Gut microbiota mediates the improved growth, flesh quality and intestinal health of largemouth bass (Micropterus salmoides) fed defatted Hermetia illucens larvae meal

Defatted Hermetia illucens larvae meal (DBM) is a promising fishmeal alternative, but its optimal application strategy and underlying mechanisms remain unclear. This study investigated the effects of levels vs . composite protein (FC) strategy of DBM on growth, flesh quality, intestinal health of largemouth bass ( Micropterus salmoides ), and the underlying role of gut microbiota. Eight experimental diets were formulated: fishmeal-based controls, and diets in which 25%, 50%, 75% fishmeal was replaced by DBM or FC. After 90-day feeding, 50% replacement with DBM and FC (i.e. 18.97-28.45% dietary DBM) significantly improved weight gain rate, specific growth rate and feed utilization compared to the controls and other groups ( P <0.05). This optimal DBM inclusion significantly increased nutrients content, antioxidant capacity (e.g. increased SOD, CAT, T-AOC activities; decreased MDA and LPO levels) in fillets. It also enhanced intestinal barrier integrity and function, as evidenced by up-regulating expression of tight junctions, anti-inflammatory factors, mucin-2 and lysozyme ( P <0.05). Metagenomic sequencing revealed that these benefits were associated with significant reshaping of gut microbiota. Diets with 18.97-28.45% DBM enriched beneficial bacteria ( Cetobacterium, Fusobacterium and Akkermansia ), while reducing potential pathogens. Functional analysis indicated that these microbial shifts advanced digestion and absorption of nutrients, promoted amino acid synthesis pathways. We conclude that the benefits of DBM depend primarily on its level rather than application strategy. Optimal dietary level of 18.97-28.45% DBM enhances the comprehensive performance of M. salmoides by modulating gut microbiota, thereby improving growth, flesh quality and health. These findings provide mechanistic basis for precise application of DBM in aquaculture.