Rhizorhina hamatipedicola n. sp. (Copepoda: Siphonostomatoida: Nicothoidae) parasitic on a tanaidacean crustacean, with a note on the effectiveness of COI and 18S data in Rhizorhina taxonomy.

A new nicothoid copepod species Rhizorhina hamatipedicola n. sp. is described, parasitic on the tanaidacean Hamatipeda kohtsukai Kakui & Hiruta (Hamatipedidae) collected at depths of 360-375 m in Sagami Bay, Japan, in the northwestern Pacific. It closely resembles Rhizorhina aesthetes Boxshall & Harrison, 1988 parasitic on an ischnomesid isopod collected at a depth of about 2910 m in the northeastern Atlantic, and Rhizorhina ohtsukai Kakui, 2016 parasitic on the leptocheliid tanaidacean Chondrochelia sublitoralis Sato, Arakawa, & Kakui collected at depths of 151-136 m in the northwestern Pacific. Although there are only minor morphological differences between these Rhizorhina species, their hosts differ at the level of order or family. The known geographical and depth distributions of R. hamatipedicola differ greatly from those of R. aesthetes. A partial nucleotide sequence of the 18S rRNA (18S) gene from R. hamatipedicola was identical to that from R. ohtsukai, but the two species differed by 13.4% and 14.9% p- and K2P distances, respectively, for the cytochrome c oxidase subunit I (COI) gene, which were much greater than intraspecific distances previously reported in Siphonostomatoida. This study suggests that an integrative approach combining morphology, sequences of COI or other genes evolving faster than 18S, and host data is necessary to reliably distinguish among morphologically similar Rhizorhina species.