Ageishi, Kentaro; Kajihara, Hiroshi. (2026). Systematic revision of the genus Thoracophelia (Annelida: Opheliidae) from Japan. Zootaxa. 5853(4): 528-544.
Up to five species of the polychaete genus Thoracophelia Ehlers, 1897 have been reported from Japanese waters: T. arctica (Grube, 1866); T. ezoensis Okuda, 1936; T. japonica (Misaka & Sato, 2003); T. minuta Jimi, Fujimoto, Takehara & Imura, 2021; and T. yasudai Okuda, 1934. Our systematic revision reduces this number to three valid species, invalidating T. minuta and T. yasudai. We conclude that T. minuta is a junior synonym of Euzonus arcticus Grube, 1866 (now T. arctica), based on identical DNA barcode sequences. Morphological observations support the previously proposed—but not universally accepted—synonymy of T. yasudai with E. arcticus. Thoracophelia arctica may possess black rod-shaped spicules suspended in the coelomic fluid, which likely disappear either during ontogeny or under specific physiological conditions. In T. ezoensis, molecularly identified specimens exhibited a wider range of intraspecific morphological variation than previously recognized, particularly in the numbers of branchial pairs and anal cirri. Energy-dispersive X-ray spectroscopy coupled with scanning electron microscopy revealed that the black rod-shaped spicules in T. japonica are rich in calcium and magnesium, distinct from the calcium- and sulfur-rich spicules previously reported for T. arctica (as T. minuta). A molecular phylogenetic analysis of the subfamily Opheliinae based on four gene markers (16S rRNA, cytochrome c oxidase subunit I, 18S rRNA, and 28S rRNA) recovered the monophyly of Thoracophelia, albeit with moderate branch support. The phylogeny suggests that the capacity for spicule formation may have arisen early in the evolution of Opheliinae but was subsequently lost in some lineages or expressed conditionally depending on ontogeny or physiology
