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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.

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.

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