Recent studies have reported that salmonids and ayu Plecoglossus altivelis altivelis (Temminck and Schlegel, 1846), both of which are salmoniform fishes, serve as important hosts for Argulus coregoni Thorell, 1864. These studies found the parasite in the upper and middle-lower reaches of large rivers, respectively, originating from high-elevation mountain areas in central Japan. However, very recently, we collected specimens of A. coregoni from cypriniform fishes at a low elevation (25 m) in the lower reaches of the Kushiko River, a small stream originating in a mountain area and a tributary of the Hasu River, in Fukui Prefecture, central Japan. The lower Kushiko River lies in a rural area surrounded by rice fields, but its water is unpolluted and running. The infected cypriniform fishes were fat minnow Rhynchocypris lagowskii steindachneri (Sauvage, 1883) (Leuciscidae) and dark chub Nipponocypris temminckii (Temminck and Schlegel, 1846) (Xenocyprididae), and these fishes dominated at the collection site. No salmonids or ayu were collected there, and other fishes (one species of amblycipitid and one species of gobiid) were rare and uninfected. These facts indicate that, despite the fact that the river flows at low elevations, A. coregoni is able to utilize the two dominant cypriniform fishes (fat minnow and dark chub) as its hosts and maintain its population there. This is in contrast to the previous reports that A. coregoni exclusively infects salmonids and ayu in Japan. This paper reports on the occurrence of A. coregoni on the two cypriniform fishes and also describes the parasite from these fish species. Fat minnow represents a new host record for the parasite.

A new species of Aquilonastra O’Loughlin in O’Loughlin and Waters, 2004 is described on the coast of Langkawi Island, Malaysia. This new species is distinguished from 32 congeners based on the number of arms, madreporites, superomarginal and subambulacral spines, the absence of secondary abactinal plates, the shape of abactinal spines, and the attachment site of gonads.

Cirrhimuraena chinensis Kaup, 1856 is recorded from the western coast of Ise Bay, Mie Prefecture, central Japan, based on a single specimen of 240 mm+ (with a damaged tail tip) in length. It represents the first record of the species from Japanese waters. The species has been known from East and southeastern Asia including China and Taiwan, the Philippines and Indonesia. However, we tentatively limit its distribution to East Asia only because the previous research suggested the population in Indonesia is genetically distinct from that in China. During comparison, some notable characters of several type specimens are provided.

Microbothrium Olsson, 1869 (Monogenea: Monopisthocotylea) is the type genus of Microbothriidae and presently comprises three accepted species that parasitize squaliform sharks. More than seven decades have passed since the species affiliated with this genus was last reported. This study describes two new Microbothrium species that parasitize the body surfaces of squalid sharks in Kyushu, southern Japan, along with their molecular data. Microbothrium myzolepis n. sp. was recovered from Squalus mitsukurii Jordan and Snyder, 1903, collected off Kuchino-erabu Island in Kagoshima Prefecture. The new species is distinguished from its congeners by the vaginal pore opening on the left side of the body, presence of a slightly curved vagina, and 1–2 turns in the sclerotized tube associated with the penis. In addition, two individuals of Cirrhigaleus barbifer Tanaka, 1912 reared at the Oita Marine Palace Aquarium (Umitamago) were found to be infected with M. cirrhigalei n. sp. This newly described species can be readily differentiated from the other four Microbothrium species by the presence of a muscular vaginal pore, simple vagina, and three turns in the sclerotized tube associated with the penis. Furthermore, the phylogenetic analysis presented in this study supports the monophyly of the Microbothriidae. However, this analysis did not support the monophyly of each subfamily, suggesting that the subfamily classification may not accurately reflect evolutionary relationships. This study provides a taxonomic key for the identification of Microbothrium species.

The checklist comprises 66 species/subspecies of the earthworm genus Metaphire Sims and Easton, 1972 in Vietnam. Of 66 species, 47 were described by samples from Vietnam and considered to be endemic species to the Vietnam fauna. The genus Metaphire is more distributed in southern Vietnam in comparison to other parts of Vietnam (32 species in the southern part, 31 species in the central part, and only 19 species in the northern part).

A new species of Cybaeus L. Koch, 1868, C. eremospelyngus sp. nov., is described from Osawa-shonyudo Cave, Niigata Prefecture, northeastern central Honshu, Japan. This species has pale body coloration and relatively small eyes, which may be adapted to subterranean environments. In addition, C. eremospelyngus sp. nov. constructs a ‘Y-shaped’ retreat with three openings. This is the first record of a Cybaeus species that builds a retreat with three openings from central Honshu.

Molecular analyses of 18 specimens of Culeolus Herdman, 1881 collected in 2022 and 2023 near the southwestern limit of the Chishima-Kamchatka (Kuril) Trench, southeastern Hokkaido, between ca. 6100 and 6200 m depth, revealed two morphologically distinct clusters, each distinguishable from the other by clear differences in the Culeolus-ridge structure on the tunic surface around the atrial aperture (viz., a continuous crest vs. a ring of conical projections). Examination of the syntypes of the deep-water NW Pacific species C. tenuis Vinogradova, 1970, characterized by some structural variations, resulted in designation of a lectotype characterized as having a continuous crest, conspecific with the former crest-furnished specimens. The cone-furnished specimens were referred to C. uschakovi Redikorzev, 1941, characterized by intraspecific variations in the completeness of the projection ring, gonadal number and position, and number of branchial folds. Culeolus tenuis was compared with other crest-furnished congeners, and C. uschakovi with other cone-furnished ones, resulting in a better understanding of the taxonomy of the genus.

The eumonostiliferous hoplonemertean genus Tetranemertes Chernyshev, 1992 consists of 10 named species worldwide, whereas only one has been reported from Japanese waters. In this paper, we i) describe Tetranemertes aurantia sp. nov., ii) transfer Emplectonema kandai Kato, 1939—the only nemertean species known to be bioluminescent—to this genus, iii) briefly mention T. unistriata Cherneva, Ellison, Zattara, Norenburg, Schwartz, Junoy, and Maslakova, 2023, the only species previously known in Japan, and iv) report two species left undescribed due to lack of information on the morphology of the stylet apparatus. Our study raised the number of known Japanese congeners to five. Tetranemertes aurantia sp. nov. is established based on a single specimen collected among bycaught samples of a fishery gill net from a depth of 10 m off Sugashima island, Mie, Japan. Each of the two species left undescribed was also represented by a single specimen, from i) a depth of 288 m off Kakeromajima island in the Amami Islands, and from ii) a tidal flat in Misaki, respectively. Our molecular phylogenetic analysis suggests that Tetranemertes is the sister taxon to Amphiporus cf. cruentatus Verrill, 1879, albeit not fully supported. It is suggested that Ommatoplea Ehrenberg, 1828 may be a senior synonym of Tetranemertes.

A single priacanthid specimen (129.4 mm in standard length), collected off Cape Moreton, Queensland, Australia at a depth of 109.7 m, was identified as Priacanthus zaiserae Starnes and Moyer, 1988 (Priacanthidae), being characterized by 13 dorsal-fin soft rays, 13 anal-fin soft rays, 25 total gill rakers on first gill arch, 93 scales in lateral series, preopercular spine not reaching to posterior margin of opercle, and posterior margin of caudal fin convex. Because P. zaiserae has been recorded to date only from the northwest Pacific region, from Japan, Taiwan, and the Philippines, the present specimen from Queensland, described here in detail, represents the first record from the Southern Hemisphere.