A new ostracod species, Semicytherura obitsuensis sp. nov., is described from the intertidal zone of the Obitsu River estuary, Chiba Prefecture, central Japan. As S. obitsuensis sp. nov. individuals were collected from muddy sand bottoms in tidal creeks, possess some carapace-features comparable to those of interstitial and infaunal ostracods, and were observed to crawl in detritus rich fine-grained sediment in the laboratory, we concluded that this new species is an infaunal species.

Two new species Composetia kumensis and C. tokashikiensis (Nereididae) are described based on specimens collected from subtropical small estuaries in the Kume-jima and Tokashiki-jima islands in the middle Ryukyu Islands, southern Japan, respectively. Both species have the following diagnostic characteristics of Composetia Hartmann-Schröder, 1985: (1) having conical paragnaths in the maxillary ring of the proboscis and lacking paragnaths or papillae in the oral ring, (2) prostomium with entire anterior margin, (3) the absence of falcigers among notochaetae, and (4) the absence of simple chaetae among upper neurochaetae. These new species share the following diagnostic characters: (1) presence of notoacicula on chaetigers 1 and 2, (2) absence of notopodial prechaetal lobe throughout body, (3) presence of neuropodial postchaetal lobe only in anterior body, (4) neuropodial falcigers all heterogomph, and (5) oral ring greatly enlarged in full-everted proboscis. However, C. tokashikiensis sp. nov. is distinguishable from C. kumensis sp. nov. by the presence of heterogomph spinigers among the upper neurochaetae around chaetiger 5. A list of all of 34 species currently belonging to Composetia and a key to Japanese species of Composetia are also provided.

By the survey on the nereidid fauna in small estuaries in creeks in the Ryukyu Islands, southern Japan, two species of Composetia Hartmann-Schröder, 1985, C. kumensis Sato, 2020 (type locality: Kume-jima island, the Ryukyu Islands) and C. tokashikiensis Sato, 2020 (type locality: Tokashiki-jima island, the Ryukyu Islands) have been newly collected from 10 sites on 5 islands, and from 20 sites on 6 islands, respectively. In 26 of a total of 28 sites, one of the two species occurred exclusively, but both of them coexisted in two sites in Oura Bay on Okinawa-jima island. Morphological variations and genetic differentiations among the local populations of the two species were examined. Molecular analyses based on the partial sequences of the mitochondrial cytochrome c oxidase subunit I gene supported that the morphologically similar two species are certainly distinct species. In C. kumensis, marked genetic differentiation was detected among populations in the following three regions in the middle Ryukyu Islands: (1) northern Kume-jima, (2) southern Kume-jima, and (3) Okinawa-jima and Kikai-jima islands. In contrast, no marked genetic differentiation was detected among any populations of C. tokashikiensis in a wide range of the middle and southern Ryukyu Islands. A single specimen of C. tokashikiensis was also collected from an euryhaline area of the lower reaches of Songkhla Lagoon, southern Thailand. A morphological description of this specimen is provided here as a new record of this species from Thailand.

Epinephelus kupangensis Tucker, Kurniasih, and Craig, 2016, previously known from the Indo-West Pacific as far north as Taiwan, was recorded from Japan for the first time on the basis of two specimens (98.1–384.6 mm SL) collected from Tosa Bay, Kochi and Okinawa prefectures. The Kochi specimen had been previously reported as the first Japanese specimen of “Kokuten-aohata”, Epinephelus amblycephalus (Bleeker, 1857). Epinephelus kupangensis differs from the latter in having slightly lower numbers of lateral-line scales (46–49 vs. 48–51) and lateral scale series 91–99 (vs. 103–120), and lacking a U-shaped white marking dorsolaterally on the head and a white band below the orbit (vs. both present). The standard Japanese name “Kokuten-aohata” should be applied to E. kupangensis, the new standard Japanese name “Yahazu-aohata” being proposed for E. amblycephalus.

The live coloration of Kopua nuimata Hardy, 1984 is described for the first time based on a recently collected specimen from south of Norfolk Island, Australia. The new color information includes new diagnostic characters of the species, including arch-shaped blotches on the lateral aspect of the body and two reddish-orange stripes on the cheek. Although K. minima (Döderlein, 1887) and K. yoko Fujiwara, Okamoto, and Motomura, 2018 share a similar live coloration with K. nuimata, K. kuiteri Hutchins, 1991 and K. vermiculata Shinohara and Katayama, 2015 differ markedly by lacking arch-shaped blotches on the body and by exhibiting reddish-orange markings on the cheek. A revised diagnosis and redescription of K. nuimata is provided based on the recently collected specimen, which represents the first Australian record of the species.

A rare lizardfish, Synodus mundyi Randall, 2009, is newly reported from the northwestern Pacific around Japan, based on a single specimen (147 mm in standard length) collected from off Muko-jima island of the Ogasawara Islands at a depth of 63 m. Prior to the present study, the species was represented by only the two type specimens collected from the Hawaiian Islands. Among Japanese congeners, it is most similar to the Indo-Pacific Synodus doaki Russell and Cressey, 1979 especially in having a very long nasal flap, but differs notably from the latter in that the tip of the pectoral fin extends well beyond a line connecting the dorsal- and pelvic-fin origins (vs. just reaching this level). The Japanese specimen is fully described, with a color photograph of the fresh specimen. A new standard Japanese name, “Kaede-eso”, is proposed for S. mundyi.

Four species of alien parasites, Dactylogyrus skrjabini Achmerow, 1954, D. hypophthalmichthys Achmerow, 1952, D. wuhuensis Lee, 1960 (Platyhelminthes: Monogenea: Dactylogyridae), and Sinergasilus lieni Yin, 1949 (Crustacea: Copepoda: Ergasilidae), were collected from the gills of the alien cyprinid, the silver carp Hypophthalmichthys molitrix (Valenciennes, 1844) in the Watarase River flowing into the Watarase Retarding Basin, Tochigi Prefecture, central Japan. These alien parasites represent new geographical records from Japan. The generic position of D. skrjabini has been doubted because its dorsal anchor shape is similar to those of Pellucidhaptor spp. (Dactylogyridae), but the species should be assigned to Dactylogyrus based on the phylogenetic analysis of 28S rDNA made in this study. The scientific name “Pseudergasilus polycolpus Markewitsch, 1939” is an incorrect subsequent spelling of P. undulatus Markewitsch, 1940, and S. lieni is the oldest available name that can be applied to the ergasilid found in this study.

Three new species of the ascothoracidan crustacean genus Dendrogaster Knipovich, 1890 (Dendrogasteridae) are described from goniasterid sea-stars in Japan. Dendrogaster komatsuae sp. nov. and D. tobasuii sp. nov. were found respectively in the coelomic cavities of Lithosoma japonica Hayashi, 1952 and two species of Mediaster: M. arcuatus (Sladen, 1889) and M. brachiatus Goto, 1914 from the Kumano-nada Sea. Dendrogaster nagasakimaruae sp. nov. was similarly found in Nymphaster euryplax Fisher, 1913 from the East China Sea. Partial DNA sequences of the cytochrome c oxidase subunit I and 16S ribosomal RNA genes were determined for D. tobasuii sp. nov. These findings represent the first records of Dendrogaster from the host family Goniasteridae in Japan.

Three forms of Japanese Eudigraphis (Penicillata, Polyxenidae), which have long been treated as subspecies of Eudigraphis takakuwai (Miyosi, 1947), i.e., E. t. takakuwai (Miyosi, 1947), E. t. nigricans (Miyosi, 1947), and E. t. kinutensis (Haga, 1950), are raised here to three distinct species. Overlap of their distributional ranges over wide areas of Japan was confirmed, and molecular phylogeny using both nuclear (ITS2) and mitochondrial (COI) DNA testified monophyly of each of the three forms. Additionally, the three forms can be easily distinguished from one another by body size, coloration, and habitat, though species identification for the specimens preserved in ethanol is often difficult due to their morphological similarity other than coloration and size. Description of some so-far neglected morphological characters were provided with some SEM photos to supplement knowledge on taxonomy and morphology of the family.