伊豆半島南端部の河口域カニ類相の特徴

抄録

In recent years, many subtropical brachyuran crab species distributed mainly along the southern Japanese coast have been recorded in the southern portion of the Izu, Miura, and Boso Peninsulas in eastern Japan near the warm Kuroshio Current axis. In this study, we conducted multiple surveys of the crab fauna in the Aono and Ogamo Rivers in the southern Izu Peninsula from 2014 to 2018, and assessed their population stability based on their occurrence frequency. We also compared the composition of the crab fauna using cluster analysis based on the presence-absence data with those at Yukashi Lagoon (Kii Peninsula), Ena Bay (Miura Peninsula), and Banzu tidal flat (Tokyo Bay). The results showed that the subtropical crabs Parasesarma bidens and Ptychognathus ishii were highly abundant in the Aono and Ogamo Rivers, and that the crab fauna in the Izu Peninsula was similar to that of the Kii Peninsula, but differed from those of the Miura Peninsula and Tokyo Bay. Similarity in the community structure between Kii and Izu Peninsulas suggested that the southern portion of the Izu Peninsula functions as a habitat for the subtropical crabs, and acts as “stepping stones” for expanding their distribution toward northeastern regions. Further, the southern Izu Peninsula had various microhabitats, such as gravel and cobble zones, mangroves, reed marshes, and soft bottoms in the estuaries, which could function as stable habitats preferred by these subtropical crabs.