Effects of nucleotide supplementation on growth, immune responses and intestinal morphology in juvenile turbot fed diets with graded levels of soybean meal (Scophthalmus maximus L.)

A 60-day feeding trial was conducted to evaluate the effects of supplemental nucleotides in diets with graded levels of soybean protein on growth, immune responses and intestinal morphology of juvenile turbot, Scophthalmus maximus L. (mean initial body weight, 9.18±0.02g). Nine isonitrogenous (50% crude protein) and isolipidic (12% crude lipid) practical diets were formulated to contain 30%, 40% and 50% soybean protein, and each soybean protein level was supplemented with three levels of nucleotides (0.0gkg−1, 0.3gkg−1 and 1.0gkg−1) from a mixed-nucleotides (sigma). Quadruplicate groups of fish were randomly fed with each diet by 20–30gkg−1 of their body weight per day. The results showed that not dietary nucleotide supplementation but soybean protein level significantly affected specific growth rate (SGR) following the 60 -day feeding trial. The immune assay showed that activity of serum lysozyme in fish decreased with increasing dietary soybean protein (P<0.05) only in the treatment at nucleotide supplementation level of 0.3gkg−1, while activity of serum superoxide dismutase (SOD) in fish increased (P<0.05) with increasing dietary soybean protein and then decreased (P>0.05) at each dietary nucleotide supplementation level. The activity of lysozyme in fish fed diets containing 30% soybean protein increased with increasing dietary nucleotide level (P>0.05) and then remarkably decreased (P<0.05). At each dietary soybean protein, activity of serum SOD was not significantly different among fish fed diets with graded supplementation levels of nucleotides (P>0.05). The higher soybean protein and nucleotide supplementation level showed a significantly higher activity of respiratory burst (P<0.05). There was a significant decrease in enterocyte height (HE), microvillus height (HMV) and fold height (HF) in distal-intestine when the replacement level increased from 40% to 50% (P<0.05). HE in distal-intestine of fish fed diets at nucleotide supplementation level of 1.0gkg−1 was higher than those without nucleotides at each soybean protein level. These results suggested that up to 40% of fish meal protein replaced by soybean protein with essential amino acid supplementation did not significantly reduce growth and feed utilization. Nucleotide supplementation did not significantly influence growth of fish fed diets with 30% to 50% soybean protein but could be helpful to improve the non-specific immune responses and the intestinal structure of turbot.