We describe a new species of polyclad flatworm, Phaenoplana kopepe sp. nov., from Chichijima island in the Ogasawara Islands, Japan. This is the first report of Phaenoplana from Japan. Phaenoplana kopepe sp. nov. is characterized by i) a vagina that curves anteriorly, ii) gonopores well-separated from each other, and iii) a Lang’s duct that is shorter than the vagina. We provide a partial sequence (610 bp) from the mitochondrial cytochrome c oxidase subunit I gene as a DNA barcode for the species.

A specimen (114.4 mm SL) of the rare ophidiid species, Benthocometes australiensis Nielsen, 2010, was collected from a deep-sea water intake at a depth of 320 m off Cape Muroto, Kochi Prefecture, Shikoku Island, Japan. The species is previously only known from the holotype trawled off northwestern Australia. Thus, the present specimen is the first record from the Northern Hemisphere and the second record of the species. New Japanese names “Suisei-ashiro-zoku” and “Muroto-suisei-ashiro” are proposed for Benthocometes and B. australiensis respectively.

The parasitic mite Riccardoella (Proriccardoella) triodopsis Fain and Klompen, 1990 was sampled from two localities in Chiba prefecture, Japan. The sampled mites were identified based on palps and leg chaetotaxy and structure of famulus on tibia I. This study is represented by the second record of R. triodopsis inhabiting the pulmonate snail and the first discovery in Japan.

A single specimen of the platycephalid fish Thysanophrys papillaris Imamura and Knapp, 1999, collected from Nha Trang, Vietnam, represents the first record of the species from the western Pacific, previous records having been restricted to the Indian Ocean (Timor, Arafura and Andaman seas). Thysanophrys papillaris is separable from other members of the genus in having 11 second dorsal-fin rays, 12 anal-fin rays, 58–75 oblique body scale rows slanting downward and forward below the lateral line, the snout longer than the orbital diameter (ratio of snout length/orbital diameter 1.1–1.3), a single (rarely 2) small papillae on the upper eye surface, the upper iris lappet with short branches, 1 preorbital spine and 3–5 suborbital spines (usually 4). Thysanophrys haploblepharis Prokofiev, 2017, recently described from the western Indian Ocean, is regarded as a junior synonym of Suggrundus cooperi (Regan, 1908), also known from the western Indian Ocean.

Two specimens (174.0 and 567.7 mm standard length) of the barracuda Sphyraena qenie Klunzinger, 1870, collected from Tanega-shima and Amami-oshima islands, northern Ryukyu Islands, Japan, represent the reliable Japanese records of the species, for which the new standard Japanese name “Tatsu-kamasu” is proposed. Sphyraena qenie is similar to Sphyraena jello Cuvier, 1829 and Sphyraena putnamae Jordan and Seale, 1905 in lacking gill rakers on the 1st gill arch, and having more than 120 lateral-line scales and indistinct dark bars on the body. Sphyraena qenie differs from S. jello in having a deeply forked caudal fin with a pair of small lobes on the posterior margin in adults (vs. moderately forked without lobes in S. jello), a blackish caudal fin (vs. yellowish), and 18–22 dark bars crossing the lateral line on the body (ca. 11 indistinct dark bands dorsolaterally in individuals smaller than 360 mm SL). Sphyraena qenie also differs from S. putnamae in caudal-fin shape and lateral body markings, the former having indistinct lobes in large adults (generally lacking distinct lobes on the posterior caudal fin) and having 18–22 dark bars crossing the lateral line on the body (ca. 15 dark chevron markings crossing the lateral line on the body). Although, S. qenie has been suspected to be a junior synonym of Sphyraena nigripinnis Temminck and Schlegel, 1843, original drawing of S. nigripinnis showed a moderately forked caudal fin with two large, well developed lobes which differ significantly from that of S. qenie. The holotype of S. nigripinnis, collected from Japan, has apparently been lost and due to no Indo-Pacific sphyraenid species clearly consistent with S. nigripinnis, the taxonomic status of S. nigripinnis still unclear.

Four species of the family Dulichiidae (Crustacea: Amphipoda) are described from Japan. The first species is Dulichia biarticulata Hirayama and Takeuchi, 1993, which is characterized by the 2-articulate accessory flagellum and the expanded basis of the pereopod 4. The occurrence of this species from Miyagi–Osaka Prefectures and Fukui Prefecture is the first record from localities other than the type locality, and the morphological characters of the female are also described for the first time. The second species, Dulichia latimana sp. nov., was collected from the Ariake Sea and Sagami Bay. In this species, the male has a robust gnathopod 2 with a short proximal projection on the propodus and the slightly expanded basis of the pereopod 4. The third species is Dulichiopsis barnardi Laubitz, 1977 from off Fukushima Prefecture, which has the eyes composed of many dispersed ommatidia, the propodi of the pereopods 5–7 with a palm and the uropod 2 outer ramus about half length of the inner ramus. The morphological characters of the male are firstly described. The last species is Metadulichia kohtsukai gen. nov., sp. nov. from Sagami Bay. This species can be distinguished by the small maxilla 1 with 7 terminal robust setae on the outer plate, the maxilla 2 inner plate bearing single mediofacial seta, and the extremely short outer ramus of the uropod 2. A key to Japanese species of the family is provided.

We describe Parakonarus kajii sp. nov. from near Iriomote Island, Ryukyu Islands, southwestern Japan. This is the first record of Parakonarus from the North Pacific. As members in this genus show great sexual dimorphism, we checked our male and female specimens for conspecificity by using partial sequences of the cytochrome c oxidase subunit I (COI) gene. The two individuals differed by two nucleotide substitutions in 661 nt (0.3% K2P distance), indicating they are the same species. This species closely resembles P. kopure reported from New Zealand, but differs in having the ventral process on the chelipedal carpus longer than wide, the inner comb-row on the chelipedal propodus composed of 16 setae, and the dactylus and unguis of pereopods 4–6 fused in the male; and the carpi of pereopods 2 and 3 without a ventrodistal spiniform seta, the propodal palm of the cheliped with two inner simple setae at the insertion of the dactylus, and article 2 of the maxillipedal palp with one mid-outer and one outer distal simple setae in the female. On the inner surface of the chelipeds in male P. kajii, we observed an elongate plate-like structure that could be a reduced ischium.

A new species of free-living marine nematode (Chromadorida: Chromadoridae), Graphonema antarcticum sp. nov., is described. The nematode was found in the shallow water of Kita-no-ura, off Syowa Station in Lützow-Holm Bay, Antarctica. Graphonema antarcticum sp. nov. is similar to G. metuliferum Kito, 1981 in the ratios of the spicules length to the body diameter at the cloaca, the well-developed gubernaculum with L-shaped lateral pieces bending at an obtuse angle and equipped with minute denticles at the distal end, and the tail length in both sexes. However, G. antarcticum sp. nov. differs from the latter in having a larger body size and presence of the lateral differentiation. The new diagnosis of the genus and the key to the species are provided. Graphonema achaeta Platonova, 1971 was transferred to Endeolophos due to the absence of the lateral pieces of gubernaculum that are present in the genus Graphonema.

In 2004, Uchida published a partial revision of the family Hesionidae in Japan, where four new genera were proposed and 13 species newly described. However, both the holotype fixations and their depository numbers were omitted in the original article, rendering the names unavailable after Article 16.4 of the International Code of Zoological Nomenclature. In this work, the names are made available by clarifying the depository of the types. We provide diagnoses for the four new genera (Synsyllidia Uchida, Uncopodarke Uchida, Parahesiocaeca Uchida, and Ichthyohesione Uchida) and 13 new species (Synsyllidia alternata Uchida, Oxydromus brevipodius Uchida, O. bunbuku Uchida, O. constrictus Uchida, O. fauveli Uchida, O. longifundus Uchida, O. okudai Uchida, O. parapallidus Uchida, Heteropodarke kiiensis Uchida, Uncopodarke intermedia Uchida, Microphthalmus itoi Uchida, Parahesiocaeca japonica Uchida, and Ichthyohesione gorgasiae Uchida). A key to species of Hesionidae recorded to date from Japan is also presented.

Riccardoella (Proriccardoella) reaumuri Fain and van Goethem, 1986 (Ereynetidae Oudemans, 1931) is one of the parasitic mites reported from lungs of land mollusks. This paper provides a morphological redescription of this species, with some information of leg setation and chaetotaxy, lyrifissures and aggenital region. Riccardoella reaumuri is distinct from and can be distinguished from the other seven species of genus Riccardoella Berlese, 1923 by the following features: all setae on idiosoma, palptarsi and legs not prolonged with apical filament; palptarsus with three setae; tibia I with five setae; femur I with one seta; trochanters I and III each with one seta; coxae I and III each with two setae; famulus forked distally and ca. 60% long of its guard seta; two pairs of pseudanal setae.

In 2017, we collected a pair (male and female) of topotypic individuals of the parasitic isopod Pseudione nephropsi Shiino, 1951 from a single nephropid lobster, Metanephrops japonicus (Tapparone-Canefri, 1873), at 310–298 m depth off the eastern coast of the Kii Peninsula, Japan. Based on these specimens, we provide a supplementary description of the male and female morphology, and present mitochondrial cytochrome c oxidase subunit I (COI) DNA barcode sequences. The sequence length amplified with primer pair LCO-1490/HCO-2198 was 658 nt, and there were four synonymous nucleotide substitutions (0.6% p-distance) between the two sequences.