Distribution patterns of the Ogasawara mitten crab Eriocheir ogasawaraensis and brachyuran crabs in the rivers and streams of Chichi-jima, Ogasawara (Bonin) Islands [Japan]

The Ogasawara mitten crab Eriocheir ogasawaraensis is a varunid species endemic to the Ogasawara (Bonin) Islands. Although this crab undertakes catadromous migration to the sea for reproduction, little is known about its ecological characteristics. Therefore, we carried out field surveys on the distribution patterns of E. ogasawaraensis and other brachyuran crabs in the rivers and streams of Chichi-jima, Ogasawara Islands, from June to July and in November, 2007. Although, the streams and rivers of Chichi-jima are generally small and the range of the tidal river area is usually very short, the upper tidal river area is an extremely important habitat for the recruitment and maintenance of the mitten crab population. The present survey revealed that E. ogasawaraensis settled in a narrowly limited area nearly dozens of meters long at the river mouth. Juvenile crabs (13-23 mm carapace width) with relatively longer legs migrated upstream to the headwaters at a high altitude. Though the distribution of E. ogasawaraensis covered the entire river area, it was strongly inclined toward the headwaters. The microhabitat of the crab was usually underwater, though an occasional adult crab wandering along the riverside wetland was also captured. This distribution pattern differs from that of its congener, E. japonica, which favors freshwater downstream areas in mainland Japan. In addition, the number of mitten crabs caught was too small for the effort made in the present survey, suggesting that the population of this species in Chichi-jima may be endangered. Three brachyuran species (Ptychognathus glaber, Varuna litterata and Chiromantes dehaani) were also widely distributed from the downstream area to the upper freshwater area in the river. Complexity of brachyuran species composition in the rivers and streams of Ogasawara Islands was generally poor compared to that in the Japanese mainland and the Ryukyu Islands.