Two specimens (15.8 and 22.4 mm in standard length) of Sueviota minersorum Greenfield, Erdmann, and Utama, 2019, a species originally described on the basis of four specimens from Misool, Raja Ampat Islands, Indonesia, were collected from Sotobanari Island, off Iriomote Island, Yaeyama Islands, southern Japan. Although S. minersorum in Japanese waters has previously been identified on underwater photographs taken at Iriomote Island, this study provides the first specimen-based record of S. minersorum from Japan, and proposes a new standard Japanese name, “Kadomori-awase-isohaze” for the species.

Microcotyle elegans Goto, 1894 (Polyopisthocotyla: Microcotylidae) was redescribed from newly collected specimens parasitic on the gills of its type host, Scombrops boops (Houttuyn, 1782) in Japan. This monogenean has not been reported since the original description, and the holotype is believed to be lost. Morphologically, this species is most similar to M. sebastis Goto, 1894 and M. caudata Goto, 1894 in terms of the haptor shape and number of clamps, but differs from M. sebastis in the length of the genital spine, and from M. caudata in the shape of the genital atrium. The phylogenetic trees for species of Microcotyle Van Beneden and Hesse, 1863 based on the partial cytochrome c oxidase subunit I gene sequences were provided. In addition, all currently known species of Microcotyle are listed, and a key to valid species of the genus is provided.

A small-type quadrannulate leech species of Orobdella Oka, 1895, O. minima sp. nov., from Mt. Hehuan in central Taiwan is described. Phylogenetic analyses using nuclear 18S rRNA, 28S 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 revealed that O. minima forms a clade with the middle-type quadrannulate O. ketagalan Nakano and Lai, 2012, which is endemic to northern Taiwan. This clade was sister to the other Taiwanese middle-type quadrannulate O. meisai Nakano and Lai, 2017. Based on the phylogenetic relationships among the Taiwanese Orobdella, their morphological characteristics are briefly discussed in a phylogenetic context. A key to the three species of Orobdella endemic to Taiwan is also provided.

Two new species of dicyemids are described from an octopus, Octopus conispadiceus (Sasaki, 1917), collected off Shiranuka, Hokkaido, Japan. Dicyemennea tsunekii sp. nov. is a large species that reaches about 1 mm in length. The vermiform stage is characterized by 33–35 peripheral cells, a conical calotte, and an axial cell that extends to the base of the metapolar cells. The infusoriform embryo consists of 39 cells; two nuclei are present in each urn cell and the refringent bodies are solid. Dicyemodeca shirarikaense sp. nov. is a medium-sized species that reaches about 2.5 mm in length. The vermiform stage is characterized by 24 peripheral cells, a cap-shaped calotte, and an axial cell that extends to the base of the propolar cells. The infusoriform embryo consists of 35 cells; two nuclei are present in each urn cell and the refringent bodies are liquid.

An adult female of Argulus coregoni Thorell, 1864 was collected from the operculum of a freshwater minnow Opsariichthys platypus (Temminck and Schlegel, 1846) (Cypriniformes: Xenocyprididae) at 15 m elevation in a stream in Oita Prefecture, Kyushu, western Japan. This adult female of A. coregoni is described herein, and freshwater minnow represents a new host record for this parasite. In central Japan, A. coregoni is known to infect salmonids and a plecoglossid Plecoglossus altivelis altivelis (Temminck and Schlegel, 1846) (both Salmoniformes) in the upper and middle-lower reaches of rivers, respectively, and to use cypriniform fishes as its hosts in low-elevation streams. In contrast, almost nothing is known about the host utilization and distribution of A. coregoni in Kyushu, where the species was collected only once from a land-locked salmonid fish at a high elevation in a mountain stream. The present collection of A. coregoni from freshwater minnow suggests that this parasite infects cypriniform fishes in low-elevation streams in Kyushu as well. Based on new information on the leg morphology of the specimen of A. coregoni collected in this study, an amended key is provided for the five species of the genus found on Japanese freshwater fishes.

During a recent deep-sea survey around the Okinawa Islands in the Ryukyu Archipelago, southern Japan, two species of commensal gobies, Lobulogobius omanensis Koumans, 1944 and Pleurosicya annandalei Hornell and Fowler, 1922, were observed using remotely operated vehicles (ROVs) on or within marine invertebrates at depths of 95–110 m, and were subsequently collected. This represents the first record of these two species in Japan. This study provides a detailed morphological description of the specimens, including their coloration when alive and fresh. DNA analysis was also performed to further characterize these species. The integration of morphological and genetic approaches is essential for the accurate identification of newly discovered species, particularly in underexplored deep-sea environments.

Axine japonica Price, 1945 (Monogenea: Axinidae) is redescribed from the gills of Cypselurus poecilopterus (Valenciennes, 1847) and C. hiraii Abe, 1953 (Exocoetidae) in Japanese waters. Although the taxonomic position of this monogenean had been frequently revised, A. japonica remains known only from the original description. The present study revealed that A. japonica has these following diagnostic characters of Axine Abildgaard, 1794: the spines of the genital atrium are in 3 groups, the spines are arranged in a single incomplete row in the median group and the cirrus is armed with a circle of spines. In the phylogenetic analysis based on the 28S rDNA, sequences for A. japonica formed a clade with strong support regardless of the hosts. The present study shows a narrow host range for A. japonica, fishes of three species of one genus.

We describe a new species of anuran in the species complex of Limnonectes kuhlii (Tschudi, 1838) from the Meratus Mountain Range of Borneo Island, Indonesia. Mitochondrial DNA analysis places the new species as a sister of L. cintalubang Matsui, Nishikawa, and Eto, 2014. The new taxon differs from all other recognized species of L. kuhlii complex by a large genetic distance and a combination of morphological, bioacoustic, and larval characteristics. This discovery provides new insights into the taxonomic diversity of the L. kuhlii complex in Borneo, particularly within the Meratus Mountain Range, and highlights the importance of conserving this unique and understudied region.

Herein, we describe a new species in the genus Hemicorallium Gray, 1867 (Coralliidae), H. ryukyuense sp. nov. An almost complete colony was found at a depth of 860 m approximately 10 km east of Okinawa Island. The colony was identified through the visual and microscopic observations of standard morphological characteristics, including colony size, the diameters of colony base and branches, the diameter and height of autozooid mounds, coenenchyme thickness, and sclerite sizes. The specimen is characterized by large autozooid mounds (1.1–2.0 mm in diameter) and a pinkish-to-reddish cortex and axis. A distinctive feature of this specimen is the absence of commensal burrows on the axis. This is the first described species of precious coral with the Ryukyu Islands as its type locality. Pairwise genetic distances indicate that H. ryukyuense constitutes a distinct evolutionary lineage. However, tree topologies differed among loci, and the interspecific distances within Hemicorallium were smaller than the divergence observed between Pleurocorallium elatius (Ridley, 1882) and P. konojoi (Kishinouye, 1903). Consequently, population-level genomic data will be needed to confirm the species status of H. ryukyuense with greater confidence.

A new deep-water snailfish, Paraliparis anomalus, is described from two specimens collected off eastern Honshu Island, Japan (635–687 m depth). It can be distinguished from congeners by the following combination of characters: vertebrae 67 (9 + 58), a single pair of pleural ribs on abdominal vertebra; dorsal-fin rays 61 or 62; anal-fin rays 56; pectoral-fin rays 27; caudal-fin rays 9 (1 + 4/4); teeth on both jaws primarily trilobed; orbit 23%–26% of head length; cephalic pore pattern 2-6-7-2; gill slit extending ventrally to third pectoral-fin ray base; pyloric caeca ≤ 9; body pink when fresh, pale after preservation. The relatively large sequence divergence in cytochrome c oxidase subunit I between P. anomalus and other morphologically similar species also supported the validity of the former.

Bigelowina phalangium (Fabricius, 1798) is the only representative of the genus Bigelowina Schotte and Manning, 1993 known from the Indo-West Pacific region. In Japanese waters, this species has been reported from the littoral environments of the Ryukyu Islands to Honshu Island. Our morphological and phylogenetic analyses of specimens of Bigelowina from Japan found two allopatric species, one from the Ryukyu Islands and another from Kyushu to Honshu Islands. The Ryukyuan species is morphologically closest to B. phalangium, with slight differences in the morphology of the raptorial claw and the color pattern of the telson. In contrast, another species from Kyushu to Honshu Islands is here described as a new species, B. komaii sp. nov. The field survey conducted in this study suggests that the host of two commensal mollusk species previously reported from the burrow of species of Bigelowina at Nakatsu, Kyushu Island, is B. komaii sp. nov., not B. phalangium. An identification key to species of Bigelowina is also provided.

A remarkable new species of the genus Arcuphantes Chamberlin and Ivie, 1943 characterized by conspicuous genital morphology, is described from northeastern Shikoku, Japan, based on both sexes, under the name of A. bandoi n. sp. The new species is distinguished by an extremely elongated, looped scape of epigyne with S-shaped undulation and a paracymbium of the male palp extending to the distal end of the cymbium. Arcuphantes bandoi n. sp. is known only from the vicinity of four rivers and their associated tributaries located north of the Yoshino-gawa River basin, within an area measuring approximately 6.0 km × 8.0 km. At the periphery of this region, the new species exhibits a non-strict parapatric distribution with Fusciphantes iharai (Saito, 1992). This paper provides a detailed description of the new species including its mating behavior and the male-female genital interaction, comparisons with other species of Arcuphantes and Fusciphantes Oi, 1960 in northeastern Shikoku, and a preliminary molecular analysis.

Cavisoma magnum (Southwell, 1927), the sole member of the genus Cavisoma Van Cleave, 1931 (type genus of Cavisomatidae), is an intestinal parasite of marine fishes in the Indo-Pacific, but molecular data are lacking. We report the first Japanese record of C. magnum, found in the nemipterid fish Pentapodus caninus (Cuvier, 1830) off Okinawa—also the first known host from Nemipteridae. Morphological traits matched previous descriptions, and we obtained new sequences for cytochrome c oxidase I, 18S rRNA, and 28S rRNA genes. Phylogenetic analyses showed that Cavisoma clusters with Rhadinorhynchus Lühe, 1911, while Filisoma Van Cleave, 1928, another genus in Cavisomatidae, is distantly related. These results support the non-monophyly of Cavisomatidae. Based on both molecular and morphological evidence, we propose resurrecting Filisomatidae Van Cleave, 1928 for Filisoma, which differs from Cavisoma and related genera by its filiform cement glands. We also present an amended diagnosis of Cavisomatidae and revise its composition.

A new species of Caprella Lamarck, 1801 (Crustacea: Amphipoda: Caprellidae) collected from the rocky coasts of Ehime Prefecture, western Japan, is described. The sampling site of the present new species is adjacent to the district where organic citrus cultivation is predominant. Of the approximately 200 species of Caprella, the present species, C. gracilenta sp. nov., is most closely related to C. rhopalochir Mayer, 1890. The type locality of C. rhopalochir is the mouth of the Amur River, located at the northern end of the Sea of Japan. In addition, C. rhopalochir was recorded until the 1970s from northern Kyushu and adjacent areas near the southwestern end of the Sea of Japan, where oceanographic conditions are largely different from the type locality of C. rhopalochir. Caprella gracilenta sp. nov. is distinguished from C. rhopalochir by the absence of forwardly curved triangular projection near the posterior end of pereonite 1, triangular projection on the dorsal face of the propodus of gnathopod 2, paired dorsal projections on pereonites 3–4, and triangular projection near the posterior end of pereonites 3–4. Of the two descriptions of C. rhopalochir from near the southwestern end of the Sea of Japan, a description from Saga Prefecture was identified as C. gracilenta sp. nov. This re-identification indicates that C. gracilenta sp. nov. is a rare species widely distributed along the western coast of Japan.

We herein provide a taxonomic revision of the species complex of Plestiodon barbouri (Van Denburgh, 1912) in the Ryukyu Archipelago, Japan. Molecular and morphological analyses showed that P. barbouri sensu lato comprised at least four distinct lineages, each warranting recognition as a separate species. Thus, we propose a new taxonomic arrangement, where we restrict P. barbouri sensu Van Denburgh to island populations of the Amami Group. In addition, we describe three new species for island populations of the Okinawa Group. The new species are distinguished from each other and all congeners by combining scalation and color patterns.