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

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We conducted a field study of the life cycle of the eusirid gammaridean amphipodSternomoera rhyacaKuribayashi, Mawatari, and Ishimaru, 1996 in a stream at Gokibiru, Hokkaido, Japan over thecourse 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. Reproductionoccurs from March-June. Mature adults drift downstream to the sea singly and in precopulatingpairs. 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 theireggs develop and hatch, after which the females also die. Juveniles remain in the stream, growinguntil they reach sexual maturity. Laboratory experiments showed that survivorship of all stageswas lowest in seawater and highest in freshwater, though juveniles survived equally well in mixedwater (50% seawater) and freshwater. Eggs developed to hatching only in freshwater; hatchlingsin seawater and mixed water died within one and 21 days, respectively. Thus,S. rhyacais welladapted to freshwater. Indeed, the only stages that required elevated salinity were copulation andsubsequent oviposition, and we speculate that freshwater inhibits the female pre-reproductivemolt. Because the life cycle ofS. rhyacahas the most ontogenetically and temporally restrictedsaltwater phase known in any catadromous animal, its origin and maintenance are of evolutionaryinterest. We discuss two alternative hypotheses for the origin of the migratory life cycle, and discussits maintenance in terms of fitness costs and benefits.
社団法人日本動物学会の論文
2006-09-25

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

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