The number of mollusc farms off the coast of Crimea and the Caucasus has increased significantly in recent years. The cultivation of the Pacific oyster Crassostrea gigas (Thunberg, 1793) requires monitoring of mollusc health and parasitological control of mariculture farms. The aim of this work was to study species composition of epibionts and endobionts, associated with shells of cultivated oyster C. gigas, as well as to identify species, damaging shells. Commercial oysters with visual shell damage were collected on a mariculture farm in the Donuzlav Bay (Crimea, the Black Sea) and brought to the laboratory alive chilled. As a result of 22 oysters’ examination, 14 macrozoobenthos species and live specimens of rock-boring mollusc Petricola lithophaga (Retzius, 1788) were found. The size of rock-borers varied 9 to 16 mm; their age was about two years. Prolonged presence of P. lithophaga inside oyster valves can cause degradation of shell calcareous layer and even death of the mollusc host; this fact is of great importance for the Black Sea mariculture. Considering P. lithophaga annual development cycle, during the period of mass larval settlement (July to October), it is recommended to inspect the shells of cultivated oysters. Further detailed studies will allow to develop measures for prevention and protection of bivalve molluscs from infestation with P. lithophaga.

The rate of linear and weight growth of triploid spat of oyster Crassostrea gigas in Donuzlav Liman (Black Sea, Crimea) was investigated. A quantitative relationship between the mass (W, g) of the whole oyster and the height of the shell (H, mm) is given (it is described by the equation W = 7 · 10 -5 · Н 3.1 , R² = 0.78), as well as a quantitative relationship between the mass (W, g) of the whole oyster and the length of the shell (L, mm) (W = 3 · 10 -4 · L 3.12 , R² = 0.65). The maximum rates of linear growth of triploid oysters T20 were registered in April (1.15 mm per day), of weight growth -in June (0.50 g per day) and August (0.61 g per day). It was shown that 27 % of triploid oysters T20 can reach commercial size after 6 months. It is recommended to use Pacific oysters of T20 size as a planting material on the shellfish farms in the Black Sea region.

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