The effect of dietary full-fat Hermetia illucens larvae meal on growth performance and intestine physiology in largemouth bass ( Micropterus salmoides)

The present study aimed to investigate the effects of different inclusion levels of full-fat Hermetia illucens larvae (HI) meal as a protein source on the growth performance, intestinal morphology, health, and microbiota of largemouth bass. Five isonitrogenous and isolipidic diets were formulated by adding HI to replace 0 % (HI0, control), 10 % (HI10), 20 % (HI20), 30 % (HI30) and 40 % (HI40) of fish meal (FM) in diets. Each diet was randomly assigned to triplicate groups of 30 fish per aquarium. Fish were fed two times daily to apparent satiation for 80 days. The results indicated that weight gain rate, specific growth rate and protein efficiency ratio were not appreciably reduced until more than 30 % of the FM protein was replaced with HI. Fish fed HI40 diet had the lowest fatty acid (C18:3, C20:5, C22:6) contents (P < 0.05). Moreover, the villi height was decreased, villi width and goblet cell number in hindgut was increased in the HI40 group compared with the other groups, respectively. Meanwhile, serum diamine oxidase activity, D-lactate and endotoxin contents increased markedly as the proportion of HI increased (P < 0.05). Notably, the increased dietary HI levels down-regulated the mRNA expression levels of cldnd1b and ocel1 in intestine. Higher malondialdehyde content and down-regulated expression of sod and gpx1a in the intestine were also observed in the HI40 group. Accordingly, the increased dietary HI levels upregulated the expression of intestinal pro-inflammatory cytokines (tumor necrosis factor-alpha and interleukin-1 beta) and down-regulated anti-inflammatory cytokines (transcriptional growth factor-beta 1 and interleukin-10) (P < 0.05). In addition, the abundance of both intestinal Lactobacillus and Streptococcus increased significantly, while the abundance of Cetobacterium decreased in the HI20 group (P < 0.05). Broken-line model analysis based on weight gain rate against dietary HI replacement level indicated that the optimum replacement level was 26.79 %. However, including too high proportion of HI could induce intestinal dysbiosis, impair the intestine physical barrier, and finally reduced growth of Micropterus salmoides.