Black soldier fly meal effects on meagre health condition: gut morphology, gut microbiota and humoral immune response

The effects of black soldier fly (Hermetia illucens) meal (HM) on gut morphology, gut microbial community, and humoral immune response was evaluated in meagre (Argyrosomus regius) juveniles. For that purpose, fish with 18.0 +/- 0.02 g were fed a control diet (CTR) without HM, and 3 other diets including 10, 20 and 30% HM (diets HM10, HM20 and HM30, respectively), replacing 17, 35 and 52% of fish meal (FM), for 9 weeks. The meagre distal intestine (DI) showed no significant histomorphologic alterations in fold height, lamina propria, submucosa width and cellularity, and enterocytes shape, between dietary treatments. Increasing numbers of eosinophilic granulocytes and intraepithelial leukocytes were observed in fish fed the HM30 diet compared to the CTR and HM10 diets. The mean DI score was higher in meagre fed the HM30 diet, denoting increased overall histomorphological alterations in that group compared to fish fed the CTR diet. No differences in gut bacterial profiles (number of operational taxonomic units, microbiota richness, diversity, and similarity indices) were observed between dietary treatments, either in allochthonous (digesta) and autochthonous (mucosa) communities. Plasma anti-protease activity increased with increasing dietary HM level, while no significant differences were observed in the plasmatic protease, peroxidase, lysozyme and bactericidal activities, nitric oxide production and total immunoglobulins. Overall, and in order to avoid gut histomorphological alterations, an inclusion level up to 10% of HM, corresponding to 17% of FM replacement, is recommended for meagre juveniles.