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To explore the effects of dietary threonine on growth and ovarian development of red swamp crayfish (Procambarus clarkii), crayfish (5.48 ± 0.19 g) were fed six isoproteic and isoenergetic diets with varying levels of threonine (7.16 g/kg (control), 9.19, 12.74, 16.44, 20.83, and 23.78 g/kg) for 8 weeks. The results showed that weight gain rate, feed conversion ratio, protein efficiency rate, protein deposition rate, and essential amino acid deposition rates obtained the optimal values when the dietary threonine level was 12.74 or 16.44 g/kg. Compared to the control group, the 12.74 g/kg group exhibited enhanced nonspecific immunity and antioxidant properties. The 16.44 g/kg group demonstrated a significant increase in the frequency of B cells and R cells in the hepatopancreas, the length and width of intestinal villi, and the activities of protease and lipase. It also showed elevated ecdysterone hormone, gonadal index (GI), cAMP content, and the relative abundance of beneficial intestinal microflora. Compared to the control group, the mRNA expression of mTOR, S6K1, 4EBP1, EcR, RXR, chitinase, PKA, Vg, cdc2, and cyclin B was significantly upregulated, and the mRNA expression of MIH was significantly downregulated in the 16.44 g/kg group. Overall, optimal dietary threonine could improve intestinal health, regulate immune function, and enhance protein utilization, molting, and growth performance of red swamp crayfish. Additionally, it improved the synthesis of yolk substance and facilitated the development of ovarian cells of female crayfish. The optimal threonine level was 14.87–16.94 g/kg (dry matter), corresponding to 42.51–48.42 g/kg of dietary protein in red swamp crayfish.

To investigate the dietary cholesterol requirements of large red swamp crayfish (Procambarus clarkii), crayfish (initial body weight: 13.49 ± 0.22 g) were hand-fed six diets containing 2.47 (C0), 4.27 (C1), 6.80 (C2), 8.77 (C3), 11.74 (C4), and 14.24 (C5) g/kg cholesterol. After 8 weeks of feeding, the maximum weight gain rate and specific growth rate occurred in group C4. The lowest feed conversion ratio was observed in group C3. Total flesh percentage increased significantly by 15.33% in group C2 compared to group C0. The increase in dietary cholesterol resulted in significant quadratic trends in concentrations of crude protein and lipid in muscle and whole body; cholesterol and free fatty acid in hemolymph, hepatopancreas, and muscle; activities of lipase and amylase in hepatopancreas and intestine; and total antioxidant capacity and catalase activity in hepatopancreas. Group C3 experienced a noteworthy increase in hemolymph glucose and total protein content compared to group C0. Additionally, malondialdehyde content and superoxide dismutase activity in hepatopancreas displayed significant linear and quadratic trends. The optimal dietary cholesterol level for large P. clarkii is between 7.42 and 10.93 g/kg, as revealed by the quadratic regression analysis.