Chitin can enhance the growth performance and immune function of juvenile Chinese mitten crabs ( <i>Eriocheir sinensis</i> )

An 8-week feeding trial was conducted to assess the effects of dietary chitin on the growth and immunity of juvenile Chinese mitten crab ( Eriocheir sinensis ). Five isonitrogenous and isolipidic diets were formulated with chitin levels of 0% (C1), 0.7% (C2), 1.4% (C3), 2.1% (C4), and 2.8% (C5), respectively. Results indicated that chitin supplementation had significant quadratic effects on final body weight, weight gain rate, whole-body crude protein content, and hepatopancreatic protease and lipase activities. These indicators in the C3 and C4 groups were significantly higher than those in the C1 group (P &lt; 0.05), while the feed conversion ratio and whole-body moisture content were significantly lower ( P &lt; 0.05). For antioxidant and non-specific immune function, the activities of T-SOD, GSH-Px, and T-AOC in C3 and C4 groups were significantly higher than those in the C1 group ( P &lt; 0.05), while the MDA content was significantly lower ( P &lt; 0.05). Only C4 showed higher ACP and AKP activities than C1 ( P &lt; 0.05). Regarding intestinal health, the C4 group exhibited significantly higher values in intestinal fold quantity, height, width, and abundance of beneficial bacteria Candidatus Bacilloplasma compared to the C1 group ( P &lt; 0.05). In terms of hepatopancreatic health, the C3 and C4 groups demonstrated intact cellular architecture with neatly arranged nuclei, whereas the other three groups showed varying degrees of structural damage. In conclusion, 1.4-2.1% chitin supplementation improved the growth performance and immunity of E. sinensis , while excessive supplementation produced adverse effects.