Abstract
Deep hydrothermal vents are highly fragmented and unstable habitats at all temporal and spatial scales. Such environmental dynamics likely play a non-negligible role in speciation. Little is, however, known about the evolutionary processes that drive population-level differentiation and vent species isolation and, more specifically, how geography and habitat specialisation interplay in the species history of divergence. In this study, the species range and divergence of Alviniconcha snails that occupy active Western Pacific vent fields was assessed by using sequence variation data of the mitochondrial Cox1 gene, RNAseq, and ddRAD-seq. Combining morphological description and sequence datasets of the three species across five basins, we confirmed that A. kojimai, A. boucheti, and A. strummeri, while partially overlapping over their range, display high levels of divergence in the three genomic compartments analysed that usually encompass values retrieved for reproductively isolated species with divergences rang from 9% to 12.5% (mtDNA) and from 2% to 3.1% (nuDNA). Moreover, the three species can be distinguished on the basis of their external morphology by observing the distribution of bristles and the shape of the columella. According to this sampling, A. boucheti and A. kojimai form an east-to-west species abundance gradient, whereas A. strummeri is restricted to the Futuna Arc/Lau and North Fiji Basins. Surprisingly, population models with both gene flow and population size heterogeneities among genomes indicated that these three species are still able to exchange genes due to secondary contacts at some localities after a long period of isolation.