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The symbiosis between a host and its microbiome is essential for host fitness, and this association is a consequence of the host’s physiology and habitat. Sinocyclocheilus, the largest cavefish diversification of the world, an emerging multi-species model system for evolutionary novelty, provides an excellent opportunity for examining correlates of host evolutionary history, habitat, and gut-microbial community diversity. From the diversification-scale patterns of habitat occupation, major phylogenetic clades (A–D), geographic distribution, and knowledge from captive-maintained Sinocyclocheilus populations, we hypothesize habitat to be the major determinant of microbiome diversity, with phylogeny playing a lesser role. For this, we subject environmental water samples and fecal samples (representative of gut-microbiome) from 24 Sinocyclocheilus species, both from the wild and after being in captivity for six months, to bacterial 16S rRNA gene profiling using Illumina sequencing. We see significant differences in the gut microbiota structure of Sinocyclocheilus, reflective of the three habitat types; gut microbiomes too, were influenced by host-related factors. There is no significant association between the gut microbiomes and host phylogeny. However, there is some microbiome related structure at clade level, with the most geographically distant clades (A and D) being the most distinct, and two geographically overlapping clades (B and C) being similar. Microbes inhabiting water were not a cause for significant differences in fish-gut microbiota, but water quality parameters was. Transferring from wild to captivity, the fish microbiomes changed significantly and became homogenized, signifying adaptability and highlighting the importance of environmental factors (habitat) in microbiome community assembly. The core microbiome of this group closely resembled that of other teleost fishes. Our results suggest that divergent selection giving rise to evolutionary novelties also includes the microbiome of these fishes, which provides a functional advantage for life in the resource-depleted cave environment.SIGNIFICANCE STATEMENTThe largest diversification of cavefishes of the world, Sinocyclocheilus, not only show that habitat, and phylogenetic clade is important in determining their gut microbiome, but also that they reach a common microbiome in captivity irrespective of their phylogenetic position, region of origin and habitat, indicating that they are adaptable in the context of microbe related changes in their environment.

It is known that animal-associated microbiomes form indispensable relationships with hosts and are responsible for many functions important for host-survival. Next-gen driven approaches documenting the remarkable diversity of microbiomes have burgeoned, with amphibians too, benefiting from such treatments. The microbiome of Gymnophiona (caecilians), one of the three amphibian orders, constituting of 3% of amphibians, however, remains almost unknown. The present study aims to address this knowledge gap through analysis of the microbiome of Ichthyophis bannanicus. As these caecilian larvae are aquatic and hence exposed to a greater propensity for bacterial microbiomic interchange, we hypothesize that bacterial phyla would overlap between gut and skin. Further, from the host-specificity patterns observed in other vertebrate taxa, we hypothesize that Gymnophiona have different dominant gut bacterial microbiomes at a higher taxonomic level when compared to the larvae of the other two amphibian orders (Anura and Caudata). We used 16S rRNA gene amplicon sequencing based on Illumina Nova sequencing platform to characterize and compare the gut (represented by faecal samples) and skin microbiome of I. bannanicus larvae (N = 13), a species distributed across South-East-Asia and the only caecilian species occurring in China. We compared our gut microbiome results with published anuran and caudate larval microbiomes. For I. bannanicus, a total of 4,053 operational taxonomic units (OTU) across 13 samples were detected. Alpha-diversity indices were significant between gut and skin samples. Non-metric multidimensional scaling analysis suggest that gut and skin samples each contained a distinct microbiome at OTU level. We record significant differences between the bacterial phyla of gut and skin samples in larvae of I. bannanicus. The study provides an overview of gut and skin bacterial microbiomes of a caecilian, while highlighting the major differences between larval microbiomes of the three amphibian orders. We find a partial overlap of gut bacterial microbiomes at phylum level for the three orders; however, the relative abundance of the dominant phyla is distinct. The skin and gut microbiomes are distinct with little overlap of species, highlighting that gut-skin axis is weak. This in turn suggests that many of the microbial species on skin and gut are functionally specialized to those locations. We also show that the skin microbiome is more diverse than the gut microbiome at species level; however, a reason for this could be a portion of the gut microbiome not being represented in faecal samples. These first microbiome information from a caecilian lay the foundation for comparative studies of the three amphibian orders.