Molecular Phylogeny and Morphology of <i>Culeolus tenuis</i> and <i>C. uschakovi</i> (Urochordata: Ascidiacea) Collected near the Chishima-Kamchatka Trench, Southeastern Hokkaido, Japan, with Special Reference to Significant D

Molecular analyses of 18 specimens of Culeolus Herdman, 1881 collected in 2022 and 2023 near the southwestern limit of the Chishima-Kamchatka (Kuril) Trench, southeastern Hokkaido, between ca. 6100 and 6200 m depth, revealed two morphologically distinct clusters, each distinguishable from the other by clear differences in the Culeolus-ridge structure on the tunic surface around the atrial aperture (viz., a continuous crest vs. a ring of conical projections). Examination of the syntypes of the deep-water NW Pacific species C. tenuis Vinogradova, 1970, characterized by some structural variations, resulted in designation of a lectotype characterized as having a continuous crest, conspecific with the former crest-furnished specimens. The cone-furnished specimens were referred to C. uschakovi Redikorzev, 1941, characterized by intraspecific variations in the completeness of the projection ring, gonadal number and position, and number of branchial folds. Culeolus tenuis was compared with other crest-furnished congeners, and C. uschakovi with other cone-furnished ones, resulting in a better understanding of the taxonomy of the genus.