Effects of dietary raffinose on growth performance, physiological indices and glycometabolism of juvenile sea cucumber (Apostichopus japonicus)

Sea cucumber (Apostichopus japonicus) is one of most valuable cultured marine species in northeast and southeast China. With the rapid development of A. japonicus breeding industry, high efficiency and environmental protection formula diet has become one of bottleneck problems which hinder the development of the industry. Raffinose is a functional oligosaccharide, which is utilized by Bifidobacterium and Lactobacillus in the intestines, and plays an important role in the digestive physiology and antioxidant properties of the body. This study investigated the effects of dietary raffinose on growth performances, physiological indices and genes related to glycometabolism of juvenile A. japonicus. Six isonitrogen and isoenergetic experimental diets were formulated to contain graded levels of raffinose, namely 0.00% (D1), 0.02% (D2), 0.03% (D3), 0.06% (D4), 0.08% (D5) and 0.11% (D6) dry diets. Each diet was randomly assigned to triplicate tanks of juvenile A. japonicus with initial body weight (11.46±0.06) g for 67 days. There were no differences in survival rate between all groups. Both weight gain rate (WG) and the specific growth rate were increased first and then decreased afterwards with the increase of dietary raffinose. All of the activities of intestinal protease, lipase, amylase and superoxide dismutase were increased first and then decreased, and were significantly higher in D3 and D4 groups than other groups. The content of malondialdehyde was first decreased and then increased, and reached the lowest value in D4 group. The heights of intestinal folds in D3-D5 groups were significantly higher than those in the D1 group, and meanwhile, inflammatory cell infiltration appeared in D5 and D6 groups. With the increase of dietary raffinose content, all of the activities of glucokinase, fructosophosphate phosphokinase and pyruvate kinase were increased first and then decreased. The activity of phosphoenolpyruvate carboxykinase were increased first and then kept stable, and D4-D6 had significantly higher figures than D1 group. The expression levels of glucokinase (GCK), fructosophosphate phosphokinase (PFK) and pyruvate kinase (PK) genes increased first and then decreased, and were significantly higher in D2-D6 groups than D1 group. Both GCK and PK genes expression reached the maximum in D4 group, and PFK gene expression levels in D4 and D5 groups were significantly higher than those in other groups. With WG as an evaluation indicator, quadratic regression analysis showed that the optimum dietary raffinose for juvenile A. japonicus was 0.063% diet. Dietary raffinose increased the glucose metabolism efficiency, improved the digestive physiology and antioxidant capacity of the body, and promoted the growth of juvenile A. japonicus.