| Summary: | Insect meals are considered promising, eco-friendly, alternative ingredients for aquafeed. Considering the dietary influence on establishment of functioning gut microbiota, the effect of the insect meal diets on the microbial ecology should be addressed. The present study assessed diet- and species-specific shifts in gut resident bacterial communities of juvenile reared <i>Dicentrarchus labrax</i> and <i>Sparus aurata</i> in response to three experimental diets with insect meals from three insects (<i>Hermetia illucens</i>, <i>Tenebrio molitor,</i><i>Musca domestica</i>), using high-throughput Illumina sequencing of the V3–V4 region of the 16S rRNA gene. The dominant phyla were Firmicutes, Proteobacteria and Actinobacteria in all dietary treatments. <i>Anaerococcus</i> sp., <i>Cutibacterium</i> sp. and <i>Pseudomonas</i> sp. in <i>D. labrax</i>, and <i>Staphylococcus</i> sp., <i>Hafnia</i> sp. and <i>Aeromonas</i> sp. in <i>S. aurata</i> were the most enriched shared species, following insect-meal inclusion. Network analysis of the dietary treatments highlighted diet-induced changes in the microbial community assemblies and revealed unique and shared microbe-to-microbe interactions. PICRUSt-predicted Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways were significantly differentiated, including genes associated with metabolic pathways. The present findings strengthen the importance of diet in microbiota configuration and underline that different insects as fish feed ingredients elicit species-specific differential responses of structural and functional dynamics in gut microbial communities.
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