The distribution of conch shell contained in clastic sediments in the mangrove swamps in the Funaura Bay, Iriomote Island, Okinawa Prefecture was studied. The sediments in the mangrove swamp are mainly composed of up to 90% sands. The sand clasts are inferred to be derived from the sandstone of Miocene Yaeyama Group. The conch shells are richer in the muddy fraction than the sandy fraction. Many Terebralia palustris inhabit the mangrove swamp. However few dead shells were also observed in the sediments. Effect of selective transportation hermit crabs is considered to be the cause of this distribution.