Overlooked since its establishment, Ascidia sydneiensis protecta Van Name, 1945, originally described as having mantle musculature comprising short parallel fibers restricted to the dorsal margin, compared with musculature along the entire margin (except for a central muscle-free area) found in the so-called “A. sydneiensis group”, is treated as a full species, Ascidia protecta. A detailed examination of the type and non-type specimens of the latter, all collected from the Gulf of California, confirmed the mantle musculature arrangement in the species, as well as revealing several new features (particularly in the alimentary tract), although not supporting a protective function of the anterior tunic to retracted siphons, as suggested in the original description. A comparison of A. protecta was made with congeneric species with similar musculature.

Five species of sea stars of the families Ophidiasteridae and Goniasteridae including two new species, Bathyferdina caelator sp. nov. and Fromia labeosa sp. nov., were collected by dredging from the mesophotic zone of the Ogasawara Islands, Japan. Bathyferdina caelator is distinguished from B. aireyae Mah, 2017 by the presence of glassy bosses (crystal bodies) on actinal and adambulacral plates and the absence of them on marginal plates. Fromia labeosa has large, elliptical pedicellariae on the actinal plate and is further distinguished from its congeners based on characters of the abactinal and superomarginal plates, granules, actinal papulae, furrow spines, and the subambulacral spines. We also provide descriptions for three mesophotic species, Fromia eusticha Fisher, 1913, Ogmaster capella (Müller and Troschel, 1842), and Tamaria tenella (Fisher, 1906), which are poorly studied in Japanese waters.

The rare Japanese deep-sea snailfish Careproctus rhodomelas Gilbert and Burke, 1912 is redescribed based on the holotype and 10 newly-collected specimens (30.0–131.4 mm standard length), with notes on morphological variations in pelvic disk size and body coloration caused by ontogenetic development. An updated distribution of the species is also provided, including the first voucher-supported records off eastern Miyakejima Island and the Hatoma Knoll in the southern part of the Okinawa Trough.

Six specimens of Anguilla luzonensis Watanabe, Aoyama, and Tsukamoto, 2009 were collected from Okinawa-jima Island, Okinawa Prefecture, southern Japan. This species was previously known from Luzon Island, Mindanao Island and Taiwan. Therefore, the present specimens represent the first records of A. luzonensis from Japan and extend the northern distribution limit of the species. A new standard Japanese name, “Uguma-unagi”, is proposed for the species.

One female specimen of Stiphodon multisquamus Wu and Ni, 1986 was collected from Arida-gawa River, Wakayama Prefecture, Japan, in November 2018. As the previously known range of this species was from the South China Sea, including Malaysia, Vietnam, and China, to the Ryukyu Archipelago in Japan, this specimen was reported as the first record from the Japanese mainland and the northernmost record of this species. Stiphodon multisquamus is an amphidromous fish of which pelagic larvae develop in marine habitats. It was believed that this specimen was transported from the southern region by ocean currents during the pelagic larval phase and recruited into the river in Wakayama Prefecture.

A single specimen of surgeonfish (116.1 mm standard length) was collected from Minamidaitojima Island, southern Japan, and identified as Acanthurus nigros Günther, 1861 on the basis of morphological and genetic analyses. This specimen represents the northernmost and westernmost record of the species, about 1500 km from the nearest island (Pagan Island, Northern Mariana Islands) with previously confirmed record for this species. The new standard Japanese name “Nangoku-kurohagi” is proposed herein.

Two new species of free-living marine nematodes, Odontophora odontophoroides sp. nov. and Parabathylaimus jare sp. nov., are described from the coastal sand of Langhovde, Lützow-Holm Bay, Dronning Maud Land, Antarctica. Odontophora odontophoroides sp. nov. is the only species in Odontophora Bütschli, 1874 that has bicuspidate odontia. Odontophora odontophoroides sp. nov. is similar to species in Odontophoroides Boucher and Helléouët, 1977 and Synodontium Cobb, 1920 in having bicuspidate odontia, but differs in having a didelphic reproductive system in females. Parabathylaimus jare sp. nov. differs from congeners in the unjointed inner labial and cephalic sensilla, the outer labial sensilla three-jointed in males and two-jointed in females, the position of the amphids, the shorter spicules, and the conical tail without long subterminal setae. Modified generic diagnoses and keys to species are included for Odontophora and Parabathylaimus De Coninck and Schuurmans Stekhoven, 1933. A new combination, Parabathylaimus arthropappus (Wieser and Hopper, 1967) comb. nov., is established.

A new species of the sand-burrowing dogielinotid amphipod genus Haustorioides Oldevig, 1958, Haustorioides furotai Ogawa, sp. nov., is described from Banzu tidal flat, east coast of Tokyo Bay, Japan. The new species is mainly characterized by a slender and triangular outer plate of the maxilliped, poorly setose antennae and pereopods, straightly acute posterodistal corner of epimeral plates 2 and 3, and an uncleft and ridged telson. Analyses of the mitochondrial cytochrome c oxidase subunit I sequences verified that H. furotai belongs to the monophyletic clade consisting of the other Haustorioides species. Additionally, Eohaustorioides Bousfield and Tzvetkova, 1982, and Parhaustorioides Ren, 2006 are synonymized with Haustorioides based on morphological analysis. An identification key to the species of the genus Haustorioides is also provided.

Two species of the paracalliopiid Amphipoda are recorded from the Ryukyu Archipelago in Japan, one of which is Mucrocalliope ryukyuensis sp. nov., the second species of the genus. This new species is very similar to the type species, M. shimantoensis Ariyama and Azuma, 2011 from the Shimanto estuary, western Japan; however, M. ryukyuensis can be distinguished from M. shimantoensis by the longer peduncular article 4 of the male antenna 2 and the basis of the male pereopod 5 with a posteromedial plumose seta. The other species is Paracalliope dichotomus Morino, 1991; its morphology in female and the coloration are firstly described.

We describe a new species of marine associated oribatid mite, Ameronothrus twitter Pfingstl and Shimano, sp. nov., from a wharf of fishing port (Choshi Outer Fishing Port), Chiba, Honshu, Japan. This species was spotted for the first time ever on photographs posted on Twitter. Ameronothrus twitter Pfingstl and Shimano, sp. nov. is characterized by a specific cuticular notogastral pattern consisting of distinct nodules that are fused to irregular ridges in the median anterior and the median posterior region. Additionally, it features a unique combination of fused lamellar keels, presence of primilateral setae on tarsus I, presence of dorsal companion setae on genu I, II, III and all tibiae and tarsal distal setae ending with a small nodule. Ameronothrus twitter Pfingstl and Shimano, sp. nov. is the second species of this genus found on Japanese islands, which represents the southernmost occurrences of this taxon in the Asian Pacific region.

Hirsutocrinus duplex, a new genus and new species of the Bathycrinidae, collected from Okinawa, Japan at a depth of 596–606 m, is described. The main diagnostic characters of the new genus are the presence of side plates in pinnules and of knobby processes on Brs 1–2. Knobby processes on secundibrachials are found for the first time. Monachocrinus A. H. Clark, 1913 shares side plates with Hirsutocrinus. It differs from the new genus in having knobby processes on IBrs 1, parallel ridges on the articular surface of knobby processes, proximal and distal arm pattern a b+c d+e f, saccules, in lacking knobby processes on IBrs 2 and Brs 1–2, pinnule on every second Br, x-shaped tube-feet plates, needle-like spines on external surface of IBrs and Brs. The cover and side plates are similar to each other in Monachocrinus, and quite different in Hirsutocrinus. Hirsutocrinus duplex is the shallowest species in the abyssal family Bathycrinidae usually known from 1100 to 9735 m. Other than H. duplex, only three among 25 nominal bathycrinid species are known from depths less than 1000 m.

Three new proturan species, Eosentomon villare sp. nov., Pseudanisentomon nasuense sp. nov., and P. villaticum sp. nov., collected from the Nasu Imperial Villa, Tochigi Prefecture, central Japan are described. Eosentomon villare sp. nov. is characterized by the absence of foretarsal sensillum b′1, a long empodium on the hind tarsus, four pairs of anterior setae on abdominal tergite VII (A1, 2, 4, and 5), two anterior and seven posterior setae on sternite VIII, and six setae on sternites IX–X. Both P. nasuense sp. nov. and P. villaticum sp. nov. also lack foretarsal sensillum b′1. Pseudanisentomon nasuense sp. nov. has long empodia on the middle and hind tarsus and five pairs of anterior setae on abdominal tergite IV. Pseudanisentomon villaticum sp. nov. is distinguished from its congeners by the number of anterior setae on abdominal tergite VII (three pairs: A2, 4, and 5), the absence of foretarsal sensillum c′ and seta x, and rudimentary setae 1 and 2 on tergite XI. In addition to descriptions of these new species, an updated key to species of Pseudanisentomon Zhang and Yin, 1984 is provided.