SPAWNING, DEVELOPMENT, AND ACQUISITION OF ZOOXANTHELLAE BYTRIDACNA SQUAMOSA(MOLLUSCA, BIVALVIA)

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Abstract
Spawning and development of the clam T. squamosa was investigated in Belau, Western Caroline Islands in February and March 1980. Five of the six species of tridacnid clams spawned in response to various stimuli, but only T. squamosa released mature eggs. Factors influencing gonad development and spawning are discussed. Development of T. squamosa was followed from post-fertilization to post-metamorphosis, with emphasis on the acquisition of the zooxanthella Symbiodinium (=Gymnodinium) microadriaticum Freudenthal. The symbiotic algae were not seen in either the fertilized eggs or trochophore stages. We found that all strains of S. microadriaticum introduced to veliger clams were taken into the stomach via the mouth. Veligers ingested motile zooxanthellae more readily than non-motile ones. Within 2-9 days after metamorphosis, zooxanthellae moved by an unknown mechanism into the developing siphonal tissues. Most of these zooxanthellae appeared to be in spaces, probably the developing haemal sinuses. However, in some cases it was difficult to tell if the zooxanthellae were intra- or extracellular. Survival and growth of veligers and juveniles with zooxanthellae was greater than those without zooxanthellae. Juveniles with zooxanthellae can survive and grow in Millipore- filtered seawater with light as the sole energy source for over 10 months, illustrating the phototrophic aspect of the association. Our observations may have practical application pertinent to spawning, development, and growth of tridacnid clams in the context of commercial mariculture.