Dactylogyrus oryziasi n. sp. (Monogenea: Dactylogyridae) is described from the gills of the southern medaka Oryzias latipes (Temminck and Schlegel, 1846) (Beloniformes: Adrianichthyidae) collected from an irrigation canal, Tokushima city, Tokushima Prefecture, Shikoku, Japan. This new species resembles Dactylogyrus rysavyi Ergens, 1970 in having a coiled penis passing through a tube of an accessory piece associated with a sigmoid rod, and shapes of haptoral parts, but these species differ from each other by turns of the penis and a connection point between the rod and tube of the accessory piece. This is the first record of Dactylogyrus from beloniform fish.

A new classification is proposed for the Japanese species of the genus Lumbrinerides Orensanz, 1973 (Annelida: Lumbrineridae), based on the observation of as many specimens as possible so as to determine the extent of intra-populational variation of each morphological character. Morphological differences apparent in material from ten local sites were compared statistically among 16 tentative, operational taxonomic units, the morphologically recognizable units then being subjected to a taxonomic consideration with other known species. By their morphology, five species were recognized from Japanese waters, three being new to science. Diagnoses and descriptions of each species are given.

Dichodactylus gen. nov. (type species Coelotes tarumii Arita, 1976) is described from western Japan. Three species are recognized: Dichodactylus shinshuensis sp. nov., D. tarumii (Arita, 1976) comb. nov. (transferred from Coelotes Blackwall, 1841), and D. satoi (Nishikawa, 2003) comb. nov. (transferred from Orumcekia Koçak and Kemal, 2008). Dichodactylus is compared with Orumcekia, especially morphological similarities in the male palps. Diagnostic and descriptive characteristics of the three species are presented including a species distribution map and genitalic illustrations.

An argulid branchiuran, Argulus quadristriatus Devaraj and Ameer Hamsa, 1977, is reported on the basis of specimens of both sexes isolated from two species of goatfish [Upeneus tragula Richardson, 1846 and Parupeneus ciliatus (Lacepède, 1802) (Perciformes: Mullidae)] and the goby Favonigobius reichei (Bleeker, 1854) (Perciformes: Gobiidae) collected in Kagoshima Bay and off both the east and west coasts of Okinawa-jima island, southern Japan. The branchiuran is easily distinguished from its nine previously recorded congeners in Japanese waters by the presence of one posterior spine on the antennule and scales ornamenting the basal half of the mouthtube. These are the first records of this branchiuran from the North Pacific Ocean including Japanese waters, and the three fish species mentioned above are new hosts for it.

The sea cucumber, Holothuria (Cystipus) dura Cherbonnier and Féral, 1981, previously known only from the Philippines and New Caledonia, is newly recorded from Japanese waters; this is also the first record of the subgenus Cystipus from Japan. The two specimens described here were collected from the sublittoral bottoms near Hahajima island (126–261 m depth) and Okinoshima island (100–120 m depth). The morphology of the Japanese specimens is briefly described. In-situ photographs of the living individuals and scanning electron micrographs (SEM) of ossicles are provided for the first time for this species. The table ossicles in the dorsal body and in the papillae are highly modified fenestrated spheroids that have not been depicted previously. Some of the ventrolateral conical papillae, especially those in the middle part of the body, were bifurcated, perhaps indicative of increased surface area against the bottom for powerful propulsion while ‘walking’. Some notes on locomotion are given, inferred from the in-situ photographs and the morphology of the ventrolateral conical papillae.

Three new sea cucumbers collected in the intertidal and subtidal zone of southern Japan, Taeniogyrus mijim sp. nov., Rowedota chippiru sp. nov., and R. motoshimaensis sp. nov., are described. All three species display a small body size (approximately 10–20 mm), light body colour (primarily a transparent solid colour), and a small number of digits in their tentacles (one pair or two). They are easily distinguished from each other and from other congeners by the number of digits and by ossicle size and shape. Among these species, the state of the tooth series on the inner margin of the wheel ossicles is an important defining feature, i.e., a continuous row in Taeniogyrus but interrupted by concavities in Rowedota. Also, each tooth is sharp in T. mijim, trapezoidal in R. chippiru, and semicircular in R. motoshimaensis. The number of teeth per radiant is 7–16 (means of 10–13 in different specimens) in T. mijim, 7–15 (means 9–12) in R. chippiru, and 8–21 (means 11–19) in R. motoshimaensis. Among these three new species, only R. chippiru has thick rod ossicles in the body wall.

Two mature specimens of the whale-fall lancelet Asymmetron inferum Nishikawa, 2004, were collected in 2016 from reducing gravel bottom near a hydrothermal vent, 196 m deep, at Oomuro Hole of Oomuro-dashi submarine volcano in the Izu Islands, Pacific coast of Japan by using the ROV Hyper-Dolphin during the RV Shinsei-maru KS-16-6 cruise of JAMSTEC. This species has so far been known exclusively from whale-fall communities off Cape Nomamisaki, SW Kyushu, Japan, 219–254 m deep. The new find suggests the dispersal stepping-stone hypothesis of whale-fall communities for the origin of vent and/or seep fauna.

Two specimens of Spottobrotula mahodadi Cohen and Nielsen, 1978 (Ophidiiformes: Ophidiidae), collected from the Andaman Sea, northeastern Indian Ocean, represent the second record of the species, previously known only from the holotype, also from the Andaman Sea. The fresh coloration of S. mahodadi is described for the first time. In addition, a single specimen of Hapalogenys bengalensis Mohapatra, Ray, and Kumar, 2013 (Perciformes: Hapalogenyidae) previously known only from its type locality off West Bengal, India (northern Bay of Bengal), was collected from the Andaman Sea and represents the first record of the species for that region.

A single specimen (31.6 mm standard length) of Plectranthias fourmanoiri Randall, 1980, previously known from Christmas Island (western Indian Ocean) and from the Mariana Islands south to Indonesia and east to the Pitcairn Islands (Pacific Ocean), was collected from Yoron-jima island, Ryukyu Islands, Japan. A full description is given and comparisons made. The specimen represents the first record of P. fourmanoiri from Japan and northernmost record of the species.

A poorly known sculpin, Atopocottus tribranchius Bolin, 1936 (Cottidae), was redescribed on the basis of 56 specimens from several localities in Japan; off Yamaguchi, Kyoto and Niigata Prefectures on the Japan Sea coast, and Kochi Prefecture on the Pacific coast, and in the northern East China Sea. The specimens from Yamaguchi, Kochi and the East China Sea represent the first specimen-based records for the species from those regions. Minute organs on the trunk surface, previously regarded as lateral-line pores, are most likely epidermal pits containing free neuromasts, and lateral-line pores are restricted to a row of embedded lateral-line scales on the anterodorsal portion of the body. Diagnostic characters of the monotypic genus, Atopocottus Bolin, 1936, are here reviewed on the basis of external and selected osteological characters, described for the first time. The genus is more similar to the Southern Hemisphere (Indonesia, Australia and New Zealand) genus Antipodocottus Bolin, 1952 than to Stlengis Jordan and Starks, 1904, known only from Japan, in having poorly developed cephalic sensory canals with few pores and the lateral-line scales in a simple canal without dorsal and ventral branches. However, Atopocottus differs from Antipodocottus in having fewer lateral-line scales (3–5 vs. 5–33 in the latter); the uppermost preopercular spine with 1–6 dorsal branches, their length and width becoming shorter and narrower, respectively, toward the spine tip, some of them with two spinous points (vs. 2–5 simple, similarly sized dorsal branches); and lacking a supraocular tentacle (vs. usually present). A more comprehensive examination of the two genera is necessary to determine their monophyly.

We employed molecular systematic analysis based on mitochondrial D-loop sequences to identify two specimens of ocean sunfish from Omani coastal waters that had previously been identified morphologically as Mola mola (Linnaeus, 1758) and M. ramsayi (Giglioli, 1883). Of the three molecular species that are recognized globally, Mola sp. A, Mola sp. B, and Mola sp. C, we confirmed that both specimens from Oman are Mola sp. A. This result suggested a broader distribution of this species than had ever been thought, but it also revealed a discrepancy between morphological and genetic characters in Mola taxonomy. We reexamined the morphology of the two specimens from Oman to clarify the extent to which the key morphological features correspond with molecular genetic identifications, and discovered errors in the original reports on both specimens.