On the cobbled shores of subtropical islands in Japan, cobble coverage seasonally fluctuates in shore-specific ways. Because of this, the densities of intertidal molluscs might seasonally vary in shore-specific ways. To examine this idea, the seasonal fluctuations in cobble coverage
and mollusc densities were compared on three cobbled shores on Ishigaki Island, Okinawa, subtropical Japan. Cobble coverage and the densities of dominant gastropods (Nerita squamulata and Monodonta labio) and bivalves (Asaphis violascens and Ruditapes variegatus)
were seasonally estimated by quadrat sampling from June 1997 to September 1998. On only one of the shores, cobble coverage remarkably dropped during a rainy season, because a brook on this shore was flooded by a heavy rain and transported sandy sediment onto cobbles. Corresponding to this,
the densities of N. squamulata, M. labio, and A. violascens sharply decreased and the density of R. variegatus markedly increased on this shore only. That is, shore-specific fluctuations in mollusc densities might be due in part to shore-specific fluctuation in
cobble coverage. It is discussed how cobbles and sandy sediment function for each species, and whether mollusc densities are affected by the other factors such as desiccation and recruitment.
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