Towards defining optimal dietary protein levels for male and female sub-adult Chinese mitten crab, Eriocheir sinensis reared in earthen ponds: performances, nutrient composition and metabolism, antioxidant capacity and immunity

Chinese mitten crab (Eriocheir sinensis) is a widely farmed and economically important aquaculture species. However, species-specific knowledge about appropriate dietary protein requirements is limited to juveniles, cultured in experimental like conditions: indoors and in clear water. Previous studies have also focused primarily only on growth performance and tissue proximate composition, but little is known about the physiological responses and immunity status of crabs fed with different amounts of dietary protein. Accordingly, this study aimed at assessing the effects of dietary protein levels on growth performance, nutrient composition, nutrient metabolism and physiological response, in sub-adult animals reared in (commercial-like) earthen ponds until harvesting size and gonad maturation. The diets had a graded protein inclusion (278, 323, 352, 391 and 420 g kg−1) and a bell-shaped trend was seen in the performance of all crabs in ponds. Regression analysis provided an estimate of ~384 g kg−1 dietary protein for maximal growth for female animals. Interestingly, female crabs fed the diets with medium protein levels (352-391 g kg−1) also subjected the lower oxidative stress. Whereas the growth performance of male crabs did not show any plateauing and continued to increase with additional dietary protein. Increasing dietary protein levels had the following effects: i) improved the activity of amino acid metabolism-related enzymes in the hepatopancreas and decreased the triglycerides (TG) and total cholesterol (TC) content in serum and hepatopancreas in both sex; ii) decreased the activity of enzymes associated with hepatic damage; iii) decreased the anti-oxidative enzyme activities as well as the content of protein and lipid oxidation products of male crabs. These results suggest that suitable protein levels in the diets improve overall growth performance by increasing protein deposition and reducing lipid deposition and damage in the hepatopancreas, as well as reducing tissue oxidative stress. In conclusion, the optimal dietary protein levels for male and female sub-adult crabs reared in earthen ponds are different: they are 385 g kg−1 for females, and at least 420 g kg−1 for males.