INTRACELLULAR DIGESTION OF SYMBIONTIC ZOOXANTHELLAE BY HOST AMOEBOCYTES IN GIANT CLAMS (BIVALVIA: TRIDACNIDAE), WITH A NOTE ON THE NUTRITIONAL ROLE OF THE HYPERTROPHIED SIPHONAL EPIDERMIS

Abstract

The question of utilization of zooxanthellae as a holozoic food source by their tridacnid clam hosts was explored utilizing techniques of electron microscopy and electron microscopical histochemistry. It is apparent from the results that older or senescent zooxanthellae are selectively culled from the algal population of the mantle edge by amoebocytes and are intracellularly digested via amoebocyte lysosomes both in the circulatory system and the interdiverticular spaces of the digestive gland. This process cannot be considered "farming," as figured by earlier work, but rather the slow systematic removal and utilization of degenerate zooxanthellae from the algal population of the clam's mantle edge. Electron photomicrographs of the microvillous surface of the hypertrophied siphons of the Tridacna revealed extensive pinocytosis of fluid and particulate material from seawater bathing the clam. It is suggested a priori that this endocytosed material contributes to the nutrition of the clam.