Sodium butyrate stimulates intestinal health and immunity by reshaping the intestinal microbiota and metabolite profile in big-belly seahorses (Hippocampus abdominalis).

This study examined the effects of sodium butyrate (SB) on growth, intestinal health and immunity of big-belly seahorses (Hippocampus abdominalis) suffering frozen diet-induced enteritis and stress. Total 750 seahorses were randomly assigned to five groups in triplicate, and were fed frozen mysids mixed with SB at 0, 1.25, 2.5, 5 and 10 mg/L (based on culture water volume) for 8 weeks, followed by an Edwardsiella tarda challenge. Results demonstrated that the SB1.25 group had a significantly higher specific growth rate, hepatic T-SOD and CAT activities, and lower MDA content than the SB0 group. Compared with the SB0 group, the SB1.25 and SB2.5 groups exhibited significantly higher activities of intestinal lipase, trypsin and alkaline phosphatase, as well as longer intestinal villus length. The SB1.25 group demonstrated upregulation of tight junction-related genes (occludin, claudin5, and zo1), increased abundance of beneficial gut bacteria (Lactobacillus and Lactococcus), and reduced abundance of potential pathogenic bacteria (Rickettsiales and Nocardia). Moreover, the levels of immune-related metabolites including Periplocin and short peptides in the SB1.25 group were significantly higher than the SB0 group. The Edwardsiella tarda challenge test revealed that the SB1.25 group exhibited the highest survival rate among all treatments, indicating that SB supplementation improves the infection resistance of big-belly seahorses. In conclusion, SB mixed into frozen mysids improves intestinal health and immunity of big-belly seahorses by regulating intestinal microbiota and metabolites, with 1.25 mg/L (culture water-based) as the optimal concentration.