A redescription of (Ascidiacea, Stolidobranchia) with an inference of its phylogenetic position within Styelidae.

Two species of styelid colonial ascidians in the genus Redikorzev, 1913 are known from the northwest Pacific. The valid status of the lesser known species, (Tokioka, 1951) (type locality: Akkeshi, Japan), is assessed here. To assess the taxonomic identity of , we compared one of the syntypes and freshly collected topotypes of with a syntype of Redikorzev, 1913 (type locality: Ul'banskij Bay, Russia). Specimens of consistently differed from the syntype of in the number of oral tentacles, the number of size-classes of transverse vessels, and the number of anal lobes. In this paper, is redescribed as distinct from , and its phylogenetic position inferred within Styelidae based on the 18S rRNA and cytochrome oxidase subunit I gene sequences. In our phylogenetic tree, formed a well-supported clade together with MacLeay, 1824. In and , a single gonad is situated on the right side of the body, which is unique among Styelidae, and thus can be a synapomorphy for this clade.