Effects of dietary protein levels on growth, muscle composition, digestive enzymes activities, hemolymph biochemical indices and ovary development of pre-adult red swamp crayfish (Procambarus clarkii)

•Effects of dietary protein levels on growth, biochemical indices and ovary development were conducted in pre-adult Procambarbus clarkia.•The optimum protein level for pre-adult crayfish was 40 % based on growth performance.•Diets with the 35–41 % protein were desirable for optimal ovarian maturation of female pre-adult crayfish. This study was conducted to evaluate the effect of dietary protein levels on growth performance, muscle composition, digestive enzymes activities, hemolymph biochemical indices and ovary development of pre-adult red swamp crayfish Procambarus clarkii (initial weight = 12.86 ± 0.10 g). Six isoenergetic diets (fish meal, soybean meal, gluten, and shrimp powder as main protein sources) were formulated to contain graded levels of 20.70 %, 24.70 %, 30.56 %, 35.77 %, 41.12 % and 44.64 % crude protein. Each diet was assigned to triplicate groups of 15 experimental crayfish (10 female and 5 male) for 8 weeks. Our results showed that weight gain (WG) increased with dietary protein supplementation from 20.7% to 35.77%, and then decreased with further increases in dietary protein. Broken-line regression analysis based on WG showed that the optimal dietary protein level for maximum growth performance of crayfish was 39.60 %. Feed conversion ratio (FCR) exhibited an opposite trend as observed in WG, with the lowest in crayfish fed the 30.56 % or 35.77 % protein diet. Protein efficiency ratio (PER) in crayfish fed the 41.12 % or 44.64 % protein diet was significantly lower than that in crayfish fed the 20.70–30.56 % protein diet. Hepatosomatic index (HSI) value of crayfish decreased with dietary protein supplementation and achieved minimum values at 35.77 % or 41.12 % protein diet. The highest crude protein content of tail muscle was found in crayfish fed the diet containing 35.77 % or 41.12 % protein. Activities of protease and lipase in both hepatopancreas and intestine were significantly affected by dietary protein levels, while amylase activity showed no significant difference. Dietary protein also significantly induced the contents of hemolymph total protein, albumin, globulin, glucose, total cholesterol and low-density lipoprotein, along with aspartate transaminase and alanine transaminase activities. Moreover, histological analysis revealed that the ovary development of most female crayfish fed the 35.77–41.12 % protein diet were at stage V, which had a higher frequency of mature oocytes than that of females fed other protein diets. Thus