Integrative taxonomy of enigmatic deep-sea true whelks in the sister-genera Enigmaticolus and Thermosipho (Gastropoda: Buccinidae)

Whelks in the sister-genera Enigmaticolus and Thermosipho (Gastropoda: Buccinidae) commonly inhabit deep-water hydrothermal vents and hydrocarbon seeps. Thermosipho desbruyeresi, originally described from the Lau Basin, was thought to occur in vents across the western Pacific, with Eosipho desbruyeresi nipponensis described from the Okinawa Trough treated as its junior synonym. However, new material collected from vents in the Okinawa Trough and seeps in the South China Sea exhibit key characteristics of Enigmaticolus. Re-examination of the types revealed that Eosipho d. nipponensis is actually morphologically distinct from Thermosipho desbruyeresi. A molecular phylogeny reconstructed using the cytochrome c oxidase subunit I (COI) gene confirmed the placement of both taxa in Enigmaticolus and supported their distinctiveness at the species level. We, therefore, rehabilitate E. d. nipponensis as Enigmaticolus nipponensis comb. nov. and transfer T. desbruyeresi to the same genus, as Enigmaticolus desbruyeresi comb. nov. Our results also revealed that Enigmaticolus monnieri described from east Africa and E. inflatus described from the South China Sea are in fact conspecific with E. nipponensis. We discuss the distribution and biogeography, as well as morphological variability, of Enigmaticolus in the light of these new findings. Thermosipho is then left with only its type species, T. auzendei from the East Pacific vents. We have revised the diagnosis for the two genera, as well as the species included in them.