; 1982a,b). However, no aspect of embryology or larval development has been published. Here we describe, for the first time, the reproductive biology, embryology and larval development of Codakia orbicularis. MATERIALS

AND METHODS

Specimens of C. orbicularis were collected monthly from Gold Rock Creek, Grand Bahama Island, Bahamas, and Key Biscayne, Florida, U,S.A. Gonad analysis was performed on 264 clams over a 20month period. After preserving clams in 7% formalin/seawater, samples of gonad tissue were removed, embedded in paraffin, and sectioned at 6 ILm. Sections were stained with hematoxylin and eosin according to standard procedures (Humason, 1972), Following examination under a compound microscope, gonad development was classified according to the following categories: Developing Gameles.-Male follicles are lined with spermatogonia and spermatocytes. Some spermatids may be present in the lumen of follicles. In females, oogonia line follicles while the lumen may be filled with angular-shaped oocytes attached to follicle walls. 424

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Ripe Gametes.-Follicles in males contain spermatozoa with pink-staining tails filling the lumen. "Sperm balls" containing hundreds of spermatozoa joined together at the head are common. A few spermatids are present. In females nearly all eggs are oval-shaped and free in the follicle lumen. A dark-staining nucleolus is clearly visible within the nucleus of the egg. Spent or Resorbed Gametes. -Spent clams have empty follicles except for residual sperm or eggs. Sperm balls deteriorate to irregularly shaped masses of agglomerated sperm. During gamete resorption, phagocytes are always present and may completely fill the gonad. Live clams were brought to the laboratory in Woods Hole, Massachusetts, and held in unfiltered seawater at temperatures matching those at collection sites (21°-31°C). Various techniques were employed to induce spawning, including thermal shock, osmotic shock and mechanical or chemical stimuli. Artificial fertilization of eggs was accomplished by carefully stripping gametes from excised bodies of ripe adult clams. Ova were collected in 8-cm diameter stacking dishes filled with filtered seawater and were fertilized with a dilute suspension of sperm. After 15 min, ova were rinsed by sieving them through 350-J.Lm Nitex screen and onto a 54-J.Lm Nitex screen. Embryos were maintained at 24°C in 2-liter Pyrex culture flasks equipped with magnetic stirrers. Since air bubbles caused veligers to stick to sides of the flask wall, magnetic stirrers were used to gently aerate cultures. After 48 h, culture flasks were removed from the stirring device and allowed to stand quietly under a light. Positively phototactic veligers swam to the surface and were slowly decanted from the flask. Veligers were maintained at 24°C in seawater passed through a 36-lLm Nitex sieve and were fed Platymonas sp. and Chlorella sp. at a concentration of I x 104 cells ml-I• Cultures were placed on magnetic stirrers and the seawater changed daily. Metamorphosed larvae were transferred to Pyrex glass trays that received slowly-flowing filtered (100 ILm) seawater. Veligers selected for sectioning were narcotized by gradual addition of a concentrated magnesium sulfate solution and embedded in paraffin using standard histological procedures. Specimens were sectioned at 8 ILm and stained with hematoxylin and eosin. RESULTS

Reproductive

Development

Histological analysis of Codakia orbicularis from Gold Rock Creek, Grand Bahama Island, showed ripe individuals present at each sampling date (Fig. 1). Gonad development began in spring, concurrent with rising water temperature, and was most evident during June and July. No gametogenesis was observed between December 1981 and March 1982, nor after August 1982. During 1981, half the population sampled was ripe by April, increasing to a major peak in October. In the following year, over half of the population was ripe by July and all individuals sampled were ripe in August and September 1982. Few ripe clams were found after November 1982 or December 1981 (Fig. 1). The spent stage of reproduction is reached when gonads have recently released gametes or are undergoing resorption. The percentage of spent animals increased dramatically in November 1981 and October 1982 following the decline in water temperature from the summer maximum. This stage predominated throughout winter. Female clams collected from the Bahamas appear to undergo rapid gonad development in early spring and remain ripe from summer through mid-fall. Male clams develop gametes later, at the beginning of summer, and are ripe by late summer. Whereas most female clams release all their gametes by late fall, male clams often remain ripe until winter. Throughout winter and spring, gonad resorption occurs in male clams, accompanied by deterioration of clumps of sperm. Only three hermaphrodite clams (1.2%) were found, all in February 1982. These large adults had spent gonads with residual sperm and eggs within separate follicles. No evidence of prot an dry was found in the Bahamian populations examined. The minimum size of C. orbicularis observed having follicles was 19.8-mm shelllength. One specimen, 25.4-mm long, possessed follicles undergoing gametogenesis. Clams greater than 30 mm consistently had ripe gonads during the spawning season.

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BULLETIN OF MARINE SCIENCE, VOL. 34, NO.3,

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Temperature

30

%

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50

25 M

A

AS

0

J

F

Months

Figure 1. Gonad development of Codakia orbicularis (Linne) and seawater temperature. Gold Rock Creek, Grand Bahama Island (. = 1981;. = 1982).

Laboratory Spawning Attempts to spawn C. orbicularis in the laboratory were made between April and November using the following stimuli: rapid and gradual temperature changes, increased salinity, concentrated phytoplankton suspensions, gonad extracts, vigorous shaking, rapid and gradual addition of hydrogen peroxide solutions and combinations of the above treatments. Results were negative. Spawning was achieved in July and October when recently collected specimens from Gold Rock Creek were placed in filtered seawater 12 to 24 h after collection. Spawning was also induced in both groups of clams by addition of 2.3 M H202 with a pH between 8.8 and 8.9. Specimens from Key Biscayne also failed to respond to the above techniques; however, ripe gametes were stripped from clams and larvae successfully reared. Embryological Development Upon release from the gonad, ova are irregular in shape, swelling in seawater to spheres approximately 108 j.tm in diameter (Fig. 2A). The inner vitelline membrane is surrounded by a clear, gelatinous layer approximately 110-j.tm thick, which forms the external capsule. Further expansion of membranes results in a capsule measuring approximately 350 j.tm in diameter. Capsules of laboratoryspawned eggs are non-sticky and slightly negatively buoyant. Former attachment to the follicle is evident as a stalk connecting the vitelline membrane with the outside capsule. Sperm measure approximately 60 j.tm in length. The head is sickle-shaped, approximately 18-j.tm long and I-j.tm thick. Sperm tails range in length from 36 to 48 j.tm. Sperm are often released as sperm balls, comprised of hundreds of sperm attached at the head. Presumably, individual sperm break away from the

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