Long-term salinity stress from megalopa to juvenile in Eriocheir japonica sinensis: Integrated physiological and multi-omics insights into osmoregulation and metabolic adaptation

The Chinese mitten crab ( Eriocheir japonica sinensis ) is euryhaline, but its long-term adaptive mechanisms to salinity stress during the critical transition from megalopa to juvenile remain unclear. Here, we reared megalopae at three salinities (0, 15, and 30 ppt) for seven months and evaluated growth, gill histology, oxidative stress, osmoregulation, free amino acids, and multi-omics responses in the resulting juvenile crabs. Rising salinity exacerbated gill tissue damage, increased apoptosis and reactive oxygen species (ROS) levels. Antioxidant enzyme activities and their gene expressions peaked at 15 ppt but were lowest at 30 ppt, indicating a non‑linear antioxidant response. Hemolymph osmolality and Na⁺, K⁺, Cl⁻ concentrations increased with salinity, accompanied by upregulation of Na⁺/K⁺-ATPase and V-ATPase but downregulation of carbonic anhydrase. Free amino acids, particularly proline, accumulated in both hemolymph and muscle under high salinity. Integrated transcriptomic and metabolomic analyses consistently enriched the amino sugar and nucleotide sugar metabolism pathway across all salinity comparisons, with BmChiNAG and nagZ showing strong correlations with key metabolites (e.g., D‑glucosamine, D‑glucose). These results reveal that long-term salinity adaptation in juvenile E. j. sinensis involves coordinated regulation of ion transport, antioxidant defense, free amino acid dynamics, and amino sugar metabolism. This study provides mechanistic insights into salinity tolerance and practical guidance for crab aquaculture in saline‑alkaline waters.