Oligochitosan‐Ameliorated Gut Microbiome and Metabolic Homeostasis in Hybrid Groupers (Epinephelus lanceolatu ♂ × Epinephelus fuscoguttatus ♀) Infected With Vibrio harveyi

Vibrio infections cause enteritis in grouper fish, leading to high mortality and stunted growth, which is a major challenge for aquaculture. Oligochitosans, marine prebiotics with bioactive properties, have proven their potential for growth promotion and immune regulation. However, the impacts of Vibrio harveyi on the gut microbiome of grouper fish and the potential of oligochitosans to modulate these effects remain poorly understood. This study investigates the influence of oligochitosan on the gut morphology, microbiota and metabolic patterns of hybrid grouper following low‐dose infection with V. harveyi . After 8 weeks of feeding, infected groupers exhibited histopathological changes in the intestine that were mitigated by oligochitosan, as evidenced by improved villus height and intestinal wall thickness. Microbiome sequencing revealed that oligochitosan enhanced α‐diversity and shifted bacterial communities, particularly increasing the abundance of Fusobacteriota and Actinobacteriota . Furthermore, the prevalence of Shewanella and Vibrio , which were more abundant in infected groupers, was reduced upon oligochitosan treatment. Metabolomic analysis indicated that lipid metabolism pathways were significantly altered in response to infection and oligochitosan intervention. Overall, oligochitosan modulates the gut microbiome and metabolite profiles, potentially protecting against V. harveyi ‐induced intestinal dysbiosis and metabolic disorders in hybrid groupers.