OBSERVATIONS ON THE SYMBIOSIS WITH ZOOXANTHELLAE AMONG THE TRIDACNIDAE (MOLLUSCA, BIVALVIA)

Abstract

The symbiosis with zooxanthellae among tridacnid bivalve molluscs has been examined in order to assess the morphological relation of the algae to the hosts tissues and the dynamics of the interactions between the algae and the clams. Light and electron microscopic evidence indicates that the zooxanthellae located in the haemal sinuses of the hypertrophied siphon are intercellular in all six tridacnid species studied. The algae pass from the siphonal tissues to the alimentary tract from which they are voided as feces. Many of the defecated symbionts are morphologically intact and photosynthetically functional. The concretions in the kidneys of tridacnids, previously thought to be the indigestible remains of zooxanthellae, are identified as phosphorite deposits, which are not unique to tridacnids. Analyses of photosynthesis-radiant flux relations in Tridacna maxima indicate that over 24 h the algae produce more oxygen than is consumed by the association, and estimates suggest that the algae may contribute more than 50% of the animals metabolic carbon requirements.