Previous studies in certain fish species have shown that a dietary addition of exogenous nucleotides (NT) could improve growth, gut integrity, stress tolerance and enhances immune response. In this contest a feeding trial was carried out to evaluate if, and to what extent, a dietary NT supplementation could elicit similar beneficial responses in a poorly domesticated species like the common sole. A basal diet (ES, control) was compared to a test preparation (EN) obtained from the basal one by adding 0.4 g/kg dry diet of Vannagen®, a commercial source of nucleotides. One hundred seventy six fish (mean body weight 122.9±9.6g) were randomly distributed in 4 tanks (1m2) at 5.4±0.7kg/m2 stocking density. Fish were fed at 1% body weight/d over 8 weeks according to a single-factor experimental design with 2 replicates per diet. At the end of the trial specific growth rate (SGR), feed conversion ratio (FCR) and survival rate were calculated per group; furthermore 8 fish per treatment were analysed for plasma lysozyme activity, levels of total proteins, immunoglobulins (IgM), respiratory burst activity of head-kidney leucocytes and for gut and liver histology. After 8 weeks of feeding, fish fed diet EN had the highest final weight and SGR resulting in a better FCR, and survival rate when compared to the controls (146.2 vs 132.4g, 0.23 vs 0.16 , 3 vs 4, and 87.8 vs 76.3%; respectively, P<0.05). Soles fed diet EN showed higher plasma levels of total proteins and IgM relative to the controls (P<0.05). No significant (P>0.05) dietary effect on respiratory burst and lysozyme activities were observed. Dietary NT supplementation didn’t affect liver histology, while the mean fold height of distal intestine was significantly improved in fish given diet EN than in those fed diet ES. The results obtained so far suggest that dietary nucleotides addition in sole diets improves growth performance, mostly through increased palatability and feed intake, and adaptive immunity as well, probably by regulating the intracellular nucleotide pool. The lack of NT effect on lysozyme and burst activity could be due either to the high among-individual response variability or to the ineffectiveness of the NT dose here tested.
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