Optimum dietary vitamin A requirement of the juvenile sea cucumber Apostichopus japonicus

Sea cucumber Apostichopus japonicus is one of most valuable marine cultured species in northeast China, and the cultured production reached 170 000 tons in 2020. With the rapid development of A. japonicus breeding industry, high efficiency and environmental protection formula diet has become one of bottleneck problem which inhibited the development of industry. Vitamin A (V_A), also called retinol, is the general name of monounsaturated alcohol or alcohol active substances, which is an extremely important and easily deficient fat soluble vitamin. V_A can maintain vision, promote cell differentiation and bone development, and participate in fish mucus secretion and fat metabolism. Meanwhile, it also can improve animal growth rate, reproductive performance and disease resistance of Cyprinus carpio var. Jian and Scophthalmus maximus, but long term excessive intake of V_A may result in bone deformities of Sparus aurata. So it is very important to determine the optimum dietary V_A in formulate diet. In order to investigate the effects of dietary V_A on growth performance, body composition, digestive metabolism and non-specific immunity for A. japonicus juveniles, an 8-week feeding experiment was conducted with A. japonicus juveniles. Six isonitrogen and isoenergetic diets with graded levels V_A3 250(D1), 5 187(D2), 7 054(D3), 8 970(D4), 12 975(D5), 16 400(D6) IU/kg were formulated by adding V_A acetate into basal diet. Each diet was assigned to triplicate tanks with 30 A. japonicus juveniles with initial body weight (15.48±0.01) g. Results showed that there was no significant effect on survival rate, but both the weight gain rate and specific growth rate were significantly increased. With the increasing of dietary V_A levels, the contents of crude lipid and V_A of body wall were increased, and crude ash contents increased firstly and thereafter declined. The contents of hydroxyproline were decreased and D1 group was significantly higher than other groups. The activity of lipase was increased with the increasing of dietary V_A levels, and D1group was significantly lower than other groups. The activity of amylase was increased first and then decreased afterwards. There were no differences in protease activities among all groups. With the increasing of dietary V_A levels, all of the activities of aspartate aminotransferase (AST), total superoxide dismutase (T-SOD) and catalase (CAT) were first elevated and then fell down, and the activities of AST and CAT reached the highest in D4 group. The activity of T-SOD of D3 and D4 groups were significantly higher than other groups. The activity of alkaline phosphatase showed an upward trend and D1 group was significantly lower than other groups. The contents of malondialdehyde were decreased. Taking the weight gain rate as the evaluation index, analysis by a linear regression equation indicated that the optimum dietary V_A requirement for A. japonicus (initial body weight 15.41g) was 10 000 IU/kg. The results of this experiment can provide a reference for the addition of V_A in the formula feed, and also provide a basis for the development of "micronutrient balance" formula feed of A. japonicus.