Modulation of immune-associated parameters and antioxidant responses in the crab (Scylla serrata) exposed to mercury

► A 14d exposure of Hg induces physiological changes in the immune system of crab. ► The approach is based on utilizing the commercial crab Scylla serrata. ► Hg provokes immunomodulation, oxidative stress and antioxidant defences in crab. ► Correlation analysis between the responses was highly significant. ► First study to show Hg modulates immune functions and antioxidant system in crabs. Organic and inorganic contaminants can suppress immune function in molluscs and crustaceans. It was postulated that metals could modulate immune function in marine crabs. To test this hypothesis, sublethal effects of mercury (Hg) on cellular immune and biochemical responses of crabs were determined. When crabs were exposed for 14d to environmentally-relevant concentrations of Hg, changes in immune-associated parameters including, total haemocyte count, lysosomal membrane stability, phenoloxidase, super oxide generation and phagocytosis were observed. Oxidative stress, as measured by lipid peroxidation, antioxidant responses, including superoxide dismutase and catalase activities and glutathione-mediated antioxidant enzymes in serum, haemocyte lysate, gills, hepatopancreas and muscle were assessed in crabs exposed to Hg. Exposure to Hg resulted in significantly lesser immune-associated parameters in haemolymph and antioxidants in all tissues studied. Conversely, GST and phenoloxidase activity, were greater in crabs exposed to Hg. Responses of antioxidant parameters (SOD, CAT and GPx) were positively correlated with immune responses, including THC, superoxide and phagocytosis. These results were postulated to be due to an immediate response of antioxidant defense to oxygen radicals generated. Overall, the results suggest that 14d exposure to environmentally realistic concentrations of Hg causes immunomodulation and potentially harmful lessened antioxidant defenses of crabs.