Effects of Dietary Nucleotides on Growth Performance, Antioxidant Capacity, Intestinal Morphology and Gut Microbiota of Swamp Eel (Monopterus albus)

This study evaluated how graded dietary nucleotide supplementation (0, 0.25, 0.5, 0.75, 1.0, and 2.0 g/kg) affects growth performance, antioxidant capacity, intestinal morphology, and gut microbiota in swamp eel (Monopterus albus) (initial body weight 10.07 ± 0.92 g). Three hundred sixty fish were randomly assigned to six diets, each in triplicate, for eight weeks. Compared with the control, nucleotide addition significantly increased final body weight, weight gain rate, and specific growth rate, and decreased feed conversion ratio (p < 0.05), with optimal results at 0.75 g/kg (HS3). Survival was 100% in all groups. Supplemented fish showed lower serum and intestinal malondialdehyde levels and higher superoxide dismutase and catalase activities (p < 0.05). Serum total protein, albumin, and triglycerides increased, whereas alanine aminotransferase, aspartate aminotransferase, and γ-glutamyl transpeptidase decreased (p < 0.05), pointing to improved hepatic and lipid metabolism. Intestinal trypsin, lipase, and amylase activities also rose markedly (p < 0.05), peaking in HS3. Histological examination revealed greater mucosal thickness and villus height (p < 0.05); in HS3, these values reached approximately 0.95 mm and 0.87 mm, respectively. Metagenomic analysis showed that 0.75–1.0 g/kg nucleotides increased alpha diversity and restructured the microbial community, enriching Bacteroidetes- and Prevotella-related taxa while reducing Proteobacteria, including Acinetobacter baumannii and Escherichia coli. LEfSe identified dose-specific discriminant taxa, and refined KEGG Level 3 pathway analysis predicted enhanced butyrate and propanoate biosynthesis, starch utilization, and purine/pyrimidine interconversion at moderate doses. Genus-level abundances of Prevotella and Bacteroides correlated inversely with serum oxidative and hepatic stress markers. Quadratic regression estimated the optimal dietary nucleotide level at 764 mg/kg (0.76 g/kg), consistent with the best-performing 0.75 g/kg group. Collectively, 0.75–0.76 g/kg dietary nucleotides optimize growth and intestinal health in M. albus through coordinated improvements in antioxidant status, digestive function, mucosal architecture, and beneficial gut microbiota remodeling.