The monotypic polynoid genus Arctonoella Buzhinskaja, 1967 comprises solely the type species A. sinagawaensis (Izuka, 1912), which was originally described from Tokyo Bay (central Japan), and subsequently recorded from China and the Russian Far East. The six specimens we collected together with the burrowing filter-feeding spoon worm Urechis unicinctus (Drasche, 1880) from three intertidal-flat sites in the Seto Inland Sea represent a new report for the western Japan, and the second for the country. Our morphological observations reveal that the shape of the cephalic peaks in the frontal prostomial margin is variable even within a local population, although this character has been considered as diagnostic for Arctonoella. This genus is closely related with Hesperonoe Chamberlin, 1919, both morphologically and phylogenetically. Hesperonoe urechis Marin and Antokhina, 2020, collected inside a burrow of U. unicinctus in the Russian Far East, is hereby deemed a junior synonym of A. sinagawaensis. This species morphologically resembles Hesperonoe adventor (Skogsberg in Fisher and MacGinitie, 1928), which inhabits spoon worm (Urechis caupo Fisher and MacGinitie, 1928 and Echiurus echiurus alaskanus Fisher, 1946) burrows along the northeastern Pacific coast of America. Therefore, we conclude that the Arctonoella-Hesperonoe species complex requires to be reviewed to determine the proper generic arrangement. Additionally, we revise the available data on the symbiotic fauna living inside the burrows of U. unicinctus and U. caupo.

Seven specimens of Paloa villadolidi Roxas and Ablan, 1940, were collected from Okinawa and Ishigaki islands in the Ryukyu Archipelago, Japan, and their morphologies are described herein. Although this species has been considered as a synonym of Odonteleotris macrodon (Bleeker, 1853) which is redescribed in this study, the former is distinguished from the latter by having a steeper jaw (upper jaw tip not reaching orbit vs. ending below orbit center in O. macrodon), a contrasting number of canine teeth on the jaws (more upper-jaw canines than lower-jaw ones vs. more lower-jaw canines), the absence of distinct fin markings (vs. many small black spots on fin rays), no spots on the caudal-fin base (vs. with a red spot), and a higher number of cephalic sensory canal pores (17 vs. 13 or 14). This study also noted a unique canine teeth arrangement in P. villadolidi, which has canine teeth only in the anterior half of the lower jaw, unlike the canine teeth arrangement of four other butid genera (Incara Rao, 1971, Odonteleotris Gill, 1863, Ophiocara Gill, 1863, and Oxyeleotris Bleeker, 1874), which have an inner row of canine teeth in the posterior half of the lower jaw. Although further study is needed to determine the taxonomic status of P. polylepis Herre, 1927, the other nominal species of the genus, the present study tentatively considered it valid based on the original description, which described it as having a deeper body than P. villadolidi. The seven Japanese specimens were identified as P. villadolidi, as they have slenderer bodies than P. polylepis, and they represent the first Japanese records of the species.

A single specimen of gecarcinid crab, Discoplax gracilipes Ng and Guinot, 2001, was collected from an anchialine pool in a cave on Hateruma Island, southern Ryukyu Archipelago, southwestern Japan. This species was previously known only from the central Philippines. The present specimen represents the first record of the species from Japan, extending its distributional range to about 1400 km northward. Additionally, the fauna of the cave on Hateruma Island briefly noted. A key to the Japanese species of the family Gecarcinidae is provided.

A species of cyclopoid copepod Anthessius cucullatus Lee and Kim, 2021 (Anthessiidae) is newly recorded based on both sexes collected from three species of the sea slug genus Aplysia Linnaeus, 1767, i.e., Ap. dactylomela Rang, 1828, Ap. juliana Quoy and Gaimard, 1832, and Ap. kurodai Baba, 1937, from coastal waters of Kagoshima Prefecture, southern Japan. The copepod is characterized by having the following characters in the female: 1) the genital double somite covered with a hood-like dorsal lobe; 2) the antenna with four unequal claws and three setae at the distal tip; 3) the maxilla has a spinulose basal lobe; 4) leg 5 with irregular marginal rows of spinules. Furthermore, the male has 5) the third endopodal segment of leg 4 bearing a curved third inner spine. The findings of An. cucullatus in this study represent the second record of the copepod from all around the world as well as the first record from Japanese waters with the detailed description of the morphological characters based on the specimens. Aplysia dactylomela and Ap. juliana are new host records of the copepod.

Monostiliferous nemerteans are mostly benthic and free-living, but a few are recognized as commensal and parasitic species. We describe Tetrastemma carneum sp. nov. based on five specimens found from demosponges collected by the use of remotely operated vehicle at a depth of 398 m, off Ofunato, Japan. The new species resembles T. appendiculatum Chernyshev, 1998 described based on specimens discovered from demosponge Esperiopsis digitata (Miklucho-Maclay, 1870) (now accepted as Amphilectus digitatus) trawled at depths of 40–60 m in Peter the Great Bay, in having a pale-colored body with four conspicuously large eyes (approximately 50 µm in the maximum width). However, it is differentiated from T. appendiculatum in possessing a mid-dorsal line on the dorsal surface of the head as well as the number of proboscis nerves. In our molecular phylogenetic tree based on partial sequences of 16S rRNA, cytochrome c oxidase subunit I, 18S rRNA, 28S rRNA, and histone H3 genes, Tetrastemma carneum sp. nov. was nested in the clade of Tetrastemma Ehrenberg, 1828 as a sister taxon to Tetrastemma sp. IP Iturup reported from Peter the Great Bay.

Argulus nobilis Thiele, 1904 is an ectoparasite of gars (Lepisosteidae) in the U.S.A. Five female specimens of A. nobilis were found in the collection of the Lake Biwa Museum, Japan. This is the first record of the species from Japan and outside the U.S.A. The specimens were collected from an unidentified gar and shortnose gar, Lepisosteus platostomus Rafinesque, 1820. Based on a detailed examination of the specimens, the adult female of A. nobilis is redescribed. The finding from shortnose gar represents a new host record for this parasite. The specimens of A. nobilis were collected in 1992, and it is unknown whether they were taken from gars soon after imported from the U.S.A. or those that had been held in Japan.

Argulus japonicus Thiele, 1900 and Argulus coregoni Thorell, 1864 are ectoparasites of freshwater fishes and are known to occur in Lake Biwa and its adjacent inland waters in Shiga Prefecture, central Japan. Since these parasites are not common in the wild in Japan, this study examined the specimens loaned from the Lake Biwa Museum and those previously reported from this prefecture in order to clarify their distributional patterns. The specimens of A. japonicus were obtained in Lake Biwa and a nearby pond, whereas those of A. coregoni were almost exclusively collected in the large rivers flowing into the lake and most of them were from mountain streams which are higher-elevated than the lake or pond. These results indicate both A. japonicus and A. coregoni has its own habitat preference dependent on different environmental conditions: A. japonicus inhabits the still or slow-flowing waters, but A. coregoni occurs in the running waters, especially in the colder, well oxygenated mountain streams.

In Japan, the native population of the Oriental White Stork Ciconia boyciana Swinhoe, 1873 had been extinct in 1986, when the last captured stork died in a breeding facility. Later on, some wild or bred storks from Russia and mainland China (clear origin unknown) were transported to Japan, and their descendants were repeatedly released into natural fields to establish wild populations in this country. In this study, the feather mite Pelargolichus orientalis Waki, Mironov, and Shimano, 2023 has been collected from a taxidermy specimen of the Oriental White Stork belonging to a Japanese native population and preserved in the Himeji City Science Museum, Japan. Pelargolichus orientalis is the only feather mite species detected and described from the introduced storks bred in a Japanese zoo in our previous study. The present study shows that P. orientalis did occur in the Japanese native host population.

A new species of Hemicorallium Gray, 1867 (Coralliidae), Hemicorallium meraboshi sp. nov., is described here. The specimen was found at a depth of 1744–1755 m, approximately 340 km east of the Honshu (mainland Japan) coast. The colony was identified by visual and microscopic observation of standard morphological characteristics (colony size, diameters of colony base and branches, diameter and height of autozooid mound, thickness of coenenchyme, sclerite sizes, etc.) along with support from molecular evidence. The unique features of this specimen are the large (more than 2 mm in diameter) autozooid mounds and the sizable sclerites in the coenenchyme (over 0.1 mm long) that are 8-radiates and multi-radiates predominantly. This new species is the northernmost record of Pacific precious corals.

Two large specimens [310–476 mm standard length (SL)] of the genus Hoplostethus Cuvier in Cuvier and Valenciennes, 1829, collected from Iwate and Okinawa prefectures, Japan, were recently discovered and identified as Hoplostethus grandperrini Roberts and Gomon, 2012 on the basis of the following characteristics: large body size, exceeding 300 mm SL; abdominal scutes strong; pectoral fins light red; caudal-fin tip without black pigmentation; upper margin of eye almost horizontally level with lateral-line origin; predorsal scales not enlarged; fin spines moderately thickened; pectoral-fin tip not reaching anal-fin origin; anterior part of oral cavity, including underside of tongue, without black pigmentation; 15–17 pectoral-fin rays; 21 predorsal scales; and 13 or 14 abdominal scutes. Furthermore, a molecular phylogenetic analysis of a partial sequence of the mtDNA COI gene from one of the specimens placed it in the same clade as H. grandperrini previously recorded from Taiwan, with an uncorrected p-distance between both specimens of 0.19%. Hoplostethus grandperrini was originally described on the basis of two specimens collected off New Caledonia, with only one additional specimen having been reported (from southern Taiwan). The Japanese specimens are the first records of the species from Japan, the fourth and fifth specimens known, and the Iwate Prefecture specimen the northernmost record of the species.

We establish a new genus of candimboidid polyclad, Chimaeriplana gen. nov., based on a new species, C. japonica sp. nov., collected from Kanagawa, the coast of Sagami Bay, Japan. Chimaeriplana japonica sp. nov. is characterized by (i) an interpolated prostatic vesicle with a seminal vesicle, (ii) a penis stylet with a split tip, (iii) numerous prostatoid organs in the male atrium, (iv) a bursa copulatrix, and (v) without Lang’s vesicle. In addition, we modify the definition of Candimboididae.

Microcotyle pacinkar n. sp. (Monogenea: Microcotylidae) is described from the gills of Sebastes taczanowskii Steindachner, 1880 (Scorpionfishes: Sebastidae) (Japanese name: Ezo-mebaru) collected from off Usujiri in Hokkaido, northern Japan in the North Pacific Ocean. The new species is morphologically most similar to M. ditrematis Yamaguti, 1940 in having an elongated body and an inverted pear-shaped genital atrium but differs from the congener in the arrangement of the spines in the genital atrium and the shapes of the anterolateral sclerites of the clamps. The phylogenetic trees for species of Microcotyle van Beneden and Hesse, 1863 based on the partial cytochrome c oxidase subunit I region were estimated using new sequences of M. pacinkar n. sp.; the new species formed a sister group with M. sebastis Goto, 1894.

Specimens of Gyrodactylus cyprini Diarova, 1964 (Monogenea: Gyrodactylidae) and Dactylogyrus extensus Mueller and Van Cleave, 1932 (Monogenea: Dactylogyridae) were obtained from the common carp Cyprinus carpio Linnaeus, 1758, which was reared in an outdoor pond at the Tamaki Field Station of the Fisheries Technology Institute, Mie Prefecture, Japan. Although G. cyprini is a well-known parasite of C. carpio in the Palearctic and USA and its existence in Japan was implied by the dermatitis examination of exported koi carp from Japan to Thailand, any specimen of G. cyprini has not been recorded in Japan. In addition, I determined some Japanese specimens previously identified as G. kherulensis Ergens, 1974 were also G. cyprini. This study obtained novel geographical records of these monogeneans, along with redescriptions of three monogenean species and molecular data of G. cyprini and D. extensus.