Morphological Characterization via Light and Electron Microscopy of the Hemocytes of two Cultured Bivalves: A Comparison Study between

The culturing of the hard clam (Meretrix lusoria) and Pacific oyster (Crassostrea gigas) constitutes some of the most economically impor-tant fisheries along the southwestern coast of Taiwan. However since 1969, hard clams have suffered a high degree of mortality each spring and/or summer (Tseng 1976). Several factors have been implicated as possible causes of this mass mortality, such as variations in temperature and salinity, industrial and pesticide pollution, and infectious diseases (Tseng 1976, Yang et al. 1978, Kou et al. 1989). The annual production of oysters and oyster larval settlement has also seriously diminished in recent years for undetermined rea-sons. This situation, involving both clams and oysters, continues, although the exact cause still remains unclear. Since bivalve hemocytes play an important role in homeostatic functions and defense mechanisms, the morphological charac-terization of hemocytes is a prerequisite to further exploring the causes of death in these two bivalves (Cheng 1981, Fisher 1986). In general, two basic cell types are recog-nized among bivalve hemocytes: agranulocyte