Effects of dietary carbohydrate level on growth, biochemical composition and glucose metabolism of juvenile sea cucumber Apostichopus japonicus (Selenka)

A 60-day feeding experiment was conducted to evaluate the effects of dietary carbohydrate level on growth performance, feed utilization, biochemical composition and glucose metabolism of juvenile sea cucumber. The sea cucumbers were fed five isonitrogenous and isolipidic diets containing graded levels of carbohydrate from 25.61% to 66.09%, to investigate the optimum dietary carbohydrate level. The results revealed that specific growth rate (SGR) and feed conversion efficiency (FCE) first significantly increased then declined as dietary carbohydrate level increased. The relatively higher protein efficiency ratios (PER) in diets with carbohydrate levels of 45.31% and 55.82% implied that carbohydrate supplied energy to gain the protein sparing effect. Apparent digestibility coefficients (ADC) of dry matter, crude protein, carbohydrate and gross energy were all significantly affected by dietary carbohydrate level. The high dietary carbohydrate level resulted in the increased glycogen biosyntheses and lipid deposition, accelerated glycolysis and decelerated gluconeogenesis to maintain glucose homoeostasis. Glycolytic enzyme activities of hexokinase (HK), pyruvate kinase (PK) and lactate dehydrogenase (LDH), and succinate dehydrogenase (SDH) showed an ascending trend as dietary carbohydrate level increased at the initial stage. However, PK, LDH, lipogenic enzyme activity of glucose-6-phosphate dehydrogenase (G-6-PD), gluconeogenic enzyme activity of fructose-1,6-bisphosphatase (FDPase), amylase activity and cortisol level significantly decreased at the highest dietary carbohydrate level of 66.09%, and glucose in coelomic fluid significantly increased, which demonstrated that excess dietary carbohydrate might lead to the inhibition of glucose metabolism. Based on the second-order polynomial regression models of SGR and FCE, it was concluded that 48.56%-49.30% dietary carbohydrate levels were optimal for Apostichopus japonicus.