A microbial sea of possibilities: current knowledge and prospects for an improved understanding of the fish microbiome

The mucosal surfaces of fish represent an important barrier that supports and regulates a diverse array of microbial assemblages and contributes to the overall health and fitness of the host. For farmed species, knowledge of how these host–microbial systems adapt and respond to various stressors is pivotal for managing health, nutrition and optimizing productivity in aquaculture. While our understanding of these communities and the factors that shape them now suggest that a diverse balanced microbiota is critical for healthy functioning in fish, the mechanisms behind these interactions are still poorly understood. Much of the existing research has focused on characterizing the taxonomic diversity of these assemblages in different fish species, across body surfaces (e.g. skin, gills and gastrointestinal tract), and in response to changing nutrition, health and environmental conditions. However, the specific functional contributions of these communities (or specific members) remain elusive, especially in farmed or diseased fish. Here, we review our current understanding of the microbiota in fish, their interplay and the likely functional involvement with the host. We also seek to address and identify gaps in knowledge and explore the future prospects for improving our understanding of these communities in aquaculture.