Capitella teleta Blake et al., 2009 is an opportunistic capitellid originally described from Massachusetts (USA), but also reported from the Mediterranean, NW Atlantic, and North Pacific, including Japan. This putatively wide distribution had not been tested with DNA sequence data; intraspecific variation in morphological characters diagnostic for the species had not been assessed with specimens from non-type localities, and the species status of the Japanese population(s) was uncertain. We examined the morphology and mitochondrial COI (cytochrome c oxidase subunit I) gene sequences of Capitella specimens from two localities (Ainan and Gamo) in Japan. Specimens from Ainan and Gamo differed from C. teleta from Massachusetts in methyl-green staining pattern, shape of the genital spines, and shape of the capillary chaetae; we concluded that these characters vary intraspecifically. Species delimitation analyses of COI sequences suggested that worms from Ainan and Massachusetts represent C. teleta; these populations share a COI haplotype. The specimens from Gamo may represent a distinct species and comprise a sister group to C. teleta s. str.; we refer to the Gamo population as Capitella aff. teleta. The average Kimura 2-parameter (K2P) distance between C. teleta s. str. and C. aff. teleta was 3.7%. The COI data indicate that C. teleta actually occurs in both the NW Atlantic and NW Pacific. Given the short planktonic larval duration of C. teleta, this broad distribution may have resulted from anthropogenic dispersal.

The morphology of two Pseudopolydora species, P. cf. reticulata Radashevsky and Hsieh, 2000 and P. achaeta Radashevsky and Hsieh, 2000 are reported from Japan for the first time. Pseudopolydora cf. reticulata was collected from Japanese tidal flats, and individuals possess the characteristic netlike pigmentation on the dorsum of anterior chaetigers and the longitudinal black band along midline of caruncle. Pseudopolydora achaeta was collected from subtidal bottom mud of Onagawa Bay, and individuals have distinctive characteristics, such as intensive black pigmentation on dorsal and ventral sides of the anterior body and nearly straight vertical rows of major spines on the fifth chaetiger. The morphology of P. cf. reticulata is very similar to that of P. cf. kempi, with which it had been confused in Japan. We analyzed the 18S and 28S rRNA gene sequences of all five Pseudopolydora species recorded from Japan and found strong evidence that they are genetically distinct. Our analysis also suggests that boring polydorids have evolved among non-boring ones; the genus Pseudopolydora, which mostly shows the non-boring form, appears to have remained in a more ancestral condition.

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