Salinity-induced modulation of hepatic morphology, enzymatic responses, muscle fatty acid composition, and gene expression in the euryhaline teleost Oryzias dancena

Salinity strongly influences osmoregulation energy metabolism, and physiological performance in euryhaline fishes. The marine medaka, Oryzias dancena is an emerging model for investigating the molecular and physiological basis of salinity adaptation. Here, fish were exposed to hypo- (5 ppt), near-isoosmotic (23 ppt), and hyperosmotic (35 ppt) salinities, and assessed integrated physiological, biochemical, and molecular responses. Hepatic histology showed pronounced vacuolization at 5 and 35 ppt, whereas 23 ppt supported more uniform hepatocellular morphology. Whole-body digestive and antioxidant enzymes exhibited salinity-dependent modulation, with elevated protease and SOD activity at 5 ppt and higher lipase activity at 23 ppt. Gene expression analysis showed upregulation of nka under salinity extremes, while lipid oxidation genes (ppar-δ, cpt1) peaked at 23 ppt, indicating a trade-off between osmoregulatory demand and lipid catabolism. Muscle fatty acid composition remained largely conserved; however, hyperosmotic stress (35 ppt) caused a significant decline in docosahexaenoic acid (DHA, 22:6n-3), together with a concomitant increase in monounsaturated fatty acids, particularly palmitoleic acid (16:1), indicating selective changes in membrane lipid composition under high salinity. Although hepatic fatty acid composition was not measured in this study, the combined evidence from hepatocellular morphology and lipid metabolic gene expression provides clear indications of salinity-dependent shifts in hepatic lipid handling. Collectively, this study provides foundational insight into the osmoregulatory and metabolic strategies of O. dancena, establishing its value as a tractable marine model for integrative studies on salinity adaptation.