An oreosomatid of the genus Allocyttus McCulloch, 1914 is fished commercially on the Emperor Seamounts. However, the species’ identity is uncertain, as is the taxonomy of the oreosomatid species of the seas around Japan, where the names Allocyttus verrucosus Gilchrist, 1906 (type locality: off South Africa) and A. folletti Myers, 1960 (type locality: off California) have both been used. From its anticipated susceptibility to over-exploitation, it is urgent to establish the correct taxonomic identity to facilitate effective management measures. Meristics, morphometrics and scale characters of the specimens from the Emperor Seamounts and Japan agreed well with data of the holotype of A. folletti and differed from those of A. verrucosus, confirming that those specimens represent A. folletti. Oreosomatids reported from the western North Pacific in the literature were identified as A. folletti. From the data of the present study and historical references, A. folletti is thought to be distinguished from A. verrucosus by the following characters: more dorsal- and anal- spines+rays (36–42 vs. 33–38 and 31–35 vs. 27–33 respectively), more total vertebrae (37–41 vs. 34–38), greater numbers of enlarged scales of dorsal- (S-DFB) and anal-fin base (S-AFB) (31–42 vs. 26–31, and 29–37 vs. 25–28 respectively), more spines on the margin of S-DFB and S-AFB (up to 7–12 vs. 3–6), a shorter preanal-fin length (53.8–63.6% vs. 64.8–83.7% of SL), a longer caudal peduncle (10.4–15.6% vs. 6.1–10.2% of SL), a shorter head (32.9–40.4% vs. 38.5–48.4% of SL), and cycloid scales on the mid-side of body (vs. ctenoid). Available data indicate that A. folletti reaches up to 537 mm SL, larger than A. verrucosus (up to ca. 325 mm SL). From the anticipated slow growth and longevity, concern is raised regarding the susceptibility of A. folletti to over-exploitation.

Seven specimens (149.0–221.2 mm standard length) of Pristipomoides amoenus (Snyder, 1911), previously known only from Okinawa-jima and Ishigaki-jima islands, southern Ryukyu Islands, Okinawa Prefecture, Japan, were collected from other regions of Japan (Amami-oshima island, Amami Islands, Kagoshima Prefecture), Taiwan (Dong-gang, Pingtung), the Philippines (Iloilo, Panay Island), and Fiji (Viti Levu Island), thereby representing the first records of the species from outside Okinawa Prefecture. The Amami-oshima and Fijian specimens also represent the northernmost and first Southern Hemisphere records, respectively, for the species. Comparison of these plus newly collected specimens from the southern Ryukyu Islands (herein described in detail) with the closely related species Pristipomoides argyrogrammicus (Valenciennes in Cuvier and Valenciennes, 1832) revealed the following hitherto unrecognized diagnostic color features of P. amoenus: a few small silvery-blue blotches present inside yellow saddles on dorsum; small silvery-blue blotches below trunk lateral line absent or indistinct; a distinct line (formed by small silvery-blue blotches) absent on lower caudal peduncle; a large silvery-blue blotch present on upper opercle, extending anteriorly beyond preopercular margin; a line formed by small silvery-blue blotches on upper caudal peduncle ending at upper caudal-fin base; a pair of lines formed by small silvery-blue blotches along dorsal-fin base (dorsal view); a larger pair of elliptical silvery-blue blotches on occipital region (all silvery-blue blotches retained as dark-brown blotches after preservation). Pristipomoides argyrogrammicus is newly recorded from the Tokara Islands, northern Ryukyu Islands, Japan.

The three heteronemertean nominal species Ascaris longissima Gunnerus, 1770, Borlasia angliae Oken, 1815, and Nemertes borlasii Cuvier, 1816 have been considered as synonymous, denoting the same species to which the valid name Lineus longissimus (Gunnerus, 1770) has been applied. These three nominal species are the type species of the genus-group names Lineus Sowerby, 1806, Borlasia Oken, 1815, and Nemertes Cuver, 1816, respectively, which are in turn the type genera of the family-group names Lineidae McIntosh, 1874, Borlasiidae Diesing, 1862, and Nemertidae Ehrenberg, 1831. Therefore, Lineus Sowerby, 1806 (currently in use) is a senior subjective synonym of Borlasia Oken, 1815 and Nemertes Cuvier, 1816 (both currently not in use), while Lineidae McIntosh, 1874 (currently in use) is a junior subjective synonym of Borlasiidae Diesing, 1862 and Nemertidae Ehrenberg, 1831 (both currently not in use); in addition, Micruridae Ehrenberg, 1831 (not in use) is also a senior synonym of Lineidae McIntosh, 1874. Borlasiidae Diesing, 1862 and Nemertidae Ehrenberg, 1831 have not been used as valid after 1899, while Lineidae McIntosh, 1874 has been used in at least 216 works published by 437 authors since 1971 until 2021, thus satisfying the conditions stipulated in Article 23.9.1 of the International Code of Zoological Nomenclature. Borlasiidae Diesing, 1862 and Nemertidae Ehrenberg, 1831 are herein declared nomina oblita with respect to Lineidae McIntosh, 1874, the latter being regarded as a nomen protectum under Article 23.9.2 of the International Code of Zoological Nomenclature. However, reversed precedence of Lineidae McIntosh, 1874 over its senior synonym Micruridae Ehrenberg, 1831 requires a ruling by the International Commission on Zoological Nomenclature, as the latter name was used as valid between 1998 and 2009

Three specimens of the White Sardine (Clupeiformes: Clupeidae) Sardinella albella (Valenciennes, 1847), collected from Nakagusuku Bay, Okinawa Island, Ryukyu Archipelago, Japan in the early 20th century and recently discovered in the fish collection of the Department of Zoology, The University Museum, The University of Tokyo, represent the first records of the species (and seventh confirmed species of Sardinella Valenciennes, 1847) from Japanese waters. The morphology of the specimens is described in detail, and a diagnostic key provided for Japanese species of Sardinella.

Twenty-four specimens (184.6–243.6 mm in standard length) of Monotaxis Anonymous [Bennett], 1830, collected from the Ryukyu Islands, southern Japan, were identified as M. heterodon (Bleeker, 1854) (Perciformes: Lethrinidae). Although a widely distributed Indo-West Pacific species, all previous records from Japanese waters have been based solely on photographs, the present specimens therefore representing the first specimen-based records of M. heterodon from Japan. A detailed comparison between M. heterodon and its only congener, M. grandoculis (Forsskål, 1775), from which it has previously been distinguished primarily by scale rows below the lateral line and fresh or live coloration, revealed new diagnostic characters, most of which are applicable to preserved specimens. In addition to previously recognized characters, the two species can be distinguished by mid-dorsal snout profile in adults (concave vs. straight), snout length (excluding lips) [8.9–11.1 (mean 9.9) % of SL vs. 10.3–12.1 (11.2) % SL], spinous anal-fin base length [4.6–5.9 (5.0) % of SL vs. 3.9–4.9 (4.4) % SL], a distinct black blotch above the pupil (absent vs. present), a dark brown stripe across the post interorbital region in preserved specimens (present vs. absent), and preserved coloration of the pectoral-fin base inner surface (blackish brown vs. light brown).

The taxonomic status of the two Japanese species of the spider genus Cybaeus L. Koch, 1868, viz., C. communis Yaginuma, 1972 and C. maculosus Yaginuma, 1972, is revisited on the basis of male and female specimens, which were collected from each type locality and the adjacent areas, along with the holotypes of both species. Mitochondrial cytochrome c oxidase subunit I and nuclear internal transcribed spacer 1 sequence data confirmed that C. communis and C. maculosus are conspecific, and C. maculosus is synonymized with C. communis. The obtained molecular phylogenies corroborate the monophyly of C. communis, C. kirigaminensis Komatsu, 1963, C. shinkaii (Komatsu, 1970), and C. daimonji Matsuda, Ihara, and Nakano, 2020.

The epistylidid ciliate Epistylis wuhanensis Wang, Zhou, Guo, and Gu, 2017 was found attached to the body surface of Lernaea cyprinacea Linnaeus, 1758 (Copepoda: Lernaeidae) parasitizing Rhinogobius similis Gill, 1859 (Perciformes: Gobiidae) from the Arida River in Wakayama Prefecture, Japan. This epistylidid was described based on live ciliates, stained specimens, and scanning electron microscope observation with molecular information. This is the first record of E. wuhanensis from Japan and the second record of the epistylidid attached to metazoans. A list of records of Epistylis Ehrenberg, 1830 species attached to metazoans in Japan is proposed.

The authorships of 14 species-group names of chilopods introduced by Yoshioki Takakuwa in 1934 are clarified. Four names, viz., Dicellophilus latifrons Takakuwa, 1934, Tygarrup moiwaensis Takakuwa, 1934, Prolamnonyx obtusus Takakuwa, 1934, and P. dentatus Takakuwa, 1934 should be attributed to Takakuwa’s work published in Japanese in April 1934 in Volume 2, Issue No. 4 of the journal Shokubutsu oyobi Dobutsu (=Botany and Zoology). The specific names of two other Mecistocephalus species, viz., M. ongi Takakuwa, 1934 and M. brevisternalis Takakuwa, 1934, should be attributed to the Takakuwa’s work published in German in April 1934, in Volume 14, Issue No. 3 of the journal Annotationes Zoologicae Japonenses. The correct attributions of six more names, viz., Geophilus monoporus Takakuwa, 1934, Nesogeophilus kozuensis Takakuwa, 1934, N. tiosianus Takakuwa, 1934, N. littoralis Takakuwa, 1934, Thalthybius tenuicollis Takakuwa, 1934, and Cryptops japonicus Takakuwa, 1934, is to Takakuwa’s work published in Japanese in September 1934, in Volume 46, Issue No. 551 of the journal Dobutsugaku Zasshi. In addition to the 14 species-group names, the correct attribution of Mecistocephalus takakuwai, which has traditionally been attributed to the work by Karl W. Verhoeff published in November 1934, is clarified. This species was described by Takakuwa in the work published in April 1934, but nonetheless, it is concluded that its correct attribution is to Takakuwa’s work published in Japanese in October 1932 in Volume 30, Issue No. 47 of the journal Hakubutsugaku Zasshi. Synonymies showing earlier authors’ attributions are given for each of these species, and, when possible, the composition of the original type series of each of these species is inferred. In 1955 some of the type material was still said to exist, but by now all of Takakuwa’s specimens of these species seem to have been lost.

Gemmaecaputia corrugata Tripathi, 1959 (Monogenea: Chauhaneidae), the type-species of Gemmaecaputia Tripathi, 1959, is redescribed from the gills of Sphyraena forsteri Cuvier, 1829 (Sphyraenidae) off Yomitan Town, Okinawa-jima island, Southern Japan, as a new country record. Sphyraena forsteri represents a new host record for this monogenean. The cox1 phylogenetic tree showed that G. corrugata and Pseudochauhanea macrorchis Lin, Liu, and Zhang, 2001 (Chauhaneidae) formed a sister group, but the 28S rDNA phylogenetic tree indicates that G. corrugata is nested in a clade comprised of Chauhaneidae and Protomicrocotylidae.

Perlomyia gakken sp. nov. (Plecoptera: Leuctridae) from Kyoto Prefecture, Honshu, Japan, is described and illustrated. This new species is distinguished from all other Perlomyia species by having a remarkably long and slender epiproct with a crochet hook shape at tip. Although, this new species is most similar to P. kappa Sivec and Stark, 2012, but can be discriminated from P. kappa by the three morphological differences: (1) cercus with small spine-like apex (long terminal spine in P. kappa), (2) epiproct slightly swollen at middle in lateral aspect (swollen basally in P. kappa), and (3) epiproct like a crochet hook shape apically in lateral aspect (expanded apically in lateral aspect in P. kappa). A checklist and type localities map for 25 currently recognised Perlomyia species are also presented.

Pristina multiseta sp. nov. is described from rice paddies in Japan. This species resembles P. aequiseta Bourne, 1891 and P. evelinae Marcus, 1943 in having proboscis and shortly bifid distal ends of dorsal needles, however P. multiseta is distinguished from these species by the short proboscis, a higher number of chaetae in dorsal bundles, and enlarged ventral chaetae from IV through VII. Thin vessels surrounding the ectal half of the atria in this species are unique in the genus.

Members of the family Flabelligeridae are distributed worldwide, inhabiting the underside of rocks or within mixed sand sediments. Some genera of the family have gelatinous tunics with hooked chaetae. The genus Flabelligera Sars, 1829 is one of the members of the gelatinous free-living groups of Flabelligeridae. A fan-shaped cephalic cage, thick gelatinous tunic, and hooked neurohooks distinguish members of this genus. In Japan, Flabelligera affinis Sars, 1829 [Japanese name: Kanten-habouki] has been known as the sole Japanese species of Flabelligera inhabiting several locations. We reviewed Japanese species of Flabelligera from several sites from Hokkaido to Honshu and concluded that the Japanese “F. affinis” contained at least five different species under the specific name. In this study, we described them as five new species. All new species were collected from the underside of rocks in shallow to deep-sea depths. We have also provided mitochondrial cytochrome c oxidase subunit I sequences for DNA barcodes of the new species.

Two new species of the aorid Amphipoda are described from Chichijima Island, the Ogasawara Islands in Japan. Aoroides macrops sp. nov. from shallow water is characterized by the heavily setose male gnathopod 1, the coxa of which bears several plumose setae anteriorly, and the uropod 2 with a short inter-ramal process. This species is very similar to A. longimerus Ren and Zheng, 1996; however, this new species can be distinguished from A. longimerus by the larger eyes, the shorter carpus of the male gnathopod 1, and the shorter carpi of the pereopods 3 and 4. Grandidierella ogasawarensis sp. nov. from river mouth has the male gnathopod 1 with two teeth on the carpus and an anterodistal projection on the propodus, the male gnathopod 2 with an elongate carpus, and the uropod 1 bearing an inter-ramal process. This new species is different from its closely related congeners (G. koa Barnard, 1977; G. insulae Myers, 1981; and G. rubroantennata Ariyama and Taru, 2017) in the separated teeth on the carpus of the male gnathopod 1.

Twenty specimens of Acentrogobius ocyurus (Jordan and Seale, 1907) were collected in Manko, Okinawa-jima Island, Japan and Sulu Sea side of Puerto Princesa City, Palawan Island, Philippines. As the species had previously been reported from the South China Sea, Andaman Sea, and northern Australian waters, these specimens from Okinawa and Palawan represent the first records of the species from Japan and the Sulu Sea. Acentrogobius ocyurus was found on muddy substrata at the lowest tidal zone of the mudflat and was often found in burrows of mangrove pistol shrimp, Alpheus richardsoni Yaldwyn, 1971. Morphological traits suggested that A. ocyurus is closely related to A. globiceps (Hora, 1923), A. kranjiensis (Herre, 1940), and Acentrogobius sp. (“Suzume-haze” in Japanese name), with several homologous features including, the transverse pattern of the sensory papillae on the cheek, caudal fin pointed in middle, a black stripe behind the eye along the anterior oculoscapular canal, bright blue spots laterally on the body, and an oblique yellow stripe across the upper part of the caudal fin.

Three infaunal apodid sea cucumbers collected in Kushimoto, Shirahama, and Susami, in the southern coast of Wakayama, Japan are described. They have different morphological characteristics in body color, ossicle arrangement, and internal organs, each other. First species, animal displays a large body size (approximately 70–150 mm) and pale body color (semi-transparent pink with anterior tip brownish) was found out. Second species, animal displays a small body size (approximately 20 mm) and pale body color (transparent yellow) with whitish wheel-papillae on three dorsal interradii was found out. Third species, animal displays a moderate body size (approximately 20–35 mm) and deep red or reddish brown body color with dark yellowish wheel-papillae on three dorsal interradii was found out. All three species having 10 tentacles, sigmoid-hook ossicles, and wheel ossicles in their body wall, turned out to be belonging to the genus Taeniogyrus Semper, 1867, and new to science.

Argulus coregoni Thorell, 1864 was collected from white-spotted char, Salvelinus leucomaenis (Pallas, 1814), red-spotted masu salmon, Oncorhynchus masou ishikawae Jordan and McGregor, 1925, masu salmon, O. m. masou (Brevoort, 1856), and a hybrid between white-spotted char and masu salmon in mountain streams, Gifu Prefecture, central Japan. The host’s body surface under and near the pectoral fins was the most common infection site for A. coregoni. The infected white-spotted char were caught at high elevations (461–873 m) in the headwater streams. The infected individuals of the two masu salmon subspecies were mostly caught in the upper river reaches, but the elevations where they were caught were lower (237–733 m and 660–707 m, respectively) than those of the white-spotted char. Since A. coregoni is also known as a parasite of ayu, Plecoglossus altivelis altivelis (Temminck and Schlegel, 1846), in the middle and lower river reaches in the prefecture, this parasite utilizes different fish species as its hosts along a river course: white-spotted char and the two masu salmon subspecies serve as the hosts, respectively, in the headwater and the middle to lower sections of the upper reaches, but in the mid- and lower river reaches, ayu is the important host.

Both adult females and males of Argulus mongolianus Tokioka, 1939 were collected from the buccal cavity wall and body surface of freshwater fishes in Lake Izunuma and Lake Uchinuma, Miyagi Prefecture, northen Honshu, Japan. The original description of the species was insufficient, based on a single female from Inner Mongolia, China, and no further description has been made to date. Thus, this paper redescribes A. mongolianus based on the newly-collected specimens, including the first description of the male. This represents the first record of A. mongolianus from Japan. The infected fishes were largemouth bass, Micropterus salmoides (Lacepède, 1802), common carp, Cyprinus carpio Linnaeus, 1758, Japanese white crucian carp, Carassius cuvieri Temminck and Schlegel, 1846, and silver crucian carp, Carassius sp. The three species other than common carp are new host records for A. mongolianus. This parasite was most probably introduced with an unspecified freshwater fish from China into Japan.

Fourteen new species of dicyemid mesozoans are described from six sepiid species (Mollusca: Cephalopoda: Sepiida) and a sepiolid species (Mollusca: Cephalopoda: Sepiolida) collected in the Kumano Sea, off the Kii Peninsula, Pacific coast of Honshu, Japan: Dicyema bacterocephalum sp. nov., Dicyema gozaense sp. nov., and Pseudicyema anemophilum sp. nov. from Sepia kobiensis Hoyle, 1855; Dicyema conocephalum sp. nov. and Dicyema tenuipoeceum sp. nov. from Sepia tenuipes Sasaki, 1929; Dicyema hyalocephalum sp. nov. from Austrorossia bipapillata (Sasaki, 1920); Dicyema lorigeroeceum sp. nov., Dicyema tympanocephalum sp. nov., and Pseudicyema cuplacephalum sp. nov. from Sepia lorigera Wülker, 1910; Dicyema miense sp. nov. and Pseudicyema jinshoae sp. nov. from Sepia subtenuipes Okutani and Horikawa, 1987; Pseudicyema daioense sp. nov. from Sepia aureomaculata Okutani and Horikawa, 1987; Dicyema shimaense sp. nov. and Pseudicyema physocaudatum sp. nov. from Sepia madokai Adam, 1939. The dicyemid fauna and their cooccurrence patterns of species are briefly discussed in relation to possible interaction of chromidinid ciliates.