Current saline-alkali aquaculture of the saline alkali-tolerant mud crab Scylla paramamosain fails to meet market demand. Given that Ca2+ and Mg2+ are closely related to growth in crustaceans, physiological responses of mud crab to different levels of these ions at varying salinity levels were explored. Long-term aquaculture experiments were conducted by adding CaCl2, MgCl2, and NaCl to the water to control Ca2+, Mg2+ and salinity levels. Three different ratios and concentrations of Ca2+ and Mg2+ were established (Sanmen area, SM group; Cangnan area, CN group; Rui'an area, RA group); in addition, four different salinity groups (3, 8, 10, and 12 ‰) were established under each of the Ca2+–Mg2+ treatment groups At Ca2+:Mg2+ ratios of 300:900 (SM) and 400:1200 mg (CN), S. paramamosain showed stronger osmotic regulation and higher Na+/K+-ATPase and Ca2+/Mg2+-ATPase activity compared with those reared at a Ca2+: Mg2+ ratio of 300:300 (RA). Indicators of hepatopancreas injury also increased in the RA group with time, indicating lipid metabolism disorders and abnormal inflammatory reactions. Immune and antioxidant enzyme activity increased with time in the SM and CN groups, but decreased in the RA group. In addition, malondialdehyde increased with time in the RA group, but remained constant in the SM and CN groups under normal salinity and decreased under low salinity. Thus, cultivation under 300: 900 or 400: 1200 Ca2+: Mg2+ ratios enhanced the immune and antioxidant abilities of mud crab, as well as its ability to alleviate damage caused by low-salt stress. Upregulation of various energy substrates in the CN group under low-salt stress indicated the mobilization of energy reserves to support osmotic regulation. At a salinity of 3 ‰, the CN group also had more pathways upregulated related to energy metabolism, ion transport, and antioxidation, whereas, at 12 ‰, more antioxidant pathways were upregulated. A 120-day aquacultural experiment confirmed that the SM and CN groups showed better growth performance compared with the RA group. Overall, these results confirm that conditions in the SM and CN groups were more suitable for the aquaculture of mud crab, providing a theoretical guidance for increasing mud crab production in saline-alkali fishery.
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