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Scylla paramamosain is a high-quality cultivar for saline-alkaline water aquaculture as a euryhaline crustacean species. However, salinity impacts the respiratory metabolism, growth, and survival of marine crustaceans. The metabolic response of crabs adapting to multiple low salinity environments has not been thoroughly studied yet, especially in inland saline-alkaline water. In this study, we analyzed metabolites in the gill and hemolymph of crabs cultured in three different low salinity environments. The results showed that membrane composition (lipids and lipid molecules) and free amino acids played an essential role in the osmoregulation of crabs, and the energy consumption accompanied as well. Meanwhile, S. paramamosain relied on ion transport and energy metabolism under acute/short-term low salinity conditions for osmoregulation. In contrast, amino acids and energy metabolism occupied a leading position in long-term low salinity. Furthermore, taurine and hypotaurine play a vital role in crabs adapting to inland saline-alkaline water. This is the first study to identify the crucial metabolites and key pathways as biomarkers to differentiate the metabolic mechanisms of S. paramamosain under multiple low salinity stress modes based on GC-MS technology, which provided novel insight into the metabolic response of S. paramamosain adapting to inland low salinity saline-alkaline water, and provided theoretical guidance for the aquaculture of S. paramamosain in the inland saline-alkaline water.

Scylla paramamosain is an important marine aquaculture crustacean on the southeastern coast of China. Given the difficulties of overcrowded coastal aquaculture space and insufficient production, inland saline-alkaline water culture has the potential to alleviate this bottleneck. This study separated 600 crabs into four groups: normal salinity (12-18 ppt, NS), coastal low salinity (3-5 ppt, CS), inland low salinity saline-alkaline (1.5 ppt, IS), and acute low salinity (23 ppt down to 3 ppt, AS), followed by a transcriptomic analysis of the gills. CS-vs-NS, IS-vs-NS, and AS-vs-NS obtained 1154, 1012, and 707 DEGs, respectively. GO analysis showed that the DEGs of the three comparison groups were mainly involved in cellular process, metabolic process, biological regulation, organelle, membrane, extracellular region, binding, and catalytic activity. The findings demonstrate that a significant number of genes are engaged in controlling metabolic processes in the osmoregulation process, and that cell membrane catalysis and different enzymes play a vital part in the environmental adaption process. KEGG enrichment analysis revealed that IS possesses a considerable number of signaling pathways that play important roles in osmoregulation when compared to NS. The cAMP signaling pathway increased the expression of CaM and Na+/K+-ATPase. These findings show that cations like Ca2+, Na+, and K+ are critical for S. paramamosain to adapt to inland low salinity saline-alkaline water, and that the cAMP signaling pathway regulates their levels. This research provides a theoretical foundation for improving the saline-alkaline aquaculture technology of S. paramamosain.