Taxonomic study on the GenusSternomoera(Crustacea: Amphipoda), with redefinition ofS. japonica(Tattersall, 1922) and description of a new species from Japan

Reassessment of the Groundwater Amphipod Paramoera relicta Synonymizes the Genus Relictomoera with Paramoera (Crustacea: Amphipoda: Pontogeneiidae)

The systematic status of the stygobitic amphipod, Paramoera relicta Uéno, 1971, was revisited based on morphological examination of the paratypes, and molecular phylogenetic analyses using a newly collected specimen from its type locality. The paratypes clearly showed that this species does not possess the "sinusoid" antennal sinus in the head, which was erroneously described in the original description of this species; P. relicta is unquestionably classified within the genus Paramoera Miers, 1875 . Molecular phylogenetic trees obtained with nuclear histone H3 and mitochondrial 16S rRNA demonstrated that P. relicta clearly belongs to the clade consisting of Paramoera species. Since the genus Relictomoera Barnard and Karaman, 1982 was erected according to the "sinusoid" characteristic of its type species P. relicta, this genus is no longer valid, and should be treated as a junior subjective synonym of Paramoera. One of the paratypes of P. relicta is herein designated as a neotype for the species to clarify its taxonomic status and emend the morphological characteristics of this pontogeneiid amphipod.

The enigmatic groundwater amphipod Awacaris kawasawai revisited: synonymisation of the genus Sternomoera, with molecular phylogenetic analyses of Awacaris and Sternomoera species (Crustacea : Amphipoda : Pontogeneiidae)

Amphipod crustaceans are dominant in subterranean habitats, and members of eight genera are endemic to groundwater environments in the Japanese Archipelago. The taxonomic status of two of these genera remains unclear, because their original descriptions were incomplete. The descriptions of the enigmatic subterranean monotypic genus Awacaris and its type species, A. kawasawai Uéno, 1971, are revisited here. Awacaris kawasawai was originally described based on specimens from a subterranean stream at Himise Cave, Tokushima Prefecture, Shikoku, Japan. Recently, a new population of A. kawasawai was found at Saruta Cave, Kochi Prefecture, Shikoku. Detailed observation of the newly collected specimens reveals the presence of sternal gills, which is the diagnostic character of the pontogeneiid genus Sternomoera, making the validity of Sternomoera open to question. Phylogenetic analyses using nuclear 28S rRNA and mitochondrial cytochrome c oxidase subunit I markers demonstrate that A. kawasawai forms a well-supported clade with the subterranean S. morinoi Tomikawa and Ishimaru, 2014. In addition, phylogenetic analysis reveals cryptic diversity in epigean species of Sternomoera. Ancestral state reconstruction suggests that catadromous Sternomoera species have evolved from freshwater ancestors. Based on our morphological and phylogenetic analysis of Awacaris and Sternomoera species, it is concluded here that Sternomoera should be treated as a subjective junior synonym of Awacaris.

Description of a new species of Sternomoera (Crustacea: Amphipoda: Pontogeneiidae) from Japan, with an analysis of the phylogenetic relationships among the Japanese species based on the 28S rRNA gene

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.

Round-trip Catadromous Migration in a Japanese Amphipod, Sternomoera rhyaca (Gammaridea: Eusiridae)

We conducted a field study of the life cycle of the eusirid gammaridean amphipod Sternomoera rhyaca Kuribayashi, Mawatari, and Ishimaru, 1996 in a stream at Gokibiru, Hokkaido, Japan over the course of two non-consecutive years. This species is biennial; it spends most of its life in freshwater, but undertakes a short catadromous migration to the sea for reproduction. Reproduction occurs from March-June. Mature adults drift downstream to the sea singly and in precopulating pairs. Copulation and oviposition in the marsupium occur in mixed water at the stream mouth. Males die after copulation; ovigerous females return upstream by walking or swimming, where their eggs develop and hatch, after which the females also die. Juveniles remain in the stream, growing until they reach sexual maturity. Laboratory experiments showed that survivorship of all stages was lowest in seawater and highest in freshwater, though juveniles survived equally well in mixed water (50% seawater) and freshwater. Eggs developed to hatching only in freshwater; hatchlings in seawater and mixed water died within one and 21 days, respectively. Thus, S. rhyaca is well adapted to freshwater. Indeed, the only stages that required elevated salinity were copulation and subsequent oviposition, and we speculate that freshwater inhibits the female pre-reproductive molt. Because the life cycle of S. rhyaca has the most ontogenetically and temporally restricted saltwater phase known in any catadromous animal, its origin and maintenance are of evolutionary interest. We discuss two alternative hypotheses for the origin of the migratory life cycle, and discuss its maintenance in terms of fitness costs and benefits.