A new species of sea anemone, Antennapeachia jambio sp. nov., was collected off Jogashima, central Japan. The presence of a single, strong siphonoglyph and physa-like aboral end, and the absence of the sphincter muscle classify this sea anemone within the family Haloclavidae. Two antenna tentacles and non-paired microcnemes categorize it into genus Antennapeachia, where only Antennapeachia setouchi Izumi and Yanagi, 2016 was included. However, unlike A. setouchi, A. jambio has two more marginal tentacles and two macrocnemes between ventral directives and ventro-lateral mesenteries. On the basis of details of these new characters of Antennapeachia, the diagnosis of the genus was revised to accommodate both species.

Haplocotyle japonica n. gen., n. sp. (Monogenea: Microbothriidae) is described from the skin and gill cavity of Rhinobatos hynnicephalus Richardson, 1846 (Elasmobranchii: Rajiformes: Rhinobatidae) in the Seto Inland Sea off Hiroshima Prefecture and the southern Sea of Japan off Fukuoka Prefecture, Japan. The new genus is closely related to Dermophthirioides Cheung and Nigrelli, 1983, Dermophthirius MacCallum, 1926, Dermopristis Kearn, Whittington, and Evans-Gowing, 2010, and Pseudoleptobothrium Young, 1967 in common morphological features (i.e., the anterior extremity of the oötype having a tetrahedral shape, the anterior aperture of a bell-shaped pharynx, and the structure of the male copulatory organ) but differs from the latter four genera by the presence of a single testis and an ovary not looping the intestinal caeca. The phylogenetic analysis based on 28S rDNA sequences suggests that the new species shows affinity with Dermophthirius and Dermopristis. All currently known species of Microbothriidae Price, 1936 are listed, and a key to 12 genera, including Haplocotyle n. gen., of the family is provided.

We describe a new capitellid species, Mastobranchus uru sp. nov., from a tidal flat on Iriomote Island, Okinawa, southern Japan. This species differs from its three congeners in having capillary chaetae only on the notopodia in chaetiger 1 and in the number and arrangement of teeth on hooks. We also provide for this new species a barcode nucleotide sequence from the mitochondrial 16S rRNA gene.

A new species of hesionid polychaetes, Leocratides kimuraorum sp. nov., is described based on material collected from the Shima Peninsula and Sagami Bay (middle Honshu, Japan), as well as museum specimens collected from Sagami Bay, Suruga Bay, and Shirahama. Leocratides kimuraorum sp. nov. is the third species in this genus and can be discriminated from the two congeners, L. filamentosus Ehlers, 1908 and L. ehlersi (Horst, 1921), by i) the length of the antennae, which are as long as palps, ii) the presence of pharyngeal terminal papillae, and iii) the absence of a papillose peristomial membrane. On the other hand, L. filamentosus, originally described from off Western Sumatra, Indonesia, was also collected from Sagami Bay, and represents a new record of this species for Japanese waters. Partial mitochondrial cytochrome c oxidase subunit I gene sequences from the holotype of L. kimuraorum sp. nov. and newly collected specimen of L. filamentosus are provided for reliable species identification in the future.

A quadrannulate leech species, Orobdella meisai sp. nov., from southern Taiwan is described. This new species is characterized by its camouflage coloration, which is unique among the known Orobdella species. Orobdella meisai is the second species of the genus to be described from Taiwan. Phylogenetic analyses using nuclear 18S rRNA, histone H3, mitochondrial cytochrome c oxidase subunit I, tRNA^Cys, tRNA^Met, 12S rRNA, tRNA^Val, 16S rRNA, tRNA^Leu and NADH dehydrogenase subunit 1 markers showed that O. meisai and another Taiwanese species, O. ketagalan Nakano and Lai, 2012, formed a supported clade. According to the results of the morphological examination and the phylogenetic position of O. meisai, morphological-phylogenetic relationships of this species and close congeners are briefly discussed.

A new species of free-living marine nematode, Oncholaimus langhovdensis sp. nov., is described from the intertidal zone of Langhovde (near Syowa Station), Dronning Maud Land, East Antarctica. It closely resembles 11 congeners in the conico-cylindrical tail shape present in males and amphid and excretory pore positions, short spicules, and Demanian system structure present in females. However, it mainly differs from these congeners in body size, de Man’s ratios, tail length and shape, and Demanian system structure present in females. Oncholaimus langhovdensis sp. nov. also resembles four congeners known only by females, but it can be distinguished from them based on the tail length and uvette position. In addition to O. langhovdensis sp. nov., two undescribed species (Tripyloididae gen. sp. and Axonolaimidae gen. sp.) and four unidentified species (Sphaerolaimus sp., Oncholaimidae gen. sp., Comesomatidae gen. sp., and Chromadorida fam. gen. sp.) were found from the same locality.

Female and male specimens of Doridicola similis Ho and Kim, 2001 (Copepoda: Poecilostomatoida: Rhynchomolgidae) from the gills of the loliginid squid Sepioteuthis sp. (called shiro-ika) (Cephalopoda: Myopsida: Loliginidae) in coastal Pacific waters of central Japan are described as a new country record. This collection represents the second record of D. similis and expands its distribution range from the Gulf of Thailand (type locality) northeast to the western North Pacific Ocean off central Japan.

The lernaeopodid copepod Tracheliastes sachalinensis Markevich, 1936 was found on the fins of three species of the subfamily Leuciscinae Bonaparte, 1846 (Cypriniformes: Cyprinidae): big-scaled redfin, Tribolodon hakonensis (Günther, 1877); Sakhalin redfin, Tribolodon sachalinensis (Nikolskii, 1899); and lake minnow Rhynchocypris percnurus (Pallas, 1814); collected in three lakes (Lake Tôro, Lake Shirarutoro, and Lake Abashiri) and two rivers (Mena River and Jirô-sawa River), Hokkaido, northern Japan. These findings represent the first records of Tracheliastes sachalinensis from Japan and also from outside of Russia, in which the copepod has hitherto been found. The species is specific to leuciscine fishes in the subarctic region of the Russian Far East and Japan. We also provide a review of parasitic copepods of freshwater fishes from Hokkaido.

Rugosuscandona scharfi gen. nov. sp. nov. is described as a new genus of the family Candonidae from groundwaters of Texas, USA. The new genus differs from its congeners based on the presence of relatively long pore canals with sensory seta on the carapace surface, 6-segmented first antenna (A1), absence of bristles (t setae untransformed) and z setae on second antenna (A2), distinctly shaped male hemipenis and clasping organs, a simple claw-like uropod, and presence of 4+2 whorls on the Zenker’s organ. Additionally, reduction in the number of setae and segments of the other soft body parts can be used to differentiate between the new genus Rugosuscandona gen. nov. and its congeners. Therefore, the new genus is currently monotypic. Taxonomic status of the new genus is discussed and compared with living genera of the family.

Three species of Grandidierella (Crustacea: Amphipoda: Aoridae) were collected from coastal areas of the Tohoku and Kanto-Tokai Districts, East Japan. The first species is G. sanrikuensis sp. nov. from Miyagi Prefecture, which is characterized by the male gnathopod 1 with carpus bearing 3 teeth (only middle tooth large). The second species is G. rubroantennata sp. nov. from Chiba Prefecture. This new species is identical with G. insulae sensu Ariyama (1996), but is different from the true G. insulae Myers, 1981 in the more flagellar articles of the antennae. The last species is G. osakaensis Ariyama, 1996, from river mouths in Kanagawa and Shizuoka Prefectures and an anchialine pool in Izu Oshima Island, Tokyo Prefecture. This species can inhabit various sandy mud bottoms in a brackish environment. Including G. japonica Stephensen, 1938, four Grandidierella species occur in the districts.

A new species of comatulid crinoid, Comanthus scintillus n. sp., is described based on specimens from southern Japan. The general appearance is similar to congeners having a small centrodorsal with few cirri, such as Comanthus gisleni Rowe, Hoggett, Birtles, and Vail, 1986, and C. suavia Rowe, Hoggett, Birtles, and Vail, 1986. However, the new species is characterized by the distribution of the terminal combs to middle arms, lack of secondary teeth on the combs, and the presence of fringing spines on the segments of the distal pinnules. Live specimens of the new species show striking coloration in which the whole body is red with scattered small yellow spots, which also makes it easily distinguishable from congeners.

Two specimens (304.0–441.4 mm total length) of Uropterygius fasciolatus (Regan, 1909) were collected from Yonaguni-jima island, the westernmost island in Japan. This species was previously known from Indonesia, Papua New Guinea, the Solomon Islands, Palau, the Tokelau Islands, and the Phoenix Islands. Therefore, the present specimens represent the first record from Japan, as well as the northernmost record of the species up until now. A new standard Japanese name, “Murakumo-kikaiutsubo”, is proposed for the species.

An old museum specimen of the rare deep-sea species Ophichthus exourus McCosker, 1999 (Anguilliformes: Ophichthidae) (685 mm in total length) collected near Saipan and a recently collected example from Okinawa, Japan (634 mm in total length) represent the first Northern Hemisphere records of the species, previously known only from the southwestern Pacific in waters off New Caledonia and Fiji, but now believed to be widely distributed in the western Pacific Ocean. A detailed morphological observation based on the above specimens and a re-examination of the holotype resulted in a revision of some species characteristics, including the number of infraorbital pores. Ophichthus exourus is distinguished from other congeners by the following combination of characters: head 8.3–10.0% of total length; upper jaw 36.1–38.1% of head length; pectoral-fin rounded, length 19.2–24.1% of head length; rear margin of eye above or slightly behind rictus; eye diameter 44.4–61.2% of snout length; horizontal length of posterior nostril dermal flap much greater than diameter of anterior nostril tube; gill opening to dorsal-fin origin greater than twice pectoral-fin length; preopercular pores 2; predorsal and total vertebrae 20–21 and 173–177, respectively; lower-jaw teeth nearly uniserial; body without bands or spots; posteriormost portion (much shorter than head length) of anal-fin membrane indistinct darkish brown. A new standard Japanese name “Gunbai-umihebi” is proposed for the species.

A single specimen (135.5 mm standard length) of the Indo-West Pacific species Halicampus mataafae (Jordan and Seale, 1906), previously known to range from the Red Sea and South Africa to Taiwan and Australia, was collected from Yoron Island, Amami Islands, Ryukyu Islands, Japan. A detailed description is given for the specimen, being the first collected from Japanese waters and northernmost record of the species.

The distributional range of Ostorhinchus cheni (Hayashi, 1990), previously recorded only from Guam, Taiwan, and Japan, is expanded to include Sri Lanka and the Philippines on the basis of two specimens (104.5–115.4 mm standard length), which are described in detail. The Sri Lankan specimen represents the first record of O. cheni from the Indian Ocean. In addition, the first underwater photograph of a live individual of O. cheni (taken at Tulamben, Bali, Indonesia) is included. The new records suggest that O. cheni is widely distributed in the Indo-West Pacific.