Phylogeography of benthic invertebrates in deep waters: A case study of Sternaspis cf. williamsae (Annelida: Sternaspidae) from the northwestern Pacific Ocean

Abstract Although the deep sea is a vast ecosystem, gene connectivity is not well understood in the northwestern Pacific Ocean, especially around the Okhotsk Sea. The Okhotsk Sea and the neighboring Pacific Ocean together are attractive regions to study the population connectivity of deep-sea organisms because these regions are connected through shallow straits except for the Bussol (ca. 2300 m) and Kruzenshtern (ca. 1900 m) Straits. Members of the family Sternaspidae (Annelida) inhabit various water depths ranging from intertidal to abyssal zones. Sternaspids are, therefore, interesting subjects for studying the evolutionary shifts in bathymetric distribution and phylogeography in deep waters. Sternaspid worms were collected from the deepest part of the Okhotsk Sea (the Kuril Basin) and the northwestern Pacific Ocean (lower bathyal and abyssal depths of the Kuril-Kamchatka Trench, off northeastern and central Japan). This molecular phylogenetic study, which is the first focusing on the Sternaspidae, suggests the presence of many cryptic species that were collected far from the type locality. Mainly based on shield morphology and molecular phylogenetic analysis using the mitochondrial COI gene, we found a wide distribution of Sternaspis cf. williamsae over the northwestern Pacific Ocean and a different species of the sternaspid Caulleryaspis cf. nuda in the Okhotsk Sea. The isolation by distance (IBD) test based on the mitochondrial COI gene and nuclear ITS2 region suggests moderate IBD for S . cf. williamsae over 1500 km from Kuril to central Japan.