RNA-Seq Reveals Adaptation Strategy in Grass Carp (Ctenopharyngodon idella) Under Hypersaline Conditions

Grass carp (<i>Ctenopharyngodon idella</i>) is a key aquaculture species, and understanding its adaptation mechanisms to saline environments is crucial for addressing the global freshwater salinization challenge. In this study, juvenile grass carp were acclimated to three salinity levels (0, 4, and 8 ppt) for 30 days, after which gill and intestinal tissues were sampled to quantify cortisol concentrations and conduct RNA-seq. Results showed that cortisol levels exhibited a salinity-dependent increase, with significantly higher concentrations in gill tissues than in intestinal tissues, suggesting that cortisol plays an important role in the salt adaptation of grass carp. RNA-seq revealed that ion transport-related genes were upregulated in gills, whereas biosynthesis, oxygen transport, and energy metabolism genes were downregulated. In the intestine, genes involved in taurine transport and intercellular junctions were highly expressed, while immune-related genes showed reduced expression. These findings suggest that high salinity suppresses respiration and energy metabolism efficiency, with ion exchange primarily occurring in gills. Functional annotation identified seven candidate genes (<i>LOC127513882</i>, <i>aqp9b</i>, <i>ca4a</i>, <i>ca5a</i>, <i>igfbp1b</i>, <i>slc12a2</i>, and <i>slc12a4</i>) as key regulators of salinity adaptation. Overall, our study provides valuable insights into the mechanisms underlying the salt tolerance of grass carp.