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Semicarbazide (SEM) is a common pollutant in the marine environment. It can cause cell mutation, carcinogenesis, and teratogenesis, threatening the health and survival of marine species. In this study, the immunotoxicity of SEM was investigated in a marine crustacean species, the mud crab Scylla paramamosain . After seven days of exposure to 0.2 μg/L, 2 μg/L, or 20 μg/L concentrations of SEM, the effects of SEM on hemocytes and immune function were analyzed in S. paramamosain . The results showed that the hemolymph color deepened and total hemocyte count (THC) was down-regulated significantly with increasing SEM concentration. Meanwhile, tissue sections showed cell infiltration, cytoplasmic vacuolization, and cell steatosis. The survival rate of hemocytes decreased after SEM treatment in vitro , which revealed that SEM showed a cytotoxic to hemocytes. Superoxide dismutase (SOD), glutathione (GSH), phenoloxidase (PO) and catalase (CAT) activity decreased with increasing SEM concentration, while the activity of glutathione peroxidase (GSH-Px) was increased compared with the control group. Similarly, intracellular ROS content increased to about 1.2-, 1.3-, and 2-fold that of the PBS group after exposure to 0.2, 2, and 20 μg/L SEM, respectively. There was a trend of significant up-regulation of p53 and caspase-3 with increasing concentrations of SEM identified by Q-PCR. Meanwhile, the hemocytes apoptosis rate were increased with the SEM concentration, which induced by caspase-3 pathway activated by SEM. The phagocytic activity of hemocytes decreased with increasing SEM concentration. The challenge experiment illustrated that SEM exposure reduced the resistance of mud crab to Vibrio alginolyticus . Our results demonstrated that SEM exposure could inhibit the immune function by causing cell damage, inducing reactive oxygen species (ROS), reducing the activity of antioxidant enzymes and inducing hemocytes apoptosis in a marine crustacean species. Synopsis SEM in seawater could inhibit the immune function and threaten the survival in crustaceans.
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