Four specimens (168.6–204.4 mm standard length) of Careproctus lycopersicus Orr, 2012, previously recorded from the Bering Sea and eastern Aleutian Islands, were collected from the southern Sea of Okhotsk (the Nemuro Strait, eastern Hokkaido, Japan). These specimens represent the first records of the species from the western North Pacific. A detailed description is provided for the specimens, including the intraspecific variations. The new standard Japanese name “Tomato-kon’nyaku-uo” is proposed for the species.

The subgenus Setihenricia Chichvarkhin and Chichvarkhina, 2017 currently consists of 32 out of 94 species of sea stars in the genus Henricia Gray, 1840. However, only 10 have been molecularly ascertained as to the phylogenetic position. Based on seven mitochondrial gene markers (16S rRNA, COI, tRNA-Ala, tRNA-Leu, tRNA-Asn, tRNA-Gln, and tRNA-Pro), we performed molecular analyses to assess the monophyly of Setihenricia using 16 species of Henricia from Japanese waters, as well as other congeners for which sequences were available in public databases. The monophyletic Setihenricia was only weakly recovered because the support values were low, but we assume this taxon may be valid in consideration of a morphological key. Henricia kinkasana Hayashi, 1940 is herein identified as a new member of Setihenricia, based on molecular and morphological evidence. Sequences in the public databases previously identified as H. compacta (Sladen, 1889) and H. obesa (Sladen, 1889) were nested in the Setihenricia clade; these two species are also likely members of Setihenricia. Our morphological assessment yielded three findings: (1) the number of the rows of spines on each inferomarginal plate, being three or more, appears to be a useful diagnostic character for Setihenricia, as has been suggested in previous studies, (2) the degree of spine tapering turned out to be less relevant in diagnosing the subgenus than previously supposed, (3) two newly introduced characters—(i) the width of adambulacral plates and (ii) the ratio of two measurements in ambulacral plates to the length of interradius—may be effective to distinguish members of this apparent Henricia subclade from other members of the genus.

We describe a new species of polyclad flatworm, Prosthiostomum torquatum sp. nov., from the rocky intertidal zone in Shirahama, on the Pacific coast of middle Honshu, Japan. Prosthiostomum torquatum is characterised by a unique dorsal colouration, which is comprised of i) numerous orange maculae and blue dots dispersed all over the dorsal surface, ii) a transverse dark-brown line in the anterior part of the body running slightly anterior to another transverse white line, both of which are slightly bent backwards at mid-point, and iii) an incomplete, mesh-like, median band made by dark-brown pigments, fading away posteriorly. By this dorsal colour pattern, the new species can be distinguished from similar congeners such as P. trilineatum Yeri and Kaburaki, 1920 and P. komaii Kato, 1944. We performed a molecular phylogenetic analysis based on 462-bp partial mitochondrial cytochrome c oxidase subunit I (COI) sequences of four species of Prosthiostomidae currently available in public databases in addition to that of P. torquatum. In the resulting tree, P. torquatum was sister to Lurymare clavocapitata Marquina, Aguado, and Noreña, 2015 originally described from Lizard Island, Australia. While these two share a similar dorsal colouration, P. torquatum can be distinguished from L. clavocapitata by i) the absence of a common muscle bulb/sheath enclosing the whole male copulatory apparatus, ii) the median mesh-like band comprised of dark-brown pigments, and iii) COI uncorrected p-distance being 0.094. As a result, our phylogenetic tree indicates the possibility that Prosthiostomum and Lurymare as currently diagnosed may not be monophyletic, and that the common muscle bulb enclosing the whole male copulatory apparatus may not be appropriate as a diagnostic character for Lurymare.

Neoheterocotyle quadrispinata n. sp. (Monogenea: Monocotylidae) is described from the gills of Rhinobatos hynnicephalus Richardson, 1846 (Rhinopristiformes: Rhinobatidae) in the central Seto Inland Sea, Hiroshima Prefecture and the southern Sea of Japan, Fukuoka Prefecture, Japan. This is the first report of a Neoheterocotyle species from Japan. The new species is morphologically most similar to N. forficata (Timofeeva, 1981) and N. rhinobatidis (Young, 1967) in having two pairs of dorsal haptoral accessory sclerites but differs from them in that the penis is curved proximally and the accessory piece has a blade-shaped process.

The primarily Indo-West Pacific echiuran Listriolobus brevirostris Chen and Yeh, 1958 was collected, sometimes abundantly, from benthos samples from northeastern Osaka Bay (from 6.4–17.5 m depth) between 1999 and 2001, or recorded in the field between 1995 and 2002. Since 2003, it has remained uncollected. These records were shown to represent the first occurrence of L. brevirostris in Japanese waters, being the first reliable evidence so far determined of the introduction of an exotic echiuran species, probably in ballast water, to any locality worldwide. Aquarium observations suggested a completely infaunal adult life mode, including feeding activity. Monthly changes in size distribution of specimens collected in 1999 indicated successful summer survival under oxygen-depleted bottom conditions in Osaka Bay. Factors behind the ephemeral occurrence of L. brevirostris in Osaka Bay were briefly considered in relation to annual fluctuations of dissolved oxygen content in bottom water, although the driving forces behind such occurrence remain to be established.

Members of the genus Glaphyrostomum Braun, 1901 (Trematoda: Brachylaimidae) are parasites of birds. However, an exception occurs in Glaphyrostomum soricis Asakawa, Kamiya and Ohbayashi, 1988, which was described from the long-clawed shrew, Sorex unguiculatus Dobson, 1890, in Hokkaido, Japan. A recent DNA barcode-based trematode survey of land snails clearly showed that Ainohelix editha (A. Adams, 1868), a bradybaenid snail indigenous to Hokkaido, serves as the first and second intermediate hosts for a species of the genus Pseudoleucochloridium Pojmańska, 1959 (Panopistidae). Its adult stage was furthermore confirmed from S. unguiculatus. A comparison of adult morphology between Pseudoleucochloridium sp. and G. soricis revealed that both should be considered the same species. However, Pseudoleucochloridium soricis comb. nov. cannot be applied because P. soricis (Sołtys, 1952) already exists as the type species of the genus. We, therefore, propose Pseudoleucochloridium ainohelicis nom. nov. as a replacement name for G. soricis.

Although Scorpaena dabryi Sauvage, 1878, originally described on the basis of two specimens from China, has recently been regarded as a senior synonym of Neomerinthe procurva Chen, 1981, examination of the specimens (herein designated as lectotype of S. dabryi) showed them to be consistent with the holotype and non-type specimens of S. miostoma Günther, 1877. Accordingly, S. dabryi is regarded as a junior synonym of S. miostoma. Morphological ontogenetic changes of some body proportions in S. miostoma are also described.

The genus-group name Curtipleon Bǎcescu, 1976 is not available due to Bǎcescu’s failure to fix its type species in the original publication. This genus-group name should be attributed to Sieg (1983)—the work accidentally validated the genus-group name under Article 13 of the International Code of Zoological Nomenclature.

We describe Teleotanais madara sp. nov. from the surfaces of mangrove roots on mudflats at Iriomote Island, Ryukyu Islands, southwestern Japan. This is the first record of any teleotanaid species from the northwestern Pacific. Teleotanais madara resembles T. gerlachi and T. gerlachi sensu Sieg (1976), but differs in having a mid-inner simple seta on antennular article 2, two ventral setae on the basal article of the uropod, the chelipedal fixed finger with one or two ventral simple setae, ventrodistal simple setae on the perepod-1 merus being half as long as the width of the merus, and the dactylus–unguis of pereopods 4–6 articulate. We also present the nucleotide sequence for part of the cytochrome c oxidase subunit I (COI) gene in T. madara for future use in DNA barcoding and phylogeny.

Two specimens (68.8–72.3 mm standard length) of Ostorhinchus chrysotaenia (Bleeker, 1851), previously recorded from Indonesia, northwestern Australia, Papua New Guinea, Solomon Islands, northern Borneo, Philippines, and Taiwan, and recently collected off Yaku Island, Osumi Islands, Kagoshima Prefecture, Japan, represent the first records of the species from Japan supported by voucher specimens, and the northernmost distributional record of the species. The holotype and an additional non-type specimen of O. chrysotaenia were determined from two Bleeker specimens registered under the same catalog number.

The cymothoid isopod Nerocila trichiura (Miers, 1877) is reported based on an ovigerous female from the ventral body surface of a flyingfish, Cypselurus hiraii Abe, 1953 (Beloniformes: Exocoetidae), in the coastal Pacific waters of central Japan. This represents the first record of N. trichiura from Japan, and C. hiraii is a new host record for this isopod. Nerocila trichiura has been reported from the tropical and middle-latitude waters of the Indian and Atlantic oceans and was recorded in 1881 from the Philippines. This paper confirms that the species occurs in the western North Pacific Ocean.

A single female specimen (197.2 mm standard length) of the armored searobin Peristedion richardsi Kawai, 2016, previously recorded only from Indonesian waters, but recently collected from Pingtung County, southern Taiwan, represents the largest example known and first Northern Hemisphere record of the species. A detailed description of the specimen is given, with notes on intraspecific variation in P. richardsi.

A new species of podocerid amphipod, Podocerus jinbe, is named and described. This new species was collected from the gill rakers of the whale shark Rhincodon typus Smith, 1828 from off Yomitan Village, Okinawa Island, Japan. This is the first record of an amphipod associated with the whale shark. Podocerus jinbe sp. nov. is morphologically similar to P. zeylanicus (Walker, 1904), but differs from the latter by its larger body size, shorter peduncular article 1 of antenna 1, longer flagellar article 1 of antenna 1, subrectangular propodus of male gnathopod 1, anteriorly concave basis of male gnathopod 2, narrow merus of female gnathopod 2, greater number of robust setae on rami of uropods 1 and 2, and greater number of long robust setae on the telson apical lobe. Additionally, a partial DNA sequence of the mitochondrial cytochrome c oxidase subunit I (COI) of this species was determined for future studies.

In the present study, we describe two new marine species, Microloxoconcha toyoshioae n. sp. and M. sublittoralis n. sp., collected from the sandy bottom in the western part of Japan, from the depths of approximately 20 to 30 m and 50 m, respectively. This represents the first record of this genus from such depths, since all 10 previously described species are known from the beach interstitial waters. We suspect the two new species are most probably also inhabiting spaces between sand grains since they have very small body and have been collected with other common interstitial ostracods belonging to Cobanocythere Hartmann, 1959; Parvocythere Hartmann, 1959; Paracobanocythere Gottwald, 1983; Parapolycope Klie, 1936; and Psammocythere Klie, 1936. Owing to their small body and fragile carapace, previous studies might have overlooked the existence of interstitial ostracods from habitats other than beach interstitial. In addition, a key to all species of the genus Microloxoconcha is provided.

Shallow-water comatulids were collected by scuba diving from Lombok and Ambon islands, Indonesia in 1992 and 1994. The collection consisted of a total of 35 species in 20 genera among Comatulidae, Himerometridae, Mariametridae, Colobometridae, and Antedonidae. Two species were newly recorded from Indonesian waters.

Iwogumoa songminjae (Paik and Yaginuma in Paik et al., 1969) is recorded for the first time from Japan on the basis of specimens collected in the Tsushima Islands, Kyushu and previously identified as I. insidiosa (L. Koch, 1878). The relationships among the Japanese species of Iwogumoa Kishida, 1955 are analyzed using mitochondrial cytochrome c oxidase subunit I with various Japanese species of Coelotes Blackwall, 1841 sensu lato as the outgroup. A new synonym is proposed through a comparison of morphological characteristics and the results of our molecular analysis: Coelotes iyoensis Nishikawa, 2009 (known only from the male) is a junior synonym of Coelotes mohrii Nishikawa, 2009 (known only from the female).

Two specimens of Lepidotrigla sp., collected in 1990 from Tosa Bay and the South China Sea, were identified as Lepidotrigla pectoralis Fowler, 1938, described from a single (holotype) specimen. A full description of the former specimens is given, including the first account of fresh coloration in the species. Lepidotrigla pectoralis is uniquely characterized among congeners in the following combination of characters; rostral projection round, comprising many minute spines; snout long, slightly convex in lateral profile; posterior half of first dorsal fin with a large black oval blotch dorsally; inner surface of pectoral fin black on upper half, whitish on lower half. The specimen collected from Tosa Bay is the first confirmed record of L. pectoralis in Japanese waters.

A new species of pseudocrangonyctid amphipod, Pseudocrangonyx uenoi, is described from cave streams in Taga town, central Honshu, Japan. Previous molecular phylogenetic studies reported the taxonomic status of the Taga populations of Pseudocrangonyx as an undescribed species. We present the diagnostic features to discriminate P. uenoi from the other congeners on the basis of morphological study. Pseudocrangonyx uenoi is closely related to P. daejeonensis Lee, Tomikawa, Nakano, and Min, 2018, but clearly distinguished from the latter by the presence of calceoli on female antenna 2, the 5-dentate lacinia mobilis of the left mandible, setation of the peduncular articles of pleopods, the number of articles of rami of the pleopods, a shorter terminal article of uropod 3, and a telson with a deeper cleft.

A new species of the thalassematid echiuran Ikedosoma abemama sp. nov. is described based on a single specimen collected from Abemama Atoll, the Gilbert Islands, Kiribati. The species is particularly distinguishable from all congeners by the absence of the proboscis lower lip. A single specimen of Ikedosoma collected from Dam Bay, Nha Trang, Vietnam is also described, being similar to the new species, although its poor condition prevented a positive identification. The first confirmed records of Ikedosoma from the tropical Pacific Ocean are included (hitherto known only from temperate waters of Japan), in addition to a summary table of diagnostic characteristics and a dichotomous key to the species of Ikedosoma.

Acoetid polychaetes were collected from shallow waters off Kochi Prefecture and Wakayama Prefecture, Japan. The specimens could be assigned to the genus Polyodontes Blainville, 1828, but are different from all the known species of the genus by the following characters: ommatophores with short neck; palps with minute papillae; parapodia without branchiae; the acicular neurochaetae always lacking aristae and having hairs only on a small area of their subdistal end; and antennae with brown dots. They are herein morphologically described as a new species and a partial 658-bp COI sequence as a DNA barcode is provided for future taxonomic studies.

Japanese species of the genus Pyracmon Holmgren, 1859 are reviewed. A new species, P. monticola sp. nov. is described. Pyracmon sepiellus (Holmgren, 1860) is newly recorded based on a specimen collected from Hokkaido. Pyracmon nigrifemur Barron and Walley, 1983 is also recorded from Kyushu for the first time. A key to Japanese species of this genus is provided.

A single specimen of Thrissina chefuensis (Günther, 1874) (88.0 mm in standard length), collected from the Japan Sea coast off Shimonoseki City, Yamaguchi Prefecture, Japan, and representing the easternmost record of the species and first from Japanese waters, is described in detail and comparisons made with closely related species. The suitability of previously proposed Japanese names is questioned, and the new name “Tairiku-katakuchi” is proposed.