Reshaping gut bacterial communities after dietary Tenebrio molitor larvae meal supplementation in three fish species

The aquaculture industry is currently looking for alternative, sustainable diets that provide similar or better growth for the reared species. We investigated whether replacing fishmeal with yellow mealworm (Tenebrio molitor) meal in the supplied diets of the gilthead sea bream (Sparus aurata), European sea bass (Dicentrarchus labrax) and rainbow trout (Oncorhynchus mykiss) causes similar shifts in the bacterial gut communities of these farmed fish species. The diversity of the gut bacterial 16S rRNA gene revealed the presence of 598 operational taxonomic units (OTUs) belonging to all of the major phyla known to exist in the gut of these three fish species, such as Proteobacteria, Bacteroidetes, Firmicutes and Actinobacteria. Although no statistically significant differences of diversity indices in all three species was found, there was a differential shift in the dominant bacteria in the gut of each species before and after the dietary meal replacement. In S. aurata and D. labrax diversity indices remained practically unchanged before and after the replacement, while for O. mykiss the five-fold increase of the Simpson dominance D index and the almost two-fold decrease of the Shannon H index, suggested that a more specialized gut bacterial community was favoured for this species after the replacement. Also, this replacement resulted in the occurrence of higher number of OTUs which were absent before the replacement in the gut of S. aurata (62.2% of all its OTUs) and D. labrax (60.0% of all its OTUs) compared to O. mykiss (33.0% of all its OTUs) suggesting that insect meal replacement resulted in novel nutritional niches in the gut of S. aurata and D. labrax compared to O. mykiss. Our results indicate that the most desirable fish diet substitution differentially affects the gut microbiota in different hosts, implying that a species-specific tailor-made approach in diet manipulations should be considered in the future.