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

Polychaetes are typically found in marine environments with limited species adapting to semi-terrestrial habitats. The genus 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 . In Japan, reports on were scarce, with only one species having been documented 40 years ago at Ishikari Beach in Hokkaido by the name of . We revisited these earlier findings and uncovered the presence of two distinct species in . One of these species is herein named sp. nov., while the other corresponds to , originally described from the Russian Far East. sp. nov. possesses a chaetal pattern similar to that of , and 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.