Hofstenia Bock, 1923, the type genus of Hofsteniidae, was originally established for H. atroviridis Bock, 1923, from Misaki, Kanagawa Prefecture, Japan. This study describes Hofstenia sugashimaensis sp. nov., discovered in water tanks at the Sugashima Marine Biological Laboratory (Nagoya University) and Toba Aquarium, both situated at the mouth of Ise Bay in Mie Prefecture, along the Pacific coast of central Japan. Morphologically, it resembles H. miamia Corrêa, 1960 (commonly known as the “three-banded panther worm”) but is distinguished by two transverse bands and dorsal midline markings based on intra- and interspecific comparisons. Phylogenetic analysis of partial sequences of the cytochrome c oxidase subunit I, 18S rRNA, and 28S rRNA supports its genetic distinction from H. miamia.

Troglodontobdella nom. nov. is herein proposed as a replacement name for the salifid leech genus Troglobdella Liu and Nakano in Liu, Li, Wu, Lin, Li, Nakano, and Liu, 2025, because the latter genus-group name is preoccupied by the acariform genus Troglobdella Oudemans, 1937.

Hazeus atrispinalis sp. nov. (Gobiidae; Clear Sand-goby; new standard Japanese name: Okojo-haze) is described based on 40 specimens (16.7–31.8 mm standard length) collected from the Ryukyu Archipelago, Japan, and confirmed based on underwater photographs from the Philippines, Indonesia, Papua New Guinea, and Republic of the Marshall Islands. The new species, which is the 10th valid species in the genus Hazeus Jordan and Snyder, 1901, is diagnosed from congeners by the following combination of characters: cheek and opercle naked; dorsal and anal-fin rays VI-I, 9 or 10; second or third spine of first dorsal fin longest, each without a filamentous tip; pectoral-fin rays 15–17; longitudinal scale rows 26 or 27; predorsal scales 6–9; snout relatively short, length 4.7%–6.4% of SL; second dorsal-fin spine short, length 7.8%–11.0% of SL; sensory papillae row a almost double, row ot uniserial, row n transverse; black markings internally along vertebral column in life; first dorsal fin with a black blotch near base in males; body without distinct dotted lines, except for mid-lateral row of five large black blotches; pelvic frenum translucent; maximum size ca. 32 mm SL. The new species most resembles H. ammophilus Allen and Erdmann, 2021, but prefers sand to sandy-mud bottoms, whereas H. ammophilus is commonly found on soft mud bottoms.

Three specimens of a rare snailfish, Menziesichthys alaid Prokofiev and Iftime, 2020 (Teleostei: Perciformes: Liparidae), collected from Rausu, Hokkaido, Japan, the southern Sea of Okhotsk and Bering Sea Slope, represent the second and third records of the species, previously known only from two type specimens caught off the northern Kuril Islands. A detailed description (including selected osteological characters) based on the former three specimens, showed the species to differ from other snailfishes as follows: vertebrae 61–68; dorsal-fin rays 61; anal-fin rays 55–57; caudal-fin rays 3 or 4; no pelvic disk; a shallowly notched pectoral fin with 11–14 rays, lower lobe longer than upper lobe; teeth simple; cephalic pores 2-6-7-1, chin pores paired in separate pits, coronal pore present; lower tip of retroarticular pointed; gill slit large, extending to lower base of pectoral fin; stomach and peritoneum black. Because the species displays many diagnostic characters of Paraliparis Collett, 1879 rather than Menziesichthys Nalbant and Mayer, 1971, it is here regarded as a member of Paraliparis.

Adults of both sexes of Argulus coregoni Thorell, 1864 were collected from the body surface of bester [a hybrid of beluga Huso huso (Linnaeus, 1758) × sterlet sturgeon H. ruthenus (Linnaeus, 1758)], common carp Cyprinus carpio Linnaeus, 1758, and rainbow trout Oncorhynchus mykiss (Walbaum, 1792) at a governmental fish hatchery in Hyogo Prefecture, central Japan. There was a marked difference in the number of A. coregoni per infected fish between these host species: rainbow trout were more heavily infected (151–224 individuals) than bester and common carp (25 and seven individuals, respectively). The parasite is described and characterized by possessing four to six plumose setae near the posterior margin of the coxa of the first leg, 59–66 supporting rods in the marginal membrane of the first maxilla, and two protrusions adorned with small spines and a digitiform projection on the ventro- and dorsoposterior margins, respectively, of the coxa of the male second leg. The collection of A. coregoni from bester represents a new host and the second record from sturgeons (the family Acipenseridae) worldwide. Common carp is reported as a host of this parasite for the first time in Japan.

Three new species of the iphimediid amphipod, Iphimedia declivis, I. macrops, and I. trigona, are described from the Pacific coast of Japan. These species can be distinguished from one another by the shapes of pereonites 1, 7, coxa 7, and pereopod 7 basis. A key to the species of the family Iphimediidae in Japan is provided.

A single specimen (45.9 mm standard length) of the deep-sea scorpionfish Phenacoscorpius megalops Fowler, 1938 (Teleostei: Scorpaenidae) was collected by ROV (remotely operated vehicle) from within an unidentified colonial sun coral [Dendrophylliidae gen. sp. (Scleractinia)] on a steep sea floor off Minamidaito Island, southern Japan. Phenacoscorpius megalops, characterized by the absence of palatine teeth, an incomplete lateral line, and a dark blotch on the dorsal fin, has been previously recorded from the western and central Pacific, including Vietnam, the South China Sea, Taiwan, the Philippines, Indonesia, Vanuatu, the Emperor Seamounts, and the Hawaiian Islands. This is the first record from Japanese waters.

The classification of two coelotine species from Japan is reviewed. Comparison of the male and female copulatory organs of multiple specimens of Tegecoelotes mizuyamae Ono, 2008 and T. echigonis Nishikawa, 2009, including the type specimens, and phylogenetic analyses based on partial sequences of the mitochondrial cytochrome c oxidase subunit I are presented. The results show that the morphology of the copulatory organs is highly similar between the two species in both sexes. Molecular phylogenetic analyses, including phylogenetic trees and p-distances, also revealed no significant genetic differentiation between them. Therefore, the two species should be regarded as conspecific.

Myxobolus donkoi Ishizaki, 1955 (Cnidaria: Myxosporea), originally described in Fukuoka Prefecture, Kyushu, Japan, was redescribed based on newly collected specimens from the intestinal wall of its type host, Odontobutis obscurus (Temminck and Schlegel, 1845) (Gobiiformes: Odontobutidae), and newly classified in the genus Cardimyxobolus Ma, Dong, and Wang, 1982 as C. donkoi n. comb. These specimens were obtained from Kumamoto Prefecture (Kyushu) and Yamaguchi Prefecture (Honshu). Cardimyxobolus japonensis Li and Sato, 2014 (Myxosporea), also described from the intestinal wall of Od. obscurus in Yamaguchi Prefecture, is regarded as a junior synonym of C. donkoi n. comb. based on identical morphological features and molecular analyses of 18S and 28S rDNA sequences. The authorship of Ortholinea, type genus of Ortholineidae, has traditionally been attributed to Shulman (1962). However, because Shulman (1962) did not meet the requirements of Article 13.3 of the International Code of Zoological Nomenclature, the authorship of the genus is attributed to Lom and Noble (1984), who first designated its type species. Because recent phylogenetic studies have clearly demonstrated that Ortholineidae is polyphyletic, this study follows previous proposals assigning Ortholinea to Myxobilatidae. Among the genera previously included in Ortholineidae, I propose transferring Cardimyxobolus and Triangula Chen and Hsieh, 1984 to Myxobolidae based on the morphology of their plasmodia and phylogenetic relationships. In addition, Neomyxobolus Chen and Hsieh, 1960, for which molecular data are currently unavailable, is tentatively assigned to Myxobilatidae.

As part of a survey of the remarkable diversity of Facetotecta (“y-larvae”) at an Okinawan (Japan) hotspot locality, a very small and relatively abundant species of this group, Hansenocaris lenticula sp. nov. (formerly designated as “Type K”), is described based on last-stage lecithotrophic nauplii and cyprids reared from the plankton. Among the now 18 formally described species of Facetotecta, this is the fourth for which both nauplii and cyprids are known, and it has the smallest lecithotrophic nauplii yet reported for this group. The nauplii of H. lenticula sp. nov. are distinguished by their small size (approximately 0.2 mm long), flat, trapezoidal labrum, very small antennal and mandibular endopods, and significantly dorso-ventrally bent body with a conically tapered trunk region ending in small, conical furcal and dorsocaudal spines, the latter being slightly smaller than the former. The cyprids can be recognized by their small size, unique labral armature (three similarly sized distal hooks), relatively low number of plates/facets on the telson (three to seven plates per row) and reduced setation of the furcal rami (two very unequal blade-like setae). Potentially diagnostic aspects of the cuticular ornamentation of both larval stages (pores, setae, ridge-bounded plates/facets) are discussed, as well as potential commonalities with other y-larvae, but a detailed comparison is postponed until more facetotectan species have been described in detail. This study reconfirms our earlier assertion that lecithotrophic y-nauplii from Okinawa are relatively easy to distinguish from each other, while the discrimination of their cyprids requires more detailed microscopy, including SEM.

An association comprising a solitary ascidian attached to the dorsal surface of a holothurian was discovered in 2022 on the bathyal seafloor (ca. 2000 m depth) of the western slope of the Chishima (Kuril-Kamchatka) Trench, off the southeastern coast of Hokkaido, Japan, using a beam trawl during the KH-22-8 cruise of the R/V Hakuho Maru. This is the second taxonomically detailed description of a deep-sea epizoic holothurian-ascidian relationship. The host holothurian was identified as Pseudostichopus mollis Théel, 1886, and the associated ascidian as Cnemidocarpa ochotense Sanamyan, 1992. The association seemed stable due to the ascidian being firmly attached to the holothurian body surface by its broad tunic attachment surface and numerous elongated tunic extensions. It was difficult to determine the nature of the association, whether commensalism, mutualism, or parasitism, but it clearly seemed to be beneficial for the attached ascidian. Furthermore, the host holothurian showed no rejection of its symbiont.

A species of octopus from the Red Sea is described based on two recently collected fisheries survey specimens and four historical specimens housed in the Natural History Museum, Vienna. The latter were collected between 1895 and 1927, including specimens from two of the Austrian Pola Expeditions of the late nineteenth century. This species is here identified as Pinnoctopus cuvierii (d’Orbigny, 1826) which is redescribed based on morphological characters and comparison with the monotypic holotype of P. cuvierii, that of congeneric species P. lechenaultii (d’Orbigny, 1826) and, from the Mediterranean Sea, a voucher specimen of P. macropus (Risso, 1826). The distributions of P. cuvierii (an Indian Ocean species) and P. macropus are separated by the barrier between the Red Sea and the Mediterranean Sea, with no evidence of any distribution overlap after the opening of the Suez Canal. The first molecular reference sequences are provided for P. cuvierii including one nuclear and four mitochondrial genes. Phylogenetic analysis and comparison with other species in the same genus reveal that P. cuvierii is most closely related to the Indo-West Pacific species P. luteus (Sasaki, 1929) and relatively distantly related to P. macropus, which has closer affinities to the Atlantic species P. furvus (Gould, 1852).

I describe two new species of the nemertean family Valenciniidae from Japanese waters: Baseodiscus jimii sp. nov. from subtidal habitats and Cephalomastax fujiwarai sp. nov. from a piece of deep-sea sunken wood. The new species in Baseodiscus Diesing, 1850 is clearly distinguishable from its congeners by unique external coloration patterns and head morphology, whereas C. fujiwarai sp. nov. is readily separated from the only previously known congener, C. brevis Iwata, 1957, by its uniform body coloration. In addition, a new record of B. aff. marmoratus (Bürger, 1890) from Japanese waters is provided in this study. Phylogenetic analyses based on a concatenated dataset of partial sequences of two mitochondrial and three nuclear genes recovered their phylogenetic positions among Valenciniidae.