Effect of graded inclusion of black soldier fly (Hermetia illucens, Linnaeus, 1758) pre-pupae meal in diets for gilthead seabream (Sparus aurata, Linnaeus, 1758) on gut microbiome and liver morphology

Over the last decades, an insect meal has received great attention for finfish diets, due to its nutritional composition and low ecological footprint. In the present study, we assessed the response of gut microbiota composition and liver histology of gilthead seabream (Sparus aurata) fed four experimental diets including the black soldier fly (Hermetia illucens) meal (HI) used to replace 0 (HI0), 25 (HI25), 35 (HI35) and 50 (HI50) percent of fish meal in a 131-day feeding trial. At the end of the experiment, a remarkable change in gut microbiota composition related to HI inclusion was observed, with a preponderance of Cyanobacteriain the control and low HI groups (HI0, HI25) while Chloroflexi became prevalent in the higher HI inclusion groups (HI35, HI50). Predictive analysis on bacterial metabolic pathways showed a clear separation between HI0-HI25 and HI35-HI50 groups. The microbiota shifts observed suggest a pivotal role of HI in inducing a bacterial-mediated physiological response in this fish species, probably due to chitin content and the fatty acid profile of this ingredient. Liver histology showed a higher hepatocyte size in fish from the HI50 group, suggesting lipid dysmetabolism due to the HI meal fatty acid profile, while a marginal adaptive response was observed in the HI25 group. In conclusion, while up to 25% inclusion of black soldier fly meal showed limited adverse effects, 50% HI dietary inclusion is not recommended in gilthead seabream diet, since possible alteration in lipid deposition, particularly at hepatic level, were highlighted in this fish species.