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.