Mediterranean Mussels (<i>Mytilus galloprovincialis</i>) Under Salinity Stress: Effects on Antioxidant Capacity and Gill Structure

Bivalve mollusks frequently experience salinity fluctuations that may drive oxidative stress (OS) in the organism. Here we investigated OS markers and histopathological changes in gills and hemolymph of Mediterranean mussels Mytilus galloprovincialis Lamarck, 1819 exposed to a wide range of salinities (6, 10, 14, 24, and 30 ppt). Mussels were captured at the shellfish farm with the salinity 18 ppt and then exposed to hypo- and hypersaline conditions in the laboratory. Indicators of redox balance in hemocytes (intracellular reactive oxygen species (ROS) levels, DNA damage) and gills (thiobarbituric acid reactive substances (TBARS), protein carbonyls (PC), activity of catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GPx) were measured. The effect of salinity stress on microstructure of gills has been evaluated as well. The results revealed induction of OS in tissues and cells of mussels for both experimental increase and decrease salinity modelings. Hemocytes showed higher sensitivity to osmotic stress compared to gills. In gills TBARS were stable in all experimental groups and PC increased only at salinity 6 ppt. The activity of SOD, CAT and GPx in gills decreased only in mussels acclimated to salinity 24 ppt and further salinisation up to 30 ppt was associated with the recovery of the activity of all enzymes. Major histopathological changes in gills upon salinity fluctuations included inflammatory reactions, circulatory alterations, regressive and progressive changes. Our findings clearly indicate that salinity fluctuations promote OS at cellular and tissue level and also affect microstructure of gills in mussels. The results provide new insights into the mechanisms of osmotic stress in bivalves.