A new species of the genus Pseudocrangonyx (Crustacea: Amphipoda: Pseudocrangonyctidae) from subterranean waters of Japan.

A new species of pseudocrangonyctid amphipod, Pseudocrangonyx asuwaensis, is described from subterranean water in a quarry on Mt. Asuwa "Nanatsuo-guchi", Fukui Prefecture, Japan. Pseudocrangonyx asuwaensis sp. nov. differs from its congeners in various morphological features, such as head without eyes, relative length of antennae 1, female antenna 2 without calceoli, length ratio of mandibular palp articles, setal numbers on maxillae 1 and 2 inner plates, absence of sternal gills, armature of urosomite 1, and shape of telson. Molecular phylogenetic analyses suggest that P. asuwaensis sp. nov. is clustered with P. komaii Tomikawa & Nakano, 2018, Pseudocrangonyx sp. 7 and Pseudocrangonyx sp. 8, and was the first of these species to diverge.

Species of the Maera-clade collected from Japan. Part 4: addenda to genera Maera Leach, 1814 and Quadrimaera Krapp-Schickel Ruffo, 2000, with revised keys to Japanese species of the clade (Crustacea: Amphipoda: Maeridae).

Two new species of Maera Leach, 1814 and Quadrimaera Krapp-Schickel Ruffo, 2000 included in the Maera-clade are described from Japan. Maera denticoxa sp. nov. was collected from Iwate and Hokkaido Prefectures and can be distinguished from its congeners by the small notches on the posteroventral margins of coxae 1-6. Quadrimaera angulata sp. nov. from north of Tanegashima Island in Kagoshima Prefecture is characterized by the distal tooth on the mandibular palp article 1, the rounded palm of the female gnathopod 2, and the angular posterodistal margin of the pereopod 7 basis. Keys to Japanese species of the Maera-clade are provided. In total, seventeen species included in the clade occur in Japan.

A new species of the genus Pseudocrangonyx (Crustacea: Amphipoda: Pseudocrangonyctidae) from Simbok Cave, Korea.

A new subterranean species of pseudocrangonyctid amphipod, Pseudocrangonyx joolaei, is described from the groundwater of a cave in South Korea. Pseudocrangonyx joolaei sp. nov. can be distinguished from its congeners by the number of sternal gills as well as a combination of the antennal sinus, the accessory flagellum of antenna 1, and the terminal article of uropod 3. Molecular phylogenetic analyses based on nuclear 28S rRNA and histone H3, and mitochondrial cytochrome c oxidase subunit I and 16S rRNA genes revealed that P. joolaei sp. nov. formed a clade with P. akatsukai Tomikawa Nakano, 2018 that inhabits limestone caves in the western Honshu island, Japan.

A new species of the genus Rhachotropis from off Amamioshima Island, northwestern Pacific (Crustacea: Amphipoda: Eusiridae).

A new species of the eusirid amphipod, Rhachotropis reiwa is described from off Amamioshima Island, northwestern Pacific. The new species differs from its congeners in having large eyes, the middorsal tooth on pereonite 7, pleonite 3 and urosomite 1 without middorsal and dorsolateral teeth, the basis of pereopod 5 strongly produced posteriorly, and pereopod 6 with the triangular basis. A key to the species of Rhachotropis from Japanese and adjacent waters is provided. Additionally, a nucleotide sequence of mitochondrial cytochrome c oxidase subunit I from its holotype was determined for the future study.

Opisa takafuminakanoi, a new species of Opisidae from Hokkaido, Japan (Crustacea: Amphipoda).

Opisa is a small amphipod genus including three species: O. eschrichtii (Krøyer, 1842) from the North Atlantic Ocean (Sars 1895; Bousfield 1987), the Arctic Ocean (Bousfield 1987), and the western Pacific Ocean (Stebbing 1906; Derzhavin 1929); O. odontochela Bousfield, 1987 from the southeast Alaska (Bousfield 1987); and O. tridentata Hurley, 1963 from the Pacific coasts of North America (Bousfield 1987). During field survey of the marine benthic fauna of Hokkaido, Japan, one of the authors (KK) collected an undescribed species of Opisa from off the southeast of Akkeshi Bay using a sledge net. This paper describes and illustrates the new species in detail.