Decoding diversity in a coral reef fish species complex with restricted range using metagenomic sequencing of gut contents.

Aim: Identification of the processes that generate and maintain species diversity within the same region can provide insight into biogeographic patterns at broader spatiotemporal scales. Hawkfishes in the genus are a unique taxon to explore with respect to niche differentiation, exhibiting diagnostic differences in coloration, and an apparent center of distribution outside of the Indo-Malay-Philippine (IMP) biodiversity hotspot for coral reef fishes. Our aim is to use next-generation sequencing methods to leverage samples of a taxon at their center of maximum diversity to explore phylogenetic relationships and a possible mechanism of coexistence.

Location: Flint Island, Southern Line Islands, Republic of Kiribati.

Methods: A comprehensive review of museum records, the primary literature, and unpublished field survey records was undertaken to determine ranges for four "arc-eye" hawkfish species in the species complex and a potential hybrid. Fish from four species were collected from Flint Island in the Southern Line Islands, Republic of Kiribati. Hindgut contents were sequenced, and subsequent metagenomic analyses were used to assess the phylogenetic relatedness of the host fish, the microbiome community structure, and prey remains for each species.

Results: Phylogenetic analyses conducted with recovered mitochondrial genomes revealed clustering of with and with , which were unexpected on the basis of previous morphological work in this species complex. Differences in taxonomic composition of gut microbial communities and presumed prey remains indicate likely separation of foraging niches.

Main Conclusions: Our findings point toward previously unidentified relationships in this cryptic species complex at its proposed center of distribution. The three species endemic to the Polynesian province (, , and ) cluster separately from the more broadly distributed on the basis of relative abundance of metazoan sequences in the gut (presumed prey remains) Discordance between gut microbial communities and phylogeny of the host fish further reinforce the hypothesis of niche separation.