Spatial and temporal fluctuations in densities of gastropods and bivalves on subtropical cobbled shores

On the cobbled shores of subtropical islands in Japan, cobble coverage seas onally fluctuates in shore-specific ways. Because of this, the densities of intertidal molluscs might seasonally vary in shore-specific ways. To exami ne this idea, the seasonal fluctuations in cobble coverage and mollusc dens ities were compared on three cobbled shores on Ishigaki Island, Okinawa, su btropical 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 J une 1997 to September 1998. On only one of the shores, cobble coverage rema rkably dropped during a rainy season, because a brook on this shore was flo oded by a heavy rain and transported sandy sediment onto cobbles. Correspon ding to this, the densities of N. squamulata, M. labio, and A. violascens s harply decreased and the density of R. variegatus markedly increased on thi s shore only. That is, shore-specific fluctuations in mollusc densities mig ht be due in part to shore-specific fluctuation in cobble coverage. It is d iscussed how cobbles and sandy sediment function for each species, and whet her mollusc densities are affected by the other factors such as desiccation and recruitment.