Fishes constitute a vital component of the aquatic ecosystem, that interacts with physical, chemical, and biological components of the ecosystem. They are found in almost all aquatic environments ranging from high mountain streams to the abyssal and hadal zones of the deepest oceans. With an origin dating back to 600 million years and an estimate of 34,600 species described till date (FishBase, 2021), fishes comprise nearly half of the total vertebrate diversity (Nelson, 2007).Fishes are associated with complex assemblages of microorganisms, comprising bacteria, archaea, viruses, yeasts, and protists, collectively known as 'microbiota' or 'microbiome' (Merrifield & Rodiles, 2015). The characteristic microbial community inhabiting a particular environment along with their genomes and theatre of activity is referred to as the 'microbiome' (Gilbert et al., 2016;Prescott, 2017). However, in certain contexts, the term 'microbiota' is used to define the collection of microorganisms and the term 'microbiome' to refers to the collection of microbial genomes (Burokas et al., 2015).Microorganisms take residence in almost every organ of the fish such as skin, gills, digestive tract, internal organs like the liver, kidney, spleen, and light-emitting organs (Austin, 2002). However, fish gut microflora is often acclaimed as an 'extra organ' owing to its significant contribution to important physiological functions of the host such as digestion, metabolism, reproduction, development, and immune response (Butt & Volkoff, 2019). Further, the composition of the gut microflora is affected by a plethora of factors such as habitat salinity, temperature, trophic level, taxonomy, feeding habits and host selective pressures within the gut, leading to the development of a unique and diverse microflora (Bevins & Salzman, 2011;Sullam et al., 2012). Therefore, it may be safely presumed that the fish gut microbiome, representing 'worlds within worlds' (Butt & Volkoff, 2019) is a biodiversity hotspot and a promising source for bioprospecting applications.
