Recent Ostracode Fauna in the Pacific off Southwest Japan

Abstract

Recent ostracode assemblages from the Pacific around Southwest Japan are analyzed and the species described. Six areas are investigated, namely, Kumano-nada, off Tanabe, Tosa Bay, Bungo-suido, Hyuga-nada and off Tanega-shima. From 80 surface sediment samples, a total of 191 species among 80 genera are identified, of which 61 selected species are systematically described and 10 species are described as new. Ostracodes in this region show strong affinities with those of the Sea of Japan and of the East China Sea. The fauna is dominated by subtropical/tropical species associated with widely distributed "Japonic elements". Three cryophilic species are found very rarely, showing a deepening of the distribution of assemblages in the area studied. On the basis of the geographical distributions of shallow water ostracode species, two distinct biogeographical discontinuities are recognized along the Pacific coast of Japan. One is located between Suruga Bay and Sendai Bay, and another between Kyushu and Tanega-shima Islands. The two locations correspond with the boundary of the warm temperate/subtropical zones and that of the subtropical/tropical zones, respectively. As a result many species are characterized as subtropical or tropical. A remarkable degree of downslope transportation of ostracode shells is evident in each area investigated. The frequent condensation of ostracode shells in the sediments, and the varying depth ranges of the same species in different areas, are considered to be the result of postmortem transportation. The preservation states of the ostracode shells are variable. The proportion of fresh, translucent shells is, in general, inversely correlated with water depth and mud content; this fact suggests that the opaqueness of ostracode shells may be the result of dissolution of the calcite crystals by sea water.