Morphological and Functional Characterization of Hemocytes in the Giant Clam Tridacna crocea

In order to understand the cellular defense system in the giant clam Tridacna crocea, which harbors symbiotic microalga zooxanthella, hemocytes of the giant clam were characterized using light and electron microscopy. Three types of hemocytes, which we named eosinophilic granular hemocytes, agranular cells, and morula-like cells, were recognized. The eosinophilic granular hemocyte contained granules approximately 0.6 &mgr;m in diameter. The agranular cell adhered to glass surfaces and contained electron-lucent granules and few electron-dense granules. The morula-like cell was packed with large (approximately 3 &mgr;m in diameter), electron-dense granules. The eosinophilic granular hemocyte was acid phosphatase positive and showed phagocytic ability against latex particles. The agranular cell and the morula-like cell lacked phagocytic ability. When the hemolymph was exposed to seawater, the hemocytes coagulated to make large clots. The agranular cells were mainly found in the core space of the clots. The morula-like cells released the large granules in the aggregation process.