Zoological Studies

Vol. 44 No. 1, 2005

Morphological Characterization via Light and Electron Microscopy of the Hemocytes of two Cultured Bivalves: A Comparison Study between the Hard Clam (Meretrix lusoria) and Pacific Oyster (Crassostrea gigas)

Su-Jung Chang, Su-Min Tseng, and Hsin-Yiu Chou*

Department of Aquaculture, National Taiwan Ocean University, Keelung, Taiwan 202

Su-Jung Chang, Su-Min Tseng, and Hsin-Yiu Chou (2005) In this study, the circulating hemocytes of the hard clam (Meretrix lusoria) were characterized by light and electron microscopic observations and simultaneously compared with those of the Pacific oyster (Crassostrea gigas). First, two cell types, granulocytes and agranulocytes, were identified based on the existence of cytoplasmic granules under light microscopy. The hemocytes were then stained and the granulocytes subclassifed into eosinophilic and basophilic granulocytes. In the oyster, three types of granulocytes were observed: eosinophilic, basophilic and an intermix. Conversely, the main type of granulocyte in the hard clam was eosinophilic granulocytes, and distinctive small and large granules were recognized. Agranulocytes in the hard clam could be subdivided into hyalinocytes and blast-like cells. Another cell type, vesicular cells, was observed in the oyster as unclassified cells. To further characterize hemocyte populations in both species, transmission electron microscopic observations were carried out. In hard clam granulocytes, abundant electron-dense cytoplasmic granules of two distinctive sizes were observed as recognized in light microscopy. However, two granulocyte types with either electron-dense or electron-lucent granules were found in the oyster, with the latter likely being basophilic granulocytes, which were seldom found in the hard clam. In addition, hemocytes with few or no cytoplasmic granules, and possessing few organelles such as mitochondria, Golgi complexes, and endoplasmic reticula, were found in the agranulocytes of both species. Blast-like cells, however, were particular in the hard clam in that they were small and had a high nucleus: cytoplasm ratio, while lacking most organelles, except mitochondria.

Key words: Morphology, Hemocyte, Hard clam, Oyster.

*Correspondence: Tel: 886-2-24622192 ext. 5214. Fax: 886-2-24634176. E-mail: hychou@mail.ntou.edu.tw