KURIHARA, T.; Ishigaki Tropical Station: Geographical distribution patterns of intertidal molluscs on rocky shores in all Japan

Japan has long coastlines. These extend from subtropical to subarctic zones, facing the Pacific, the Sea of Japan, and many inland seas. In such wide and complicated areas, however, the diversity and taxonomic composition of marine organisms have rarely been described quantitatively. My aim was to describe quantitatively the diversity and composition of rocky-shore molluscs in all Japan. I analyzed a part of the 3rd Marine Organisms Biological Environment Survey Report by the Japan Environment Agency. The report evaluated the densities of mollusc species by throwing quadrats at several heights on 55 Japanese rocky shores during 1984 – 86. I grouped these shores into 9 regions on the Pacific, Sea-of-Japan, and Seto-Inland-Sea coasts: P1 to P5, J1 to J3 (a lower numerical order denotes a lower latitude), and SI. In total, the regions harbored 271 species, 191 genera, and 146 families reliably identified. The Shannon diversity indices calculated at each taxonomic level were often lower at lower latitudes unexpectedly: J1 < J2, P2 < P3, and SI < J1 (latitude; SI < J1). To explain such low diversities in J1, P2 and SI, I sought environmental factors (e.g. rock type) being monotonous in such regions but failed. The Bray-Curtis dissimilarity indices calculated between the 9 regions regarding the taxonomic compositions generally corresponded to the region-to-region spatial distances. Yet, disproportionally were the dissimilarities high between SI and the other regions and low between J2 and J3. I related the peculiarity of SI to its environments typical for inland seas (e.g. eutrophication); and the similarity between J2 and J3 to their similar environmental factors (e.g. water temperature).