四国太平洋岸に共存するスナガニ属3種の季節的な分布パターンおよび食性

抄録

On sandy beaches of southern Shikoku, one temperate-zone crab, Ocypode stimpsoni, and two tropical and subtropical crabs, O. sinensis and O. ceratophthalma, co-occur, dwelling in individual burrows. Their distribution patterns, food habits, and physiological condition were investigated seasonally. Among the three species, O. sinensis was distributed in the highest zone vegetated with Vitex rotundifolia and Calystegia soldanella and was well segregated from the others except during its juvenile stage. It fed on terrestrial invertebrates and terrestrial plants. The distribution of O. ceratophthalma overlapped with that of O. stimpsoni; O. ceratophthalma mainly fed on terrestrial and aquatic invertebrates whereas O. stimpsoni fed on detritus and diatoms together with terrestrial and aquatic invertebrates. Ocypode ceratophthalma was poor in hepatopancreas index late in autumn and rarely overwintered, with the result that almost the entire population consisted of immature individuals. It is likely that the population of O. ceratophthalma in southern Shikoku is maintained by a supply of larvae from more southern regions. It is possible that O. stimpsoni, which is competitively inferior to O. ceratophthalma due to its smaller adult size, may become extinct in southern Shikoku if the recent rise in air temperature during winter enables more individuals of O. ceratophthalma to thrive there.