Abundances and size structures of crabs of the genus Ocypode were investigated seasonally from 2000 to 2003 in Nakusanohama-beach, Wakayama, middle Japan. One temperate species, Ocypode stimpsoni, and two subtropical species, O. sinensis and O. ceratophthalma, were collected, with the number being largest in O. ceratophthalma and smallest in O. sinensis. Ocypode stimpsoni was dominant among the crabs collected from spring to summer, whereas O. ceratophthalma was dominant from summer to autumn. In O. stimpsoni, collected crabs were mostly large-sized, with few newly-settled small crabs, throughout the year. In O. ceratophthalma, newly-settled small crabs occurred from June through August and grew up to be middle-sized in the late autumn, but did not attain the matured size and notoverwinter. Newly-settled small O. sinensis occurred from July to November, and some of them overwintered and attained the mature size in the next year. Judging from the seasonal pattern of population structures of O. stimpsoni and O. ceratophthalma and widely overlapped habitats between the two species, it was assumed that abundance of O. stimpsoni was negatively influenced by O. ceratophthalma.
