Diplocirrus nicolaji (Annelida: Flabelligeridae) from Japan, detailed morphological observation and DNA barcoding

Diplocirrus nicolaji is a flabelligerid worm originally described from shallow waters of Peter the Great Bay, Sea of Japan. Later, this species was also recorded from Japan, but detailed data were not provided; thus the occurrence of D. nicolaji in Japan needs to be confirmed based upon additional material. The collection of 20 D. nicolaji individuals from four localities along Japanese coasts (the Sea of Japan and western Pacific Ocean), allowed us to provide detailed morphological observations using stereoscopic and scanning electron microscopy. Partial cytochrome c oxidase subunit I sequences were obtained for phylogenetic analysis. Although the morphological analysis detected a few variations in palp length and body colour in ethanol among the local populations, the phylogenetic analysis confirmed their conspecificity with little genetic divergence. This is the first report of D. nicolaji from the western Pacific Ocean and extends its distribution southward.