A biological diversity of mudflat benthic molluscs in the Isahaya Bay, Nagasaki Prefecture

Abstract

Detailed aspect of mudflat benthic ecosystem was analyzed for dried-up tidal mudflats of Isahaya Bay. Many molluscan species have been found there, for example, gastropod potamids Cerithideopsilla djadjariensis, bivalves Merisca capsoides, Tegillarca granosa, fixo-sessile Crassostrea gigas or C. ariakensis and so on. The most abundant species is T. granosa, which occurs at density of up to 60/m². The distribution of molluscan assemblages is controlled not only by the mud content but also by the width of tidal flats. In relatively extensive tidal flats, the distribution of each species hardly overlap with each other. On the other hand, in relatively narrow tidal flats, they overlap with each other. Molluscan species highly utilize the restricted space of mudflats to form definite stratam. Regarding endobiontic bivalves, deeply digging species and shallowly digging ones co-exist within the bottom, whereas the surface of tidal flats is occupied by potamids species. And Crassostrea gigas or C. ariakensis occupies the space above the tidal flats. Besides such spatial segregation, temporal segregation occurs according to cyclical ebb and flood tidal processes. Detritus feeder M. capsoides possibly eats the sediments transferred from water to bottom by filteration of T. granosa and its excreta. In consideration of evolution of tidal flat association, molluscan fauna in the Isahaya Bay is comparable with the Miocene so-called Arcid-Potamid fauna and so on.