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.