Dietary protein requirement of Chinese hook snout carp (Opsariichthys bidens) and transcriptomic analysis of the liver in response to different protein diets

The Chinese hook snout carp (Opsariichthys bidens) is a newly developed economic fish species for its good taste, high nutritional content, and suitability for aquaculture in southeastern China, but its dietary protein requirement has not been investigated. In this study, the fish (initial weight 12.45 ± 0.21 g) were fed five isoenergetic diets with varying protein levels (35–47 %, P35-P47) for 65 days. Growth performance increased with higher dietary protein levels up to 41 % and then plateaued at higher concentrations. On the basis of the second regression analysis, the dietary protein requirement of Chinese hook snout carp was estimated to be 42.97 %. Different dietary protein levels did not significantly influence the muscle crude protein or amino acid profile (P > 0.05) but significantly affected the contents of some fatty acids (P < 0.05). The P47 group had a significantly lower hepatic malonaldehyde content than the other groups. The liver samples from the P35, P41, and P47 groups were subsequently used for transcriptomic analysis. There were 43 and 106 downregulated and 172 and 190 upregulated differentially expressed genes (DEGs) in the P41 vs P35 and P47 vs P41 comparisons, respectively. DEGs associated with nutrient metabolism and immune responses provide insights for optimizing protein levels in aquaculture feed formulations. Compared with those in the P41 group, some genes related to the digestive system (e.g., carboxypeptidases, elastase 2, trypsin-2, and bile salt-activated lipase) and immune response (e.g., NF-kappa B inhibitor alpha, tumor necrosis factor alpha-induced protein 3, complement C3, and C7) were significantly upregulated in the P47 group. In contrast, the P35 group presented lower amino acid and lipid metabolism (e.g., stearoyl-CoA desaturase, glutamine synthetase, and bile salt-activated lipase) expression levels. The present study revealed that 42.97 % dietary protein was optimal for the growth of Chinese hook snout carp. The low protein diet inhibited hepatic protein and lipid metabolism, whereas high dietary protein inhibited nutrient digestion but promoted an immune response on the basis of transcriptomic analysis.