Hepatic physiological and transcriptomic responses of grass carp (Ctenopharyngodon idella) to long-term salinity...

Salinity profoundly impacts fish survival, distribution, and physiological functions. Grass carp (Ctenopharyngodon idella), a vital aquaculture species, offers a representative model to investigate salinity adaptation mechanisms, providing crucial theoretical support for aquaculture practices. While the liver is a key metabolic organ in fish salinity adaptation, its specific regulatory role in the high-salinity tolerance of grass carp remains unclear. In this study, we investigated the effects of long-term salinity stress (0parts per thousand [ppt], 4 ppt, and 8 for one month) on grass carp liver. At 4 ppt salinity, mild hepatocyte edema and acinar cell hyperplasia were observed, while at 8 ppt salinity, irregular hepatocyte shapes, dilated hepatic sinusoids, and focal inflammatory cell infiltration were detected. Transcriptome sequencing showed that increasing salinity led to significant upregulation of genes involved in steroid synthesis, lipid metabolism, and cholesterol synthesis, while immune-related gene expression trended downward. Notably, LOC127523424 (carbonic anhydrase 4), igfbp7 (insulin-like growth factor-binding protein 7), and taurine transporter genes (slc6a6b, slc6a6a) all increased with rising salinity, suggesting their involvement in enhancing hyperosmotic stress adaptation. In conclusion, our findings confirm a significant impact of salinity on the grass carp liver. Long-term high-salinity adaptation appears to reduce immunity, while the liver enhances high-salinity tolerance through increased steroid synthesis and metabolism.