The validity of Eudistoma okai Sanamyan, 2022, proposed as a new replacement name for E. parvum (Oka, 1927), as originally described in the binomen Distoma parva (incorrect original spelling; corrected as D. parvum) and thus a junior primary homonym of D. parvum Sluiter, 1900, is confirmed. The larval morphology (including 3 attachment processes, 4 pairs of lateral ampullae, and a single median ampulla between the ventral-most and middle processes) is revised, based on a re-examination of probable syntypes and other museum specimens of “E. parvum” from Misaki, as well as the examination of new material from the Pacific coast of Japan, and differs from a previous larval description, which lacked the median ampulla. In addition, re-examination of Japanese colonies previously identified as “E. parvum” showed many to have been mis-identified. A morphological comparison of E. okai with seven congeneric Japanese species is included.

Pseudorhombus elevatus Ogilby, 1912 and P. quinquocellatus Weber and de Beaufort, 1929 (Teleostei: Paralichthyidae) are newly recorded from Japan, based on 35 and two specimens, respectively, from Okinawa Island, Ryukyu Islands. Although both species have previously been recorded from Taiwan, no Japanese records are known to date. Pseudorhombus elevatus is characterized, and readily distinguished from Japanese congeners, by numerous minute teeth on both jaws, three dark blotches along the straight section of the ocular-side trunk lateral line, and dorsal- and anal-fin ray numbers. Pseudorhombus quinquocellatus resembles P. pentophthalmus Günther, 1862 and P. oculocirris Amaoka, 1969, all sharing five dark ocelli on the ocular-side body (two pairs of ocelli above and below the lateral line plus a single ocellus on the posterior portion of the lateral line). However, the former is characterized by strong caniniform teeth on both jaws (vs. no strong caniniform teeth in P. pentophthalmus and P. oculocirris); fewer lower gill rakers (9–11 in Japanese specimens) (vs. 15–21); and two dark skin flaps along the gill opening below the pectoral-fin base (vs. absent). The status of the type specimens of P. quinquocellatus was also considered, all three being regarded as syntypes, in the absence of a formal lectotype proposal. New standard Japanese names, “Maru-ganzō-birame” and “Niten-ganzō-birame,” are proposed for P. elevatus and P. quinquocellatus, respectively, the name “Itsutsume-ganzō” now being referred to P. megalops Fowler, 1934.

Henneguya postexilis Minchew, 1977 (Cnidaria: Myxobolidae) is described as a novel record for Japan. It was found in the gills of non-native Ictalurus punctatus (Rafinesque, 1818) (Siluriformes: Ictaluridae), which were collected from the Omoi River, a tributary of the Tone River system in Tochigi Prefecture, central Honshu. This myxozoan species is native to North America and its discovery from Japan in this study is the second case reported from a non-native region. Until now, H. postexilis has only been observed in I. punctatus, suggesting that it is an introduced alien species in Japan, likely accompanying its host.

Here we report the discovery of a single specimen of the eurysquillid mantis shrimp, Manningia andamanensis Ghosh, 1975, from Okinawa Island, southwestern Japan. This specimen is the first member of the superfamily Eurysquilloidea to be recorded from Japanese waters, and is the second record ever for this species. This new record significantly extends the range of M. andamanensis from the North Andaman Islands northeastward to Japan. Live coloration for this species is revealed in detail for the first time.

Two post-metamorphic adult females of the sphyriid copepod Lophoura cornuta (C. B. Wilson, 1919) were collected from Kaup’s arrowtooth eel Synaphobranchus kaupii Johnson, 1862 (Anguilliformes: Synaphobranchidae) at two deep-sea sites in the western North Pacific off the south coast of Hokkaido, northern Japan. This paper reports on the morphology of L. cornuta based on the adult female specimens collected as the second record of the species from Japan. One of the collection sites is near the type locality of the parasite, and the infected fish were captured at depths of 550 and 620 m on the continental slope. Although the type host of L. cornuta was reported as “Synaphobranchus affinis Günther, 1877”, this fish may be S. kaupii because the latter species predominates among synaphobranchid eels in the western North Pacific off Hokkaido. An identification key to the four nominal species of the genus Lophoura Kӧlliker in Gegenbaur, Kӧlliker, and Müller, 1853 reported from Japan is also provided.

Cirrodrilus makinoi (Yamaguchi, 1934) has not been identified since it was originally described from the Japanese crayfish, Cambaroides japonicus (De Haan, 1841) on Hokkaido Island, northern Japan. A critical morphological comparison of the original and new material of C. makinoi with those of closely related C. tsugarensis Gelder and Ohtaka, 2000, from Honshu Island, reveals there are no differentiating morphological character to separate the two species. A molecular study using the mitochondrial cytochrome c oxidase subunit I gene supports this view, therefore, C. tsugarensis becomes a subjective junior synonym of C. makinoi. Another species resembling C. makinoi, C. homodontus Yamaguchi, 1932 is confirmed to be a distinct species and both are redescribed based on the original and new material.

An unidentified colonial ascidian called gaikotsu-panda-hoya in Japanese, literally meaning ‘skeleton panda ascidian,’ has been attracting SCUBA divers’ attention for the past few years since its strange appearance was introduced on the Internet by a diving shop in Kumejima Island, Japan. To confirm the taxonomic status of this species, fresh samples were collected from a diving point off the coast of Kumejima Island. Our morphological examination revealed that they represent a new species, herein described as Clavelina ossipandae sp. nov., which can be distinguished from 44 congeners in the genus Clavelina Savigny, 1816 by the combination of the following seven characteristics: i) colony consisting of completely free zooids, ii) zooids up to 20 mm in length, iii) in the living state, zooids transparent, with laterally elongated white patch between oral and atrial siphons, as well as four black markings, one between siphons, one mid-dorsally, and the other two situated laterally in a pair on the anterior part of the body, iv) transverse vessels white, v) endostyle black, vi) 10–14 stigmatal rows, and vii) two longitudinal muscular bands running from the abdomen to the endostyle on each side. Partial sequences (810 bp) of the cytochrome c oxidase subunit I gene from the holotype and one of the paratypes differed at 10 sites from each other (1.26% K2P distance) but were the same when translated into amino acids. A phylogenetic tree supported that this species is included in the genus Clavelina.

We describe Cruranthura viridis sp. nov. based on four specimens collected from the subtidal zone near Irabu Island, Miyako Islands, Okinawa, Japan. This species differs from four congeneric species in having (1) small body size (2.53–3.38 mm), (2) moss green body color, (3) eyes, (4) pleonite 1 longer than pleonite 2, (5) telson length 2.06–2.33 times telson width, (6) propodus of pereopod 1 with 6–10 inner spiniform setae, and (7) the endopods of pleopods 2–5 uniarticulate, with plumose setae. We also determined the partial sequence of the cytochrome c oxidase subunit I (COI) gene from the holotype specimen. We discuss differences in piercing mouthparts among paranthurid genera.

A single specimen of a scaleless dragonfish in the genus Grammatostomias Goode and Bean, 1896 (Teleostei: Stomiiformes: Stomiidae) was collected off the Ogasawara Islands, Japan at a depth between 40–780 m. The specimen conformed to G. circularis Morrow, 1959 in having the following characters: dorsal-fin rays 21; anal-fin rays 23; pectoral-fin rays 9; the first pectoral-fin ray not isolated; and entire luminous loop on lateral body surface very thin, almost circular and without zigzags. The likelihood of the distribution pattern of small areas of luminous tissue on the head being diagnostic for each species of Grammatostomias is briefly considered. Grammatostomias circularis has previously been recorded only from the Atlantic, the present specimen representing the first record of the species from the Pacific Ocean, and first record of a species of Grammatostomias from Japan.

Four ovigerous females of the parasitic copepod Pseudosarcotretes omorii gen. et sp. nov. (Siphonostomatoida: Pennellidae) were found on North Pacific lightfish Maurolicus japonicus Ishikawa, 1915, collected from Suruga Bay, eastern Japan. Pseudosarcotretes gen. nov. shares some features with pennellids parasitic on mesopelagic fish such as Cardiodectes Wilson, 1917, Protosarcotretes Ohtsuka, Lindsay, and Izawa, 2018, and Sarcotretes Jungersen, 1911. The morphological differences between the new genus and other related genera are having a pair of lateral cephalothoracic holdfast, the presence of distinct caudal rami, and the absence of leg 4. The hatching stage of the new genus is the copepodid. A molecular phylogenetic analysis suggested that the reduction in the number of developmental stages in pennellid life cycles had occurred at least in two cases. According to the molecular data, the new genus consists of a clade accommodating Cardiodectes, Lernaeenicus Lesueur, 1824 and Pennella Oken, 1815. All these genera have a distinct cephalothoracic holdfast embedded into the host’s muscle tissues, which seems to be advantageous for expanding the range of the hosts to small mesopelagic fish.

The distributional range of Parascorpaena poseidon Chou and Liao, 2022, previously recorded only from Taiwan, is extended into the eastern Indian and western Pacific oceans (Japan, Philippines, Thailand, Indonesia, Sri Lanka, and India), on the basis of 52 specimens (35.3–123.3 mm standard length). A revised diagnosis for the species, determined from eastern Indian and western Pacific ocean specimens, is as follows: usually 16 or 17 (rarely 15) pectoral-fin rays; 43–49 scale rows in longitudinal series; 22–26 pored lateral-line scales; 6–8 (usually 7) scale rows between 6th dorsal-fin spine base and lateral line; 6–8 (usually 7) scale rows between last dorsal-fin spine base and lateral line; 4–6 + 7–9 + 1–3 = 14–17 gill rakers; penultimate dorsal-fin spine length 60.0%–83.3% of last dorsal-fin spine length; occipital pit fairly deep; suborbital bone with two ridges, origin of first posterior to origin of second, first with a single spine, second with two - all spines of equal size; supraocular tentacle absent or very short; base of dorsal-fin soft ray covered with small scales; head and body uniformly brown when fresh; all fins with randomly distributed blackish spots; membranes of spinous portion of dorsal fin without a distinct black blotch in males; body size relatively large, maximum recorded size 128.8 mm SL. Several body proportions changing with growth, including body width, head width, snout length, interorbital width at vertical midline of eye, upper-jaw length, maxilla depth, and postorbital length (all significantly increasing), and orbit diameter and separation of opercular spine tips significantly decreasing. The new Japanese standard name “Mitsumata-nettaifusakasago” is proposed for P. poseidon.

Three bryozoan species are described from the continental shelf east of the Boso Peninsula, Japan. One is a new species of Tessaradoma Norman, 1869, resembling Arctic and Atlantic species. Its colony form is, however, composed of zooids facing one side. The other two are species described from Sagami Bay as Hippothoa connata Ortmann, 1890 and Diporula coronula Ortmann, 1890. The former is assigned to the genus Cribellopora Gautier, 1957, having cribellate pseudopores on the frontal wall and ooecium. The latter is provisionally placed in the genus Hippomenella Canu and Bassler, 1917. The biogeography of the genera and species groups including the three species in this study is discussed. The boreal origin of the Japanese Tessaradoma is inferred, and the two species established by Ortmann are related to the bryozoan fauna of Australia and New Zealand.

The new snailfish Careproctus io is described on the basis of three specimens collected from the western North Pacific, off northeastern Honshu Island, Japan. The new species can be distinguished from congeners by the following characters: vertebrae 40–42; dorsal-fin rays 36 or 37; anal-fin rays 30; pectoral fin deeply notched with 28 or 29 rays, lower lobe reaching to anal-fin origin; large pelvic disk 34.2%–34.5% HL (10.3%–10.9% SL); teeth strongly trilobed on both jaws, inner teeth weakly trilobed or shouldered; cephalic pore pattern 2-6-7-2, chin pores paired; gill slit above pectoral-fin base; body bright red, non-variegated in life. Species of Careproctus Krøyer, 1862 typically have fewer pectoral-fin rays than anal-fin rays, although some recently-described species, including the present new species, have similar ray counts in both fins. The relationships between such counts, as well as pelvic disk size, are investigated for various snailfish, genera, and the generic limits of Careproctus discussed.

Some of the Indonesian species of the genus Atholus Thomson, 1859 are re-examined based on museum specimens. Atholus famulus (Lewis, 1892), A. myrmidon (Marseul, 1861) and A. singalanus (Marseul, 1880) are re-described, and SEM micrographs and illustrations of available genitalia are provided. Additional Indonesian records of A. bifrons (Marseul, 1854), A. coelestis (Marseul, 1857), A. philippinensis (Marseul, 1854) and A. torquatus (Marseul, 1854) are reported. A key to the Indonesian species is provided.

The chromodorid nudibranch Goniobranchus aureopurpureus (Collingwood, 1881) is characterized by numerous yellow spots on a white mantle that is bordered with purple spots. It is believed to be widely distributed in both the Northern and Southern Hemispheres in the West Pacific, showing color varieties in a more-or-less geographically coherent manner. Chromodoris variata Risbec, 1928 (type locality: New Caledonia) has been considered a synonym. In addition, three, similar-looking congeners have been proposed as synonymous with G. aureopurpureus: G. albopustulosus (Pease, 1860) (type locality: Hawaii), G. alius (Rudman, 1987) (type locality: Tanzania), and G. rufomaculatus (Pease, 1871) (type locality: French Polynesia). To test the synonymies proposed above, we performed maximum-likelihood and Bayesian-inference analyses utilizing partial sequences of the cytochrome c oxidase subunit I (COI) and 16S rRNA genes. The taxon sampling included a specimen of G. aureopurpureus newly collected in Hokkaido, Japan, along with other sequences of Goniobranchus deposited in public databases (31 specimens representing 22 species), including one identified as ‘Chromodoris aureopurpurea’ from New South Wales, Australia; G. albopustulosus from Hawaii; and two ‘Chromodoris rufomaculata’ from Guam and the Mariana Islands. Goniobranchus aureopurpureus from Japan and from Australia were sister taxa in the resulting trees but differed from each other by 8.6% in terms of 658-bp COI, suggesting that these two represent closely-related but different biological entities. For the Australian taxon, Goniobranchus cf. variatus comb. nov. is proposed herein. Goniobranchus albopustulosus and G. rufomaculatus were not closely related to the G. aureopurpureus–G. cf. variatus clade, rejecting the hypothesis that they are synonymous with either of these species. However, G. albopustulosus and G. rufomaculatus appeared to comprise a single biological entity, warranting further investigation of this potential synonymy. The placement of the Indian Ocean G. alius also remains to be tested.

Bicotyle reticulata (Goto, 1894) (Monogenea: Heteraxinidae) is redescribed from the gills of Pampus punctatissimus (Temminck and Schlegel, 1845) in the Seto Inland Sea, Japan. Although its taxonomic position had been frequently revised, B. reticulata remains solely documented from the original description, 130 years ago. The present study revealed that B. reticulata exhibits subsequent diagnostic characters of Bicotyle Tripathi, 1956: clamps on either side of the haptor unequal in size and number, the genital atrium armed with several spines, and a convoluted germarium. The phylogenetic analysis based on 28S rDNA sequences suggests that B. reticulata and Heteraxinidae constitute a monophyletic group. All currently known species of Bicotyle are listed, and a key to six species of the genus is provided.

The Okinawa Rail, Hypotaenidia okinawae (Yamashina and Mano, 1981), is a flightless bird restricted to the northern part of Okinawa Island, Ryukyu Islands, Japan. A new feather mite species, Metanalges (M.) agachi sp. n. (Analgidae: Megniniinae), is described from the Okinawa Rail. The new species is distinguished from the closest species, M. (M.) curtus Gaud and Mouchet, 1959, in having, in males, legs III extending slightly beyond the bases of setae h2, and setae ps1 approximately equidistant from setae h2 and h3, and in females, ventral setae 4a almost extending to the posterior margin of opisthosoma. This is the first report of feather mites associated with this avian host and the first record of the genus Metanalges Trouessart, 1919 in Japan. As the Okinawa Rail is considered to be at high risk of population decline, the new feather mite is thought to be under the risk of extinction.