Nutritional regulation of osmoregulatory and metabolic responses to low salinity in juvenile black seabream (Acanthopagrus schlegelii): Roles of dietary DHA and EPA

DHA and EPA are pivotal for fish physiological functions. This study investigated the effects of dietary DHA+EPA levels on osmoregulation, lipid utilization, and energy metabolism in juvenile black seabream ( Acanthopagrus schlegelii ) under low salinity environment (5 psu). Different dietary levels of DHA + EPA were formulated (D1 = 10.48, D2 = 18.53, D3 = 25.77, D4 = 31.47, D5 = 38.81 mg/g; DHA/EPA ≈ 1:1) were fed to fish (initial body weight: 15.08 ± 0.02 g) for 8 weeks. Results showed that WG and SGR were markedly higher in D5 group compared to D1. The DHA+EPA contents in liver and muscle were markedly elevated with increasing dietary levels, peaking in D5. Gill expression of osmoregulatory genes ( ostf1 , aqp1 ) was markedly downregulated in the D3 group relative to D1, indicating improved osmoregulatory adaptation. Hepatic expression of lipolysis-related genes ( atgl , hsl , lpl ) peaked in the D2, whereas lipogenesis-related genes ( fas , accα ), showed the lowest expression in D4. The D2 group also exhibited the highest mRNA levels of mitochondrial energy metabolism-related genes and an increased number of hepatic mitochondria. The D4 group significantly elevated hepatic antioxidants (T-AOC, SOD, GSH-Px, GSH), while D3 showed highest serum T-AOC and SCHR, accompanied by the lowest MDA levels in both liver and serum. Furthermore, hepatic antioxidant gene expression was markedly upregulated at dietary DHA+EPA levels of 18.53–25.77 mg/g. These findings show that moderate dietary DHA+EPA supplementation (18.53–25.77 mg/g) promotes mitochondrial function, antioxidant defense, and osmoregulatory efficiency in juvenile black seabream under low-salinity conditions.