Morphological and Functional Characterization of Hemocytes of the Black Sea Bivalves: Pacific Oyster (Crassostrea gigas) and Mussel (Mytilus galloprovincialis)

To understand the role that hemocytes play in processes of cellular immunity of bivalve mollusks, they should be accurately classified based on their morphological and physiological characteristics. The circulating hemocytes of the cultured Pacific oyster (Crassostrea gigas) and marine mussel (Mytilus galloprovincialis) were investigated using light microscopy and flow cytometry. In the mussel, two cell types, granulocytes and agranulocytes, were identified based on the presence of two subpopulations of cells differing by size and granularity level on light scatter plots. Light microscopic examination confirmed the presence of cells with cytoplasmic granules and cells without granulation in hemolymph of the mussel. In the oyster, light microscopy and flow cytometry revealed three types of hemocytes: agranulocytes, hyalinocytes, and granulocytes. The cells in the hemolymph of both species were mainly represented by agranular cells, which constituted 78.4 ± 8.9 % in the mussel and 86.7 ± 2.7 % (agranulocytes and hyalinocytes) in the oyster. Agranulocytes were the smallest cell type in the mussel and oyster. They were round-shaped and had large nuclei and narrow cytoplasm. Hyalinocytes of the oyster were larger and irregularly shaped, with eccentric nuclei. Granulocytes of both species contained numerous eosinophilic, basophilic, and mixed granules and formed pseudopodia. Flow cytometry showed that the agranular hemocytes of both species produced considerably fewer reactive oxygen species compared to granulocytes. Morphological and functional characterization of hemocytes of cultivated species improves the analysis of physiological state of bivalve mollusks farmed in the Black Sea region