We have described a new genus and five new species of free-living marine flatworms in Schizorhynchia (Platyhelminthes: Rhabdocoela: Kalyptorhynchia) based on the material collected from sandy intertidal zones around Hokkaido, Northern Japan. These include Freddius tricaudatus gen. et sp. nov. (Cheliplanidae) and Proschizorhynchella caudociliata sp. nov., P. magnoliae sp. nov., P. shibazakii sp. nov., and P. shuttlecock sp. nov. (Schizorhynchidae). Based on this study, the number of confirmed species in the suborder Kalyptorhynchia discovered in Japanese waters has increased from one to six. Freddius tricaudatus gen. et sp. nov. possesses proboscis hooks that are so peculiar among Cheliplanidae that they warrant the establishment of a new genus.

Seven (including six new) species of interstitial hoplonemerteans in the genus Ototyphlonemertes Diesing, 1863 are described from southern Vietnam. The descriptions are based on light microscopic examination of internal structures as well as three methods of molecular species delimitation analyses, i) Automatic Barcode Gap Discovery (ABGD), ii) Poisson Tree Process (PTP), and iii) statistical parsimony, on the basis of ~658-bp cytochrome c oxidase subunit I gene sequences that have been i) newly determined from the Vietnamese specimens and ii) deposited in the public databases for other Ototyphlonemertes species. Three taxa are circumscribed as ‘species groups’ and placed at an infra-subgeneric, supra-specific rank, each represented by O. duplex Bürger, 1895, O. macintoshi Bürger, 1895, and O. parmula Corrêa, 1950. Of the seven Vietnamese species, one is identified as Ototyphlonemertes (species group macintoshi) ani Chernyshev, 2007, which is molecularly confirmed to be distributed from Vietnam (Phu Quoc and Nha Trang) to Japan (Okinawa and Wakayama). The other six are new to science, to which formal taxonomic descriptions are given: these are O. (duplex) chernyshevi sp. nov., O. (duplex) envalli sp. nov., O. (duplex) norenburgi sp. nov., O. (parmula) lei sp. nov., O. (parmula) nakaoae sp. nov., and O. (parmula) tsukagoshii sp. nov.

A new species of flabelligerid polychaete, Lamispina ammophila sp. nov., is described from off the coast of Shimoda, Japan. The species can be discriminated from the other congeners by the following features: i) dorsal region of body being adhered to by sediment particles, ii) lamispines without accessory tooth, present from chaetiger 4 and succeeding chaetigers, and iii) cephalic cage 1.5–2.0 times as long as body width. A partial mitochondrial cytochrome c oxidase subunit I (COI) gene sequence from the paratype is provided as a DNA barcode for the new species.

A quadrannulate leech species, Orobdella angustata sp. nov., from Kii Peninsula, Honshu, Japan is described. This new species is characterized by its peculiar male atrial cornua. Phylogenetic analyses using nuclear 18S rRNA, histone H3, mitochondrial cytochrome c oxidase subunit I, tRNA^Cys, tRNA^Met, 12S rRNA, tRNA^Val, 16S rRNA, tRNA^Leu and NADH dehydrogenase subunit 1 markers showed that O. angustata is genetically close to the small quadrannulate species, O. brachyepididymis Nakano, 2016, O. kanaekoikeae Nakano, 2017 and O. naraharaetmagarum Nakano 2016, distributed in Chugoku District of Honshu and Shikoku, Japan.

A new species of the talitrid amphipod, Brevitalitrus pangkorensis, from a backshore forest of Pangkor Island, Malaysia, is named and described. This is the first record of Brevitalitrus Bousfield, 1971 from Malaysia. Brevitalitrus pangkorensis is morphologically similar to B. hortulanus (Calman, 1912), B. strinatii Stock, 1997, and B. wolffi Bousfield, 1971 in the characteristics of the uropods 1 and 2, and telson. However, the former is distinguished from the latter three species in having peduncle of pleopod 2 without distal plumose setae, relatively longer rami of pleopods 1 and 2, rami of pleopod 3 not distinctly articulated, inner and outer rami of uropod 1 with four and three marginal robust setae, respectively, outer margin of inner ramus of uropod 2 with one robust seta, and long robust seta on peduncle of uropod 3. Additionally, a partial DNA sequence of nuclear histone H3 from the holotype was determined. A key to the species of Brevitalitrus is also provided.

We establish the new anarthrurid genus Tsuranarthrura gen. nov. based on the new species T. shinsei sp. nov. collected from 1890 m depth off the eastern coast of Japan, northwestern Pacific Ocean. Tsuranarthrura gen. nov. is the third paratanaoid genus with members having a fused segment composed of pereonite 6 and the pleon, a character state previously restricted to the genera Coalecerotanais Larsen, 2003 (Family incertae sedis) and Metagathotanais Bird and Holdich, 1988 (Family Agathotanaidae). Among anarthrurid genera, Tsuranarthrura gen. nov. is similar to Anarthrura Sars, 1882, Anarthrurella Bird, 2004, and Crenicarpus Drumm and Bird, 2016 in having the labrum not laterally compressed and the antenna with naked fourth and fifth articles from the distal end. However it differs from the latter three in having pereonite 6 fused with the pleon, the maxillipedal endite with a distal seta, the chelipedal merus with a ventral simple seta, and the merus naked on pereopods 2 and 3. We also present the nucleotide sequence for part of the cytochrome c oxidase subunit I (COI) gene in T. shinsei for future use in DNA barcoding or phylogeny.

A new species of the genus Lasconotus Erichson, 1845 from Ishigaki-jima Island, Southwest Japan, is described. Lasconotus akitai sp. nov. is compared with the three previously known species of Lasconotus in Japan: L. niponius (Lewis, 1879), L. okadai Aoki, 2011, and L. sculpturatus (Sharp, 1885). The new species is characterized by small body size, large eyes, antennal segments III and IV same in size, four sharp ridges on pronotum, and bubble-shaped sculpture on ventral side of body. A key to the four Japanese Lasconotus-species is also given.

A molecular analysis based on mitochondrial COI+tRNA(Leu)+COII gene sequences has shown that unidentified larvae collected in Hunan, Hubei and Sichuan Provinces, China, belong to Onycholyda flavicostalis Shinohara, 2012, and O. sichuanica Shinohara, Naito and Huang, 1988 (Hymenoptera: Pamphiliidae), and that the latter species is conspecific with O. fanjingshanica Jiang, Wei and Zhu, 2004. Onycholyda fanjingshanica is thus synonymized with O. sichuanica. The first host plant records are Rubus adenophorus Rolfe (Rosaceae) for O. flavicostalis and Rubus spp. for O. sichuanica. The larvae of O. flavicostalis and O. sichuanica are briefly described and O. flavicostalis is newly recorded from Hubei and Sichuan Provinces.

A recent proposal to reduce the binomen Asymmetron lucayanum Andrews, 1893 to the synonymy of Brachiostoma pelagicum Günther, 1889 lacked supporting evidence, and is considered flawed due to the uncertain taxonomic status of the latter. Examination of the holotype of B. pelagicum was hindered by its highly deteriorated state, to the extent that gonad condition and hence generic affinity could not be unequivocally determined. However, the description of gonads on both sides of the body in the original description suggests correct placement of the species in Branchiostoma. Furthermore, a published figure of a likely syntype (pelagic larva) of A. lucayanum showed the specimen to possess an elongated urostyloid process, such being absent in the (pelagic) holotype of B. pelagicum comparable in size and morphology with the former. It is proposed that Asymmetron lucayanum Andrews, 1893 be reinstated as a valid species.

Lepadichthys akiko Allen and Erdmann, 2012, previously known only from the holotype from West Papua, Indonesia, is recorded from Japan and Palau for the first time, based on underwater photographs and two specimens, respectively. The latter (12.3 and 14.0 mm standard length) revealed new diagnostic characters of the species: viz., 3 of 5 gill arches with 2 filaments and 4 or 5 gill rakers on each arch; single nasal and postocular canal pores; and no lacrimal, preopercular or mandibular canal pores. The new specimens are described in detail, including a revised diagnosis and comparisons made with congeners.

Pterois antennata (Bloch, 1787) (Scorpaenidae: Pteroinae) was redescribed on the basis of 243 specimens from throughout the Indo-Pacific region, including the lectotype of Scorpaena antennata and holotype of Pterois ellioti Meek, 1897, a junior synonym of the former. The species is diagnosed by the following combination of characters: dorsal-fin rays usually XII, 11; pectoral-fin rays usually 17; more than ten narrow (but of irregular width) transverse lateral bands on body, including caudal peduncle; pectoral-fin membranes with 3–47 blotches; posterior portion of upper pectoral-fin rays (free of membrane) without bands. Geographic variation in the number of pectoral-fin blotches and ontogenetic morphological changes within P. antennata are also discussed in detail. The first record of Pterois paucispinula Matsunuma and Motomura, 2014 from French Polynesia reported.

Ammolabrus dicrus Randall and Carlson, 1997, originally described from 14 specimens (21.0–94.1 mm standard length; SL) from Oahu, has to date been collected only from Wake Atoll and the Hawaiian Islands. Two juvenile specimens (18.4–21.4 mm SL) of A. dicrus, collected at a depth of 12 m off Taketomi Island, Yaeyama Islands (Ryukyu Islands) and described here in detail, represent the first specimen-based records from Japan. Ontogenetic changes and geographic variations are noted.

The rarely caught grenadier, Coelorinchus sheni Chiou, Shao and Iwamoto, 2004, is newly recorded from Japanese waters, based on a single specimen (196 mm in head length, 692+ mm in total length) collected from the East China Sea off the north-west coast of Ie-jima island, Okinawa, at a depth of 650 m. It belongs to the C. tokiensis group, and readily differs from other Japanese congeners in having a series of dark saddles on the body, a fully-scaled underside of the head, and a short light organ anterior to the anus (its anterior tip falling far short of a line connecting the inner pelvic-fin bases), and in lacking a complete bony support of the lateral nasal ridge. The morphology of the Japanese specimen is fully described, with the first report of the species’ fresh coloration. Localities of type specimens of C. sheni are also emended. Coelorinchus sheni was previously known only from Taiwan; thus the Japanese specimen represents the first record of the species from outside Taiwanese waters and the northernmost record of the species.