The burrowing sea anemone, Metedwardsia akkeshi (Uchida, 1932), which was originally described from Lake Akkeshi, Hokkaido, and is the only species in the genus Metedwardsia Carlgren, 1947, was again collected from Akkeshi in 2015 and 2017, many decades after it was first described, and was also collected from Lake Furen and Lake Onne-to, both in Hokkaido, in the 2010s. Although the original description of M. akkeshi was comprehensive, it lacked information about the cnidom and detailed figures of each body part. Here, we re-describe M. akkeshi after thorough analyses of the newly collected specimens in detail, and based on these topotypic specimens, we also re-diagnose the genus Metedwardsia by providing more detailed information about characteristic morphological features and the cnidom. In addition, we discuss the possible location of “Akkeshi Cove”, the site where M. akkeshi was originally collected by Tohru Uchida.

Eleven new species of dicyemid mesozoans are described from two species of octopuses (Mollusca: Cephalopoda: Octopoda): Dicyema cryptocephalum sp. nov., Dicyema petalocephalum sp. nov., Dicyemennea acetabulum sp. nov., Dicyemennea anteronucleatum sp. nov., Dicyemennea mcconnaugheyi sp. nov., Dicyemennea megalosomum sp. nov., and Dicyemennea leptocephalum sp. nov. from Octopus longispadiceus (Sasaki, 1917) (Octopoda: Octopodidae) collected in the Sea of Japan, Honshu, Japan and four species, Dicyemennea desmocephalum sp. nov., Dicyemennea moritakii sp. nov., Dicyemennea tobaense sp. nov., and Dicyemodeca kukii sp. nov. from O. tenuicirrus (Sasaki, 1929) (Octopoda: Octopodidae) collected in the Kumano-nada Sea, off the Kii Peninsula, Pacific coast of Honshu, Japan. Dicyemnnea megalosomum sp. nov., at length 15,200 µm, is the largest of these newly described species. Dicyemennea moritakii sp. nov. has 47 peripheral cells that is shown in the most of all the described species. Some species demonstrate morphological characters in the calotte (anterior part) of their adult stage: thin and irregular in Dicyemennea leptocephalum sp. nov.; disc- or wheel-like in Dicyemennea acetabulum sp. nov., Dicyemennea desmocephalum sp. nov., Dicyemennea megalosomum sp. nov., and Dicyemennea moritakii sp. nov.; cap-shaped in Dicyemodeca kukii sp. nov.; conical in D. anteronucleatum sp. nov., Dicyemennea mcconnaugheyi sp. nov., and Dicyemennea tobaense sp. nov.; and with petal-like parapolar projections in Dicyema cryptocephalum sp. nov. and Dicyema petalocephalum sp. nov. Species Dicyemennea desmocephalum sp. nov. and Dicyemennea leptocephalum sp. nov. have an unusual property in which individuals adhere to each other at the periphery of their calotte. The co-occurrence patterns of dicyemid species with distinct morphological characters are briefly discussed in relation to adaptations to the renal habitat.

Tangenarchopsis nom. nov. is herein proposed for the digenean genus Tangiopsis Skrjabin and Guschanskaja, 1955 since the latter generic name is preoccupied by the hemipteran genus Tangiopsis Uhler, 1901.

Three new species of macrodasyidan gastrotrichs are described from the coasts of Hokkaido, northern Japan. Cephalodasys mahoae sp. nov. differs from congeners in having the oocytes developing from posterior to anterior. Turbanella cuspidata sp. nov. is characterized by a pair of small, ventrolateral projective organs at U06. Turbanella lobata sp. nov. is unique among congeners in having paired lateral lobes on the neck. We inferred the phylogenetic position of C. mahoae sp. nov. by maximum-likelihood analysis and Bayesian inference based on 18S rRNA, 28S rRNA, and COI gene sequences from 28 species of macrodasyids. In the resulting trees, C. mahoae sp. nov. formed a clade with an unidentified species of Cephalodasys Remane, 1926, but not with C. turbanelloides (Boaden, 1960).

A new kinorhynch species, Echinoderes antalyaensis sp. nov., is described based on specimens from Antalya, Turkey, eastern Mediterranean Sea. Echinoderes antalyaensis sp. nov. is characterized by the presence of a middorsal acicular spine on segment 4, laterodorsal tubes on segment 10, lateral accessory tubes on segments 5 and 8, lateroventral tubes on segment 2, and lateroventral acicular spines on segments 6–8, and by the absence of type-2 glandular cell outlets. The morphology of the ornaments of outer oral styles among Echinoderidae and the value of the character in the future taxonomic studies are discussed. The new species is the third Echinoderes species from Turkish waters, and the 14th species from the Mediterranean Sea.

Tanarctus ittanmomen sp. nov. (Heterotardigrada: Arthrotardigrada: Tanarctidae) is described based on specimens collected at Ohama Beach, Miyato-jima, Oku-Matsushima, Japan. The new species is distinguished from its congeners by its characteristic leg IV appendages. Each appendage consists of one long main branch, usually more than double the body length, with varying numbers of flexible, mostly long secondary branches. All branches have blunt tips.

We report the first record of the ammotheid pycnogonid species Hemichela nanhaiensis Wang et al., 2015, from Japanese waters. Four specimens of H. nanhaiensis were collected from the East China Sea at depths of 405–635 m, including the first females ever found. We redescribe male and female H. nanhaiensis from these specimens. We found that H. nanhaiensis is sexually dimorphic in the number of tiny lateral outgrowths on the lateral processes; length of the lateral process tubercles and oviger articles 4 and 5; ratio of the terminal claw to article 10 in the oviger; presence or absence of a proximal reversed spine on oviger article 5; thickness of the leg femur; and number of gonopores.

A new finless ophichthid eel, Apterichtus soyoae, is described based on a single specimen collected from off Tori-shima island, Zunan Islands of Izu Islands, southern Japan. The new species is similar to A. moseri and A. klazingai in its numbers of supratemporal pores, preopercular pores, and vertebrae. The new species differs from A. moseri in having more supraorbital pores (1+6 vs. 1+4), the number of branchings of the supraorbital canal (1 vs. 0), shape of the snout (distinctly pointed vs. relatively blunt), eye size (50% of snout length vs. 35–45%; 8.8% of head length vs. 6.3–8.0%), and the number of vomerine teeth (1 vs. 2–5). Apterichtus soyoae can also be distinguished from A. klazingai by the number of branchings of the supraorbital canal (1 vs. 2), the number of infraorbital pores (7 vs. 9), and the location of the lower jaw tip (anterior to a vertical through anterior margin of eye vs. posterior to the vertical). The number of supraorbital pores and branchings of the canal are discussed.

A single large specimen (613.0 mm standard length) of Epinephelus magniscuttis Postel, Fourmanoir, and Guézé, 1963, previously known from scattered localities of the Indo-West Pacific and the Philippines as the northernmost record, was collected at a depth of 50 m off Tanega-shima island in the Osumi Islands, southern Japan. The specimen represents the first record from Japan and the northernmost record of the species as well as the largest record as a museum specimen. The new standard Japanese name “Uguisu-gomadara-hata” is proposed for the species.

A single specimen (57.6 mm standard length) of Suttonia lineata Gosline, 1960, recently collected from Iwo Island, Volcano Islands, Japan and described herein, represents the first confirmed record of S. lineata from Japan and northernmost record of the species, which was previously known from Christmas and Cocos Keeling islands in the eastern Indian Ocean and various localities in the Pacific Ocean. The new standard Japanese name Kazan-kurenai-togemegisu, is proposed for the species.

Six specimens (48.1–75.1 mm standard length) of Opistognathus solorensis Bleeker, 1853, previously recorded from Palau, Papua New Guinea, Timor Leste, Indonesia, Brunei, the Philippines and Taiwan, were collected from the Osumi Islands, southern Japan. The specimens, which are described and compared with previous reports, represent the first records of the species from Japanese waters, a Tanega-shima island specimen being the northernmost known record of the species. The new standard Japanese name “Hoshikage-ago-amadai” is proposed for the species.

The distributional range of Xeniamia atrithorax Fraser and Prokofiev, 2016, previously recorded only from central Vietnam, is extended northward to Taiwan on the basis of seven specimens (20.5–31.1 mm standard length). A detailed description is provided for the specimens, including fresh coloration, which was previously unknown for the species.

Twelve specimens (131.5–180.7 mm in standard length: SL) of Epigonus lifouensis Okamoto and Motomura, 2013 (Perciformes: Epigonidae) were collected from off Sumatra, Indonesia. Also, a single specimen (67.1 mm SL) of E. pectinifer Mayer, 1974 was collected off Java, Indonesia. These Indonesian specimens represent the first records of both species from the eastern Indian Ocean.

A single male specimen (137.1 mm standard length) of the damselfish Abudefduf nigrimargo Wibowo et al., 2018, previously recorded only from Taiwan, was collected from Suwanose Island, Tokara Islands, southern Japan. The specimen represents the northernmost record for the species and first record from Japanese waters. The new Japanese name “Amime-oyabitcha” is proposed.

Amphidromous gobies of Sicyopterus (Gobiidae: Sicydiinae) are distributed in tropical, subtropical, and temperate streams in the Indo-Pacific region. Two species, Sicyopterus japonicus (Tanaka, 1909) and Sicyopterus lagocephalus (Pallas, 1770), are known from Japan. In the present study, two specimens of an additional species were collected in Okinawa Island, in southern Japan. We compared morphologies of the type series of Sicyopterus longifilis de Beaufort, 1912 and Sicyopterus brevis de Beaufort, 1912 collected in Indonesia, with the specimens from Okinawa Island to revise the taxonomy of these species and to identify the Okinawan specimens. Syntypes of S. longifilis and S. brevis share many characters, including a unique mouth morphology. Although fin morphologies, tooth number, and the shape of the urogenital papilla differ between S. longifilis and S. brevis syntypes, these amount to normal sexual dimorphism of sicydiine gobies. De Beaufort collected his specimens at the same locality on the same date. We conclude that the syntypes of S. longifilis and S. brevis are actually males and females of same species. Therefore, they are subjective synonyms, and we give precedence to the name S. longifilis as the first reviser under Article 24.2 of the International Code of Zoological Nomenclature (International Commission on Zoological Nomenclature 1999). The two specimens from Okinawa Island were identified as S. longifilis and this is the first record of this species from Japan.