An adult male specimen of Argulus coregoni Thorell, 1864 was collected from the body surface of a dark chub Nipponocypris temminckii (Temminck and Schlegel, 1846) at 15 m elevation in a small stream in central Japan. The specimen collected is herein described and corresponds well to the descriptions of A. coregoni from European and East Asian countries. This represents the first record for A. coregoni from a fish of the cypriniform family Xenocyprididae in Japan, where this species usually occurs in higher-elevation mountain streams and infects fishes in two salmoniform families (Salmonidae and Plecoglossidae). The male of A. coregoni is characterized by the presence of two protrusions adorned with small spines and a digitiform projection on the ventro- and dorsoposterior margins, respectively, of the coxa of the second leg and the abdominal lobes have pointed posterior ends. In addition to these morphological characters, the number of plumose setae on the posterior margin of the coxa of the first leg and the number of supporting rods in the sucker membrane of the first maxilla are useful for distinguishing A. coregoni from a morphologically similar congeneric species, A. japonicus Thiele, 1900, which parasitizes cypriniform fishes in Japan. Based on literature published between 1936 and 2023, this paper also gives a list of the hosts of A. coregoni reported from East Asia, including the Russian Far East, China, Malaysia, and Japan. To date, 31 species and three subspecies of fishes are known as hosts of this parasite in East Asia, and these fishes belong to 16 families and eight orders, which indicates that the species is not a host-specific parasite. In order to further understand the host utilization of A. coregoni in Japan, it is necessary to study its occurrence on fishes of various taxonomic groups in rivers of different lengths.

A small amphipod, Curidia japonica sp. nov., is described from Mie and Wakayama Prefectures, central Japan. The discovery is the first record of the family Ochlesidae from the Northwest Pacific. This new species can be clearly distinguished from its congeners by the very short posterodistal process of the antenna 2 peduncular article 4. A key to the species of Curidia Thomas, 1983 in the world is provided.

Japanese Mantispidae are revised, redescribed and illustrated with details of male and female terminalia. The following seven species are recognised: Austroclimaciella quadrituberculata (Westwood, 1852); Euclimacia badia Okamoto, 1910; Eumantispa harmandi (Navás, 1909); Tuberonotha strenua (Gerstaecker, 1894); Necyla shirozui (Nakahara, 1961); Mantispilla japonica (McLachlan, 1875); and M. transversa Stitz, 1913. In addition, we propose synonymising A. habutsuella (Okamoto, 1910) and A. subfusca (Nakahara, 1912) with A. quadrituberculata and M. j. diminuta Matsumura, 1907 with M. japonica.

The tubulanid palaeonemertean Parahubrechtia rayi Chernyshev, Polyakova, and Sun, 2022 has been only known from the type locality (4–6 m depth) in the Sea of Japan, Peter the Great Bay, Russia. Our nemertean faunal surveys at shallow subtidal zones yielded a couple of anterior body fragments of whitish tubulanids, each from i) Moroiso Cove (6 m depth) in Sagami Bay, middle Honshu, and ii) Akkeshi Bay (4 m depth), eastern Hokkaido. Partial 658-bp sequences of the mitochondrial cytochrome c oxidase subunit I gene from these specimens differed by up to 1.2% p-distance from four sequences of P. rayi from the type locality. This expands the species’ known distribution by ~1000 km eastward and southeastward to the Pacific coasts of Japan.

Leeches of the predatory genus Whitmania Blanchard, 1888 were collected from Yonaguni Island in the Ryukyu Islands, Japan, and are identified as W. laevis (Baird, 1869). This represents the first record of this species from the Ryukyu Islands. Molecular phylogenetic analyses, which were performed using mitochondrial cytochrome c oxidase subunit I, tRNA^Cys, tRNA^Met, 12S rRNA, tRNA^Val, and 16S rRNA markers, reveal that the Yonaguni population of W. laevis is genetically close to the Taiwanese populations. The mitochondrial phylogeny also shows that W. laevis is a sister species of W. edentula (Whitman, 1886), which is indigenous to the Japanese Archipelago.

Between 1992 and 2016, 11 specimens (31.1–112.8 mm standard length) of Corythoichthys intestinalis (Ramsay, 1881), collected from Amami-oshima island and Ie-jima island, Ryukyu Islands, southern Japan, were identified from the following combination of characters: superior trunk ridge discontinuous with superior tail ridge; lateral trunk ridge straight, ending near anal ring; inferior trunk ridge and tail ridges continuous; 15–16 trunk rings; 49–51 total rings; 26–30 dorsal-fin rays; posterior tail rings with prominent dark markings on dorsum; anal ring without a dark blotch ventrally; and dorsal fin without spots. The species was previously recorded from eastern Borneo to Samoa and Tonga but not from Japan. Hence, the present specimens represent the first Japanese and northernmost records of C. intestinalis. The new standard Japanese name, “Ryukyu-ishiyoji,” is proposed for the species.

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.

Two genera of the poorly known family Cetomimidae (flabby whalefishes), Cetomimus Goode and Bean, 1895 and Gyrinomimus Parr, 1934, are morphologically similar but differ in jaw tooth row (indistinct diagonal rows in Cetomimus vs. distinct longitudinal rows in Gyrinomimus) and vomerine tooth plate morphology (domed, round or rarely oval vs. flat, rectangular or oval). Three examples of these genera, recently collected off the Pacific coast of Honshu Island, Japan, were respectively identified as Cetomimus hempeli Maul, 1969 (two specimens; characterized by a rounded and broad head, a reduced fourth gill arch and small slit between the third and fourth arches, and cavernous tissue present around the anus, 1–3 anal-fin rays, and dorsal fin origin, but absent from the caudal peduncle) and Gyrinomimus bruuni Rofen, 1959 (single specimen; characterized by 20 dorsal- and anal-fin rays, three or four jaw tooth rows, the dorsal-fin base length 18.2% of the standard length, the anal-fin base with lappets, and the lateral-line pores without flaps). Previously, C. hempeli had been known only from the Atlantic Ocean, the present specimens are the first reliable records from the Pacific Ocean, and the specimen collected off the Fukushima Prefecture coast is the northernmost record of the species. The specimen of G. bruuni also represents the first reliable record of this species from Japan, and the northernmost record of the species. New standard Japanese names are proposed for both species, “Seiun-kujirauo” for C. hempeli, and “Hitaguro-kujirauo” for G. bruuni.

“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.