Optimal Dietary Isoleucine Promotes Growth, Regulates Lipid Metabolism, and Modulates Hepatopancreatic Microbiota in Juvenile mud crab (Scylla paramamosain).

This study investigated the effects of dietary isoleucine on growth performance, digestive and metabolic enzyme activities, antioxidant capacity, and hepatopancreatic microbiota in juvenile Scylla paramamosain. Five isonitrogenous and isolipidic diets were formulated with graded isoleucine levels: 0.97% (Ile 1), 1.19% (Ile 2), 1.57% (Ile 3), 1.74% (Ile 4), and 2.09% (Ile 5). A total of 560 juveniles (initial weight: 15.07 ± 1.37 mg) were fed for 8 weeks. Results showed that weight gain rate (WGR) and specific growth rate (SGR) significantly increased with dietary isoleucine levels up to 1.57% and declined thereafter. Broken-line regression analysis based on WGR estimated the optimal dietary isoleucine requirement at 1.43%. Hepatopancreatic alanine aminotransferase (ALT) activity was highest in the 1.57% group, whereas trypsin and lipase activities did not differ significantly among treatments. Dietary isoleucine levels influenced lipid metabolism, with excessive supplementation (2.09%) associated with reduced growth and increased whole-body crude lipid content, accompanied by upregulated expression of fas, srebp1, acc and downregulated hsl expression. Appropriate dietary isoleucine levels (1.19%-1.57%) increased hepatopancreatic superoxide dismutase (SOD) activity and reduced malondialdehyde (MDA) content. Hepatopancreatic microbial α-diversity and overall community structure did not differ significantly among treatments, although Thiothrix showed the highest relative abundance in the 1.57% group. PICRUSt2 analysis suggested differences in predicted microbial metabolic functions among treatments. Overall, the estimated dietary isoleucine requirement for juvenile S. paramamosain was 1.43%, while excessive dietary isoleucine impaired growth and was associated with increased lipid deposition.