Immunological and histological responses to sulfide in the crab Charybdis japonica

In this study, immunological and histological responses of the crab Charybdis japonica to sulfide stress were investigated. The 24, 48, 72, and 96-h LC50 values of Na2S were 6.634, 4.703, 3.886, and 2.190mM, respectively. Based on these results, the crabs were exposed to 0.25, 0.50, 0.75, 1.00, and 1.25mM of Na2S over a 15-day period, and sampled at 0, 1, 3, 5, 10, and 15 day for analyzing changes in immunity-related indicators in the hemolymph (including total hemocyte (THC), hemocyanin content, the activities of the phenoloxidase (PO), lysozyme (LSZ), superoxide dismutase (SOD) and catalase (CAT), and malondialdehyde (MDA) content), and the histological structures of major organs (gill, hepatopancreas, and stomach). The results showed that the activities of most immune-related factors declined after an initial rise under Na2S-induced stress, with the exception of MDA content that showed an overall increasing trend. After exposure to Na2S for 15 days, most of the measured indices were lower in treatment groups than in the control. Significant negative correlations was found between Na2S concentration and the activities of LSZ, SOD, CAT (p<0.01), while MDA content was found to be positively correlated with Na2S concentration (p<0.05). Noticeable changes in the histological structure of the main organs of C. japonica were observed upon exposure to high concentration of Na2S: the outer chitin layer of the gills became thin and partialy ruptured; hemocytes in the gill hemocoel were severely vacuolized; the morphology of glandular epithelium was irregular; the rough endoplasmic reticulum became swollen and reduced in number; and several large vacuoles and some residual bodies were observed in the gastric epithelium. These results indicate that stress induced by high concentration of sulfide can significantly affect the activities of immune-related enzymes and the organ structure of C. japonica. Furthermore, changes in the activities of LSZ, SOD, and CAT, and the content of MDA may be used as indices for evaluating the immune state of C. japonica under sulfide stress.