Bigelowina phalangium (Fabricius, 1798) is the only representative of the genus Bigelowina Schotte and Manning, 1993 known from the Indo-West Pacific region. In Japanese waters, this species has been reported from the littoral environments of the Ryukyu Islands to Honshu Island. Our morphological and phylogenetic analyses of specimens of Bigelowina from Japan found two allopatric species, one from the Ryukyu Islands and another from Kyushu to Honshu Islands. The Ryukyuan species is morphologically closest to B. phalangium, with slight differences in the morphology of the raptorial claw and the color pattern of the telson. In contrast, another species from Kyushu to Honshu Islands is here described as a new species, B. komaii sp. nov. The field survey conducted in this study suggests that the host of two commensal mollusk species previously reported from the burrow of species of Bigelowina at Nakatsu, Kyushu Island, is B. komaii sp. nov., not B. phalangium. An identification key to species of Bigelowina is also provided.

“Cirriformia tentaculata” is considered to be distributed throughout Japanese waters and is used as an indicator species of organic pollution in coastal regions. However, previous studies indicated that “C. tentaculata” from Japan contained cryptic or sibling species. In order to solve this taxonomic problem, we collected specimens of Cirriformia Hartman, 1936 from throughout Japan and conducted taxonomic analyses. Molecular phylogenetic analyses showed that the Cirriformia worms collected could be divided into twelve phylogroups. We conducted detailed morphological analyses of each phylogroup and found morphological differences between each. Ten species were considered new to science, and the remaining two had the morphology according to the original description of C. tentaculata (Montagu, 1808). Physio-chemical characteristics of their habitats were not identical between each species, which suggested that the so-called “C. tentaculata” in Japanese waters is not adequate as a pollutant indicator.

A new species of the genus Nesometra Virgili, Poliseno, Fujita, Pratama, Fernández-Silva, and Reimer, 2023 is described based on the specimens collected from the Amami Islands and Ogasawara Islands, Japan and Cunningham Island, Australia. Nesometra integra n. sp. is distinguished from its single congener, N. sesokonis (Obuchi, Kogo, and Fujita, 2009), by having complete proximal pinnulation and cirri with reduced medial constriction on its distal part.