Molecular and Morphological Assessment of Juvenile and Adult Forms in the Giant Worm Cf. (Annelida: Eunicidae) and Its Phylogenetic Position in the Family.

(Pallas, 1788) is a large polychaete worm (up to 3 m in length) and the type species of the genus. In Japan, a similar but potentially different species, cf. , is distributed mainly in the rocky shores of the temperate and warm Pacific coasts. Juveniles and adults were suggested to be distinguished by their body color. The juvenile form was previously regarded as distinct species, Izuka, 1912 and Fauvel, 1936, although they are now considered synonymous with . . In this study, we revisited the validity of the present taxonomy based on morphological observations including SEM and microCT, and three molecular markers (cytochrome oxidase subunit I [COI], 16S rRNA, and histone H3 genes) and investigated the phylogenetic position of cf. in the family Eunicidae using the combined dataset of three genes (COI + 16S rRNA + 18S rRNA). The adult and juvenile forms were different in body size, color, the distribution of the branchiae and subacicular hooks, and maxillae shape, but not in other characteristics. One individual showed an intermediate body color between the two forms. The adult and juvenile forms shared major haplotypes and the maximum K2P genetic distance of COI was 1.7%, which can be considered within intraspecific variation. In the phylogenetic tree based on the combined gene dataset, cf. was closely related to Quatrefages, 1866 and cf. Ehlers, 1887, which are large species from the Mediterranean Sea and the Caribbean Sea, respectively.