Global barcoding of the deep-sea snail genus <i>Phymorhynchus</i> reveals surprising distribution ranges and genetic diversity

For rare fauna inhabiting insular habitats, specimens from geographically distant locations are often assumed to be distinct species without a detailed assessment of their genetic diversity. The raphitomid snail genus Phymorhynchus includes a species complex considered endemic to deep-sea hot vents and cold seeps. Despite small morphological differences, existing species hypotheses largely relied on geographic distance to support separation. Here, we present a molecular barcoding study for this complex with a total of 180 specimens sampled across the Atlantic, Pacific and Indian oceans. Our delimitation analyses identified six distinct species supported by morphology, notably egg-capsule characters. They have broader distributions than previously realized, with P. starmeri ranging across the Indo-West Pacific and P. moskalevi apparently cosmopolitan, both showing complex genetic subgroupings. These species are probably also distributed outside vents and seeps, supported by several ambient records. Our results challenge the assumption of fine-scale geographic separation in species inhabiting deep-water insular habitats.