In this paper, we describe two new species of dajid isopods found enveloping the antennules of decapod hosts: Akrophryxus acinaces sp. nov. parasitizing Pycnoplax surugensis (Rathbun, 1932) (off Pacific coast of central Japan, approximately 200 m depth) and A. pallipalicus sp. nov. parasitizing Parapalicus armatus Castro, 2000 (South China Sea, approximately 410 m depth). Females of both species exhibit the characteristic spheroid shape found in the genus Akrophryxus Williams and Boyko, 2021. Females of the three species in Akrophryxus can be distinguished by several characters, including form of the pleon and oostegite 5. Males can be distinguished by head/pereomere 1 to pereomere 2 ratio and posterolateral margin shape, and pereopod 6 dactylus and ischium size. The three species of Akrophryxus not only differ in morphological characters but parasitize species belonging to three different heterotreme brachyuran families. The first described cryptoniscus larval stage for any species in the genus is described for A. acinaces sp. nov. We also describe a new cryptoniscoid, Chimaeroniscus spheramator gen. et sp. nov., the first hyperparasite described from any dajid host (A. pallipalicus sp. nov. on the host Pa. armatus). This hyperparasite is a putative egg predator of its dajid host and exhibits a unique combination of generic level characters including: pleon markedly narrower than pereon, antennule with large teeth on basal segment and with setal brush, 3 or 4 coxal teeth on all pereomeres, pereopods 3–7 isomorphic and dissimilar from pereopods 1 and 2, telson with smooth distomedial projection, and uropodal exopods half-length of endopods. Finally, we address some taxonomic issues within Cryptoniscoidea, moving two genera (Neritoniscus Schultz, 1977 and Cryptocisus Schultz, 1977) known only by cryptoniscus larval characters from Cryptoniscoidea incertae sedis to Dajidae and synonymize Cryptonus Schultz, 1977 with Holophryxus Richardson, 1905.

A giant phyllosoma larva, assigned to the final stage, was collected from off Peru. This specimen was determined to be Projasus bahamondei George, 1976 by DNA barcoding, and morphological description was given for the first time in the genus Projasus George and Grindley, 1964. The present phyllosoma was characteristic of having very wide cephalic shield (about 2.3 times of thorax width), median rostrum, eminent setose exopods on the second and third maxillipeds, and five stout spines on basial endite of maxillule. Projasus phyllosoma has morphological characteristics of both ‘Silentes’ and ‘Stridentes’ groups in the family Palinuridae.

We describe a new species of the rare and deep-sea family Myxasteridae from Japanese waters. This is the first record of this family in the western North Pacific. The new species, Asthenactis agni n. sp., differs from all congeners in having seven arms with an R/r ratio of 7.2, primary radial plates with convex distal edges, and two to three actinolateral spines that are up to 4.7 mm long.

A taxonomic review of Albula Scopoli, 1777 (Albuliformes: Albulidae) in Japanese and adjacent waters, based on morphology and mtDNA cytochrome b sequence data, resulted in the recognition of four species: Albula argentea (Forster, 1801), Albula glossodonta (Forsskål, 1775), Albula koreana Kwun and Kim, 2011, and Albula oligolepis Hidaka, Iwatsuki, and Randall, 2008. Although Japanese ichthyologists have long considered A. glossodonta and a second Albula species (referred to by the Japanese name “Sotoiwashi”) to be distributed in Japanese waters, the latter having been reported as A. koreana or Albula sp. in recent literature, the present study revealed that, in fact, “Sotoiwashi” included three species, viz., A. argentea, A. koreana and A. oligolepis. Examined specimens of the latter three species represent the first reliable records of all three from Japanese waters, with comparative specimens of A. koreana from Vietnam and Malaysia also representing distributional range extensions (formerly known only from Korea and Taiwan). Albula koreana is readily distinguished from Japanese congeners by the striking yellow stripe on the cheek (just behind the mouth) in the former, a large dark blotch in front of the nostril, a dark oval blotch under an arc-shaped dark band on the snout tip, and greater numbers of body scales and vertebrae. As has been previously demonstrated, A. argentea and A. oligolepis are distinguished by pored lateral-line scale numbers (68–74 in the former vs. 61–67 in the latter) and total vertebrae (68–75 vs. 64–70). Updated distributional information shows A. argentea to be distributed from Indonesia east to French Polynesia, and north to Japan and Korea (there being no reliable records from Sri Lanka, Madagascar or the Hawaiian Islands); A. koreana in waters off Korea, Japan, Taiwan, Vietnam and Malaysia (east coast of Malay Peninsula); and A. oligolepis from the east coast of Africa to the Coral Sea, and north to Japan.

Specimens of the coastal marine fish leech Limnotrachelobdella okae (Moore, 1924) were collected from the pelvic fins and lateral body surface of big-scaled redfin, Pseudaspius hakonensis (Günther, 1877), in two brackish water lakes, Lake Tofutsu and Lake Abashiri, on the Sea of Okhotsk coast, Hokkaido, northern Japan. The external morphology of the specimens is briefly described, and the two lakes represent new locality records for the leech. Similarly to coastal marine waters, inshore brackish water lakes are suggested to be a habitat in which L. okae can complete its life cycle. Seasonal occurrence of the species may differ between subarctic Hokkaido and three temperate regions of Japan (Honshu, Shikoku, and Kyushu). The role of big-scaled redfin and its congener, Pacific redfin, Pseudaspius brandtii (Dybowski, 1872), in transporting L. okae alive from the coastal sea to brackish and fresh water bodies is discussed in terms of their diadromous migration ecology.

Two specimens of a rare deep-sea anglerfish, Himantolophus azurlucens Beebe and Crane, 1947, were collected off the Pacific coast of northern Japan, representing the first records from the western North Pacific. These specimens represent the second and third records of the species since the original description, which was based on a single specimen from the eastern North Pacific; we determined that a previous record from the Atlantic was erroneous. We herein provide a description of the specimens and comments on intraspecific variation of the escal appendages. The new Japanese name “Masamori-chouchin-anko” is proposed for H. azurlucens. In addition, we provide DNA barcode sequences of the recently collected specimens of Himantolophus Reinhardt, 1837, including H. azurlucens and the fourth record of H. borealis Kharin, 1984, and demonstrate remarkably low intraspecific genetic variation of cytochrome c oxidase subunit I within the genus.

A single female specimen (25.6 mm in standard length) of the thread-like Indo-Pacific pygmy syngnathid Kyonemichthys rumengani Gomon, 2007 was collected from fringing reef at eight meters depth from Okinawa Island in the Ryukyu Archipelago of southern Japan. It represents the first specimen of this species to be housed in a museum fish collection in Japan, where for the first time it is available for molecular analysis. We assessed the morphological hypothesis that previously suggested Kyonemichthys Gomon, 2007 is allied with the Indo-Pacific pygmy pipehorse genera Acentronura Kaup, 1853 and Idiotropiscis Whitley, 1947 based on similar characteristics of the head angled slightly ventrally from the abdominal axis, dermal appendages, and flexible tail lacking a caudal fin. However, Kyonemichthys differs from these genera in having a dorsal-fin origin on the tail versus the trunk, a characteristic shared by two Indo-Pacific pipefish genera: the morphologically similar Urocampus Günther, 1870 and the distinct worm-like Siokunichthys Herald, 1953. We therefore investigated the evolutionary relationships of K. rumengani within Syngnathidae based on the genetic divergence of the mitochondrial CO1 gene (uncorrected p-distances) and a phylogenetic hypothesis generated from the analysis of three partial mitochondrial genes (12S, 16S, and CO1). Genetic analyses demonstrated that Kyonemichthys and Urocampus are closely related and form a strongly supported clade that excludes the phylogenetically distant Acentronura, Idiotropiscis, and Siokunichthys. Furthermore, morphological comparisons of K. rumengani with members of Urocampus revealed numerous synapomorphies distinct from the pygmy pipehorses, including meristic characters, trunk and tail ridge configurations, placement of dorsal fin on the tail, and shape of the prehensile tail. Therefore, based on the genetic and morphological characteristics, we suggest that Kyonemichthys is sister to Urocampus and is allied with pipefishes rather than with pygmy pipehorses. In addition, the Japanese standard name “Hari-youji” was proposed for K. rumengani.

Two large (ca. 0.5 mm long), rare, and probably closely related species of Facetotecta (y-larvae), Hansenocaris cristalabri sp. nov. and Hansenocaris aquila sp. nov., are described on the basis of last-stage lecithotrophic nauplii reared from plankton at Sesoko Island, Okinawa, Japan. The two species resemble each other in having a labrum with a row of spines and a long, attenuate trunk region that terminates in a long, heavily spinose dorsocaudal spine. The labrum of H. cristalabri sp. nov. has an enormous, cockscomb-like ventral process that bears a row of distally directed, dagger-like spines along its anterior side, while the spine-bearing keel of the labrum of H. aquila sp. nov. extends posteriorly into a robust, eagle-like beak. The labral “crest” of H. cristalabri sp. nov. has no equivalent in any other described y-larva, nor in any other crustacean nauplius; its possible functions are discussed. Another diagnostic feature of H. cristalabri sp. nov., absent in H. aquila sp. nov., is a pair of shallow, rounded notches bounded by sharp spinules on the far posteriolateral margins of the cephalic shield. Both new species have longitudinal spine rows on the trunk dorsum, two rows in H. cristalabri sp. nov. and four in H. aquila sp. nov., something not previously documented for y-nauplii. The plate arrangement of the cephalic shield in H. cristalabri sp. nov. is described in detail, with an attempt to homologize the pattern with that of other y-nauplii (especially Hansenocaris furcifera Itô, 1989). The body surface of H. cristalabri sp. nov. has fewer setae and pores than any other late- or last-stage facetotectan nauplius described to date, suggesting paedomorphic development. A formal diagnosis is presented for the family-group taxon Hansenocarididae fam. nov.; this name, while already in use, has until now been nomenclaturally unavailable.

Two new gnaphosid spider species, Cladothela bicolor Suzuki, sp. nov. and Micaria longimana Suzuki, sp. nov. are described on the basis of both sexes, from material collected in the Okinawa Islands in the Ryukyu Islands, Japan. Illustrations and photographs of both species, as well as a map of the sampling locations are provided.

Two species of diplogyniid mites, Lobogynium sudhiri (Datta, 1985) and Neolobogynium sorae sp. nov., were found on histerid, synteliid, and geotrupid beetles in Japan. Two species of beetles are reported as new hosts of L. sudhiri. Neolobogynium sorae shows notable sexual dimorphosis in dorsal chaetotaxy. Key to species of Neolobogynium Hicks, 1957 is provided.

Bathycongrus castlei Smith and Ho, 2018, previously recorded only from Taiwan, is newly reported from Japan, based on two specimens (352+, 362 mm total length) collected from coastal waters in Tosa Bay, Kochi Prefecture at depths ranging from 128–315 m. Bathycongrus castlei differs from two congeners previously reported from Japan, Bathycongrus retrotinctus (Jordan and Snyder, 1901) and Bathycongrus wallacei (Castle, 1968), in having an intermediate number of total vertebrae (159–162 in B. castlei vs. 143–151 in B. retrotinctus and 172–181 in B. wallacei) and dark margins on the entire surface of the dorsal, anal and caudal fins (vs. only to the posterior end of the medial fins in B. retrotinctus and B. wallacei), 1 or 2 slightly enlarged teeth with a few small teeth at the front and sides and 6–9 stout or nearly granular teeth behind on the vomer [vs. 2 (sometimes 1) enlarged teeth surrounded by several smaller teeth in B. retrotinctus and 2 (rarely 1 or 3) enlarged teeth on midline, with 2 smaller teeth directly behind, side by side, and a few additional small teeth in the front and behind, no teeth laterally in B. wallacei]. A detailed description and fresh coloration are given for the specimens and a new standard Japanese name “Heri-tsumaguro-anago” is proposed for B. castlei.