In this study we describe one new species of Dactylogyrus Diesing, 1850 and three new species of Bivaginogyrus Gussev and Gerasev in Gussev, 1985 (Monogenea: Dactylogyridae) from two freshwater fishes Gnathopogon elongatus elongatus (Temminck and Schlegel, 1846) and G. caerulescens (Sauvage, 1883) (Cypriniformes: Gobionidae) endemic to Japan. Further, G. elongatus is a new host record for Paradiplozoon skrjabini Achmerov, 1974 (Monogenea: Diplozoidae). Dactylogyrus tamoroko n. sp. is most similar to D. chenjiagengi Zhang in Zhang, Zhou, and Wen, 1990, D. clypeatus Gussev, 1955, and D. gnathopogonis Yamaguti, 1963; it differs from these three congeners by the bifurcate base of the accessory piece. Bivaginogyrus japonicus n. sp. is close to D. polylepidis Alvarez Pellitero, Simon Vicente, and Gonzalez Lanza, 1981, but it can be separated from the latter species based on its uniform-sized marginal hooks. The new species was connected to the cartilage tissue of the gill using filament-like structures, which are assumed to be derived from secretions of the reservoirs associated with each marginal hook. Bivaginogyrus cingulatus n. sp. resembles D. navicularis Gussev, 1955, D. trullaeformis Gussev, 1955, and D. imberbis Wu and Wang, 1983, but the new species is distinguished from these three species by having its inconspicuous outer roots. Bivaginogyrus exiguus n. sp. is comparable to D. pandus Wu and Wang, 1983 and D. colpodes Wu, Wang, and Song, 1983, but differs from these two species in having a shorter penis. Molecular phylogenetic analysis indicates that Bivaginogyrus is a paraphyletic group and shows affinity to “Dactylogyrus” species parasitic on gobionid fishes. However, most of those closely related Dactylogyrus species are yet to be described for their internal morphology and should accordingly be redescribed. The three new species of Bivaginogyrus are assumed to have been co-introduced along with G. e. elongatus into non-native regions of this host.

The taxonomic status of millipedes of the genus Spirobolus Brandt, 1833, referred to as “Yaeyama-maruyasude” from the Yaeyama Islands, Ryukyu Islands, Japan, was unresolved. We assess the taxonomic status of these Yaeyama Spirobolus sp. using an integrated morphological and molecular approach, and describe them as a new species, S. akamma sp. nov., for which partial sequences of the nuclear 28S ribosomal RNA, mitochondrial cytochrome c oxidase subunit I, and 16S ribosomal RNA markers are provided. This new species differs from continental China and Taiwan endemic congeners in anterior gonopod morphology (in having an elongate and subtriangular coxa, and a pentagonal mesal sternal process), posterior gonopod morphology (in having a coronoid prefemoral endite with rounded distal end, and an elongate telopodite), and in having four serrations on the cyphopod lateral flange.

We establish a new genus of candimboidid polyclad, Chimaeriplana gen. nov., based on a new species, C. japonica sp. nov., collected from Kanagawa, the coast of Sagami Bay, Japan. Chimaeriplana japonica sp. nov. is characterized by (i) an interpolated prostatic vesicle with a seminal vesicle, (ii) a penis stylet with a split tip, (iii) numerous prostatoid organs in the male atrium, (iv) a bursa copulatrix, and (v) without Lang’s vesicle. In addition, we modify the definition of Candimboididae.

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.

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.

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.

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.

Members of the family Haloclavidae, belonging to the order Actiniaria, are characterized by the presence of a large siphonoglyph next to their actinopharynx and an aboral end without a basal disc. Members of the genus Haloclava Verrill, 1899 have been reported primarily from Europe and America, and have not yet been described from Japanese waters based on the collected specimen. In this study, I describe a new species, Haloclava hercules sp. nov., from the Pacific coast of Japan. This new species is chiefly characterized by cudgel-like blunt massive tentacles with knob-like acrospheres in the outer tentacular cycle and gigantic basitrichs over 250 µm in length, which are the largest known from sea anemones.