Salinity stress is an emerging key factor affecting aquatic animals in coastal waters. In bivalves, physiological responses to anisosmotic conditions are frequently associated with excessive reactive oxygen species (ROS) formation, lipid peroxidation processes and alterations of the antioxidant enzyme activity in tissues. In this study, we investigated whether short-time exposure to high (20-40 ‰) or low (6-14 ‰) salinity induces oxidative stress in hemocytes and gills of Mediterranean mussel (Mytilus galloprovincialis). Our findings showed that both hypoosmotic and hyperosmotic stress enhanced formation of ROS in hemocytes, which was accompanied by an increase in DNA damage. Experimental acclimation to low salinity (6‰, 10 ‰) led to a decrease of the activity of superoxide dismutase but did not affect catalase activity in gills. Exposure to high salinity (24 ‰) inhibited activities of both antioxidant enzymes, whereas at 30 ‰ activity of both enzymes recovered up to controls. We found that exposure to high salinity was associated with lower risks of oxidative stress in mussels, and changes in redox balance are observed in hemocytes and gills even if the range of salinity changes is in range of species adaptive potential. Taken together, this study suggests that short-time salinity fluctuations promote oxidative stress in bivalves.

Hemolymph cellular composition, morphology and functional properties of the Pacific oyster (Crassostrea gigas) hemocytes were studied. Three hemocyte types (agranulocytes, hyalinocytes and granulocytes) were described in hemolymph. The morphology of each type was characterized by light microscopy and flow cytometry. Agranular cells (agranulocytes and hyalinocytes) were the dominant type of cells in hemolymph; their number was 86.7±2.7% of total cell count. Under hypoxia the number of agranulocytes increased (37.4% for control group versus 95.3% for hypoxic probes), whereas granulocyte and hyalinocyte number decreased up to 3.9% and 0.7% in hypoxic specimens respectively. The spontaneous ROS production decreased in each hemocyte type after exposure to hypoxia. Low dissolved oxygen did not influence hemocyte proliferation and mortality level.