Revisiting Stygocapitella (Annelida, Parergodrilidae) in Japan, with insights into their amphi-Pacific...

Polychaetes are typically found in marine environments with limited species adapting to semi-terrestrial habitats. The genus Stygocapitella comprises interstitial polychaetes dwelling in sandy beach areas around or above the high-water line. Based on molecular data, previous studies suggested the presence of multiple cryptic species in some different localities in the world lumped together as Stygocapitella subterranea. In Japan, reports on Stygocapitella were scarce, with only one species having been documented 40 years ago at Ishikari Beach in Hokkaido by the name of S. subterranea. We revisited these earlier findings and uncovered the presence of two distinct species in Stygocapitella. One of these species is herein named Stygocapitella itoi sp. nov., while the other corresponds to S. budaevae, originally described from the Russian Far East. Stygocapitella itoi sp. nov. possesses a chaetal pattern similar to that of S. australis, S. furcata and S. pacifica but can be distinguished from the congeners by two characters: a slightly forked pygidium and forked chaetae consisting of two teeth and two outer prongs. Our multi-locus phylogenetic analysis showed close relationships across the Pacific Ocean in two separated lineages in the genus, suggesting ancient dispersal or allopatric speciation after vicariance events.