A subterranean species of pseudocrangonyctid amphipod, Pseudocrangonyx gudariensis Tomikawa & Sato, sp. n., is described from the spring-fed stream Gudari-numa in Hakkoda Mountains, Aomori Prefecture, northern Japan. Pseudocrangonyx gudariensis is morphologically similar to P. coreanus Uéno, 1966 and P. febras Sidorov, 2009 based on its relatively small body size, small number of articles of rami of pleopods, and urosomite 1 without basal setae. However, P. gudariensis is distinguished from those species based on the following characteristics: from P. coreanus, antenna 2 of female without calceoli, palmar margins of gnathopods 1 and 2 with distally notched robust setae, inner margin of inner ramus of uropod 2 with 4 robust setae, and basal part of inner ramus of uropod 2 without slender seta; and from P. febras, carpus of gnathopod 2 without serrate robust setae on posterodistal corners, peduncle of pleopods 1 and 2 with setae, and longer article 2 of uropod 3. Phylogenetic analyses using nuclear 28S rRNA and histone H3, and mitochondrial cytochrome c oxidase subunit I and 16S rRNA markers showed that P. gudariensis is placed among known Pseudocrangonyx Akatsuka and Komai, 1922 species. However, its exact phylogenetic position within the genus could not be determined. The polyphyly of the Japanese Pseudocrangonyx species indicates that multiple colonization events of Pseudocrangonyx ancestors to the Japanese Archipelago could have occurred. The reliability of the past Pseudocrangonyx records from Japan is briefly discussed.

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A new species of amphipod, Sternomoera morinoi Tomikawa and Ishimaru, is described from subterranean aquatic habitats in Shiga Prefecture, Japan. In addition, Relictomoera tsushimana (Uéno, 1971) from a well on the island of Tsushima in Japan is transferred to Sternomoera and redescribed based on the holotype. Sternomoera morinoi sp. nov. is most similar to S. tsushimana (Uéno, 1971) comb, nov., but is distinguished by having many fine setae on the body, a shorter antenna 1, fewer C-setae on mandibular palp article 3, a shorter mandibular palp article 3, sparsely setose anterior margins of coxae 1-4, a different armature of the palmar margins of gnathopods 1 and 2, and no long setae on the posterior margin of the basis in gnathopod 2 and pereopods 3 and 4. The phylogenetic relationships among the Japanese species of Sternomoera are also estimated, based on partial DNA sequences of the nuclear 28S rRNA gene.

A new subterranean species of pseudocrangonyctid amphipod, Pseudocrangonyx daejeonensis sp. n. is described from the interstitial waters in Daejeon, Korea. Pseudocrangonyx daejeonensis sp. n. is distinguished from three morphologically similar congeners, P. coreanus Uéno, 1966, P. febras Sidorov, 2009, and P. gudariensis Tomikawa & Sato, 2016, by the characteristics of antenna 1, antenna 2, mandible, gnathopod 2, pleopods, uropods 1–2, and telson. 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. daejeonensis is a sister species of the unnamed Pseudocrangonyx sp. 3 inhabiting central Japan.

Amphipods belonging to the crangonyctoid genus Pseudocrangonyx Akatsuka & Komai, 1922 constitute a major component of the subterranean environments in east Asia. The true species diversity of this group has been unsettled due to the lack of molecular data for P. shikokunis P. kyotonis Akatsuka & and the taxonomic status of the misidentified populations of these two species. The status of the misidentified populations is herein clarified. Morphological comparisons among the specimens of these populations and the name-bearing types of P. shikokunis and P. kyotonis demonstrate the two are distinctive species. Phylogenetic analyses using partial sequences of nuclear 28S rRNA and histone H3, mitochondrial cytochrome c oxidase subunit I, and 16S rRNA genes also confirm that each of the two populations represents a unique clade within the species of Pseudocrangonyx. Accordingly, the population indigenous to the limestone caves in western Japan, which was previously identified as P. shikokunis, is described as P. akatsukai n. sp., and that reported as P. kyotonis from central Japan is described as P. komaii n. sp. The phylogenetic relationships within P. akatsukai n. sp. and an unidentified Pseudocrangonyx species elucidate the complex stygofaunal relationships in western Japan (western Honshu, Shikoku, and Kyushu). A key to Pseudocrangonyx species is also provided.