Physiological and Molecular Analyses of Low-Salinity Stress Response in the Cuttlefish(Sepia pharaonis) Juveniles

As a stenohaline species, the survival of Sepia pharaonis can be affected by salinity significantly. This study aimed to explore the function of decreasing salinity on the survival of Sepia pharaonis, which can provide an advanced production guide on the culture of S. pharaonis in the rainy season. Salinity was gradually decreased from 29 to 22 within 48 h to acclimate S. pharaonis to a low-salinity environment. After ten days of breeding under low-salinity of 22, the death rate was high. In this process, changes in tissue and cell structures in the larval liver, biochemical indicators, and osmoregulation-related gene expression were examined. Interestingly, hepatocytes in the low-salinity group were irregular, had dissolved tissue inclusions, and contained vacuolized cells. Therefore, low salinity caused severe damages at a cellular level that can elevate the mortality rate. A gradual decline in salinity limited the full adaptation of S. pharaonis. Biochemical indicators and osmoregulation-related gene expression changed similarly. For instance,the trend of malondialdehyde(MAD) as a product of lipid peroxidation reflected the degree of damage to the body by free radicals.The antioxidant system of S. pharaonis could cope with oxidative stress caused by the change in salinity to a certain extent. Osmoregulation-related genes' expression also showed an optimistic result, that is, S. pharaonis responded positively to the change in salinity by adjusting the expression of osmoregulation-related genes. Conversely, the increase in mortality at day 10 also proved the weak adaptation capability of S. pharaonis. This study indicated that S. pharaonis can adapt to a low-salinity environment with a limited extent.