A Regional Biogeography of Shore Molluscs: Influence of the Kuroshio Current and the Two Capes

Abstract

The regional pattern of shore molluscan assemblages was investigated along a 650-km coastline of the Kii Peninsula in central Japan. Hydrography of the coastal waters differed between three subareas, the Inner Kii Channel (IKC), the Outer Kii Channel (OKC), and the Sea of Kumano (SOK), which are demarcated by the two prominent capes, Cape Hinomisaki and Cape Shionomisaki. The water of OKC was warm, saline, and transparent compared to that of the other subareas, due to the influence of the intruding Kuroshio current. Corresponding to this hydrographic difference, the composition of shore malacofauna differed between OKC and the other subareas. Species diversity and relative abundance of the southern species (< 35° N) increased southward and were higher in the OKC than in the other subareas. Multivariate analysis on various environmental and biotic parameters detected significant relationships between the temperature and geographic groups of species, and between substratum geology and trophic groups. The present and previous results elucidated the following biogeographic process in the study area. The Kuroshio washes the coast of the OKC, and enriches the southern species by increasing the temperature and providing their larvae; thus the diversified warm-water malacofauna of the OKC intervenes between the colderwater malacofaunas of the IKC and SOK. This major pattern is further modified by shore geology and the degree of wave exposure at local scales. The present survey method, a semi-quantitative timed search for intertidal assemblages required less time than, and yet yielded results similar to, the widely used dispersed quadrat survey for the parameters selected in the present analysis.