Diversity and biogeography of freshwater crabs (Crustacea: Brachyura: Potamidae, Gecarcinucidae) from East Asia

The diversity and biogeography of the two families of freshwater crabs in East Asia (China, Taiwan, Ryukyus and mainland Japan) were studied. The richness in different areas, as well as the distributions of all 40 genera and 311 species were analysed, the patterns examined and possible mechanisms discussed. The published data of freshwater crab species and their distributions in East Asia were analysed by using clustering, diversity index and graphical methods. Six biogeographical districts were recognized for the East Asian freshwater crabs based on similarity of distribution: (I) Hengduanshan Area, including Yunnan and Tibet; (II) Karst Area of Guizhou; (III) Yangtze River Basin, extending to Yellow River basin, with three subdivisions; (IV) Wuyishan Area and Pearl River Basin in Southeast and South China; (V) Hainan Island; and (VI) East Asian Islands, with subdivisions of Taiwan, Ryukyus and mainland Japan. The most diverse genera are the potamids Sinopotamon (84 species) and Geothelphusa (54 species), as well as the gecarcinucid Somanniathelphusa (27 species). Yunnan has the highest species richness (48 species) and diversity (Shannon index 2.21) in continental China, while Taiwan has the highest species richness (42 species) of the island systems. The distribution of the species of the potamid Sinopotamon and other genera along China's longest drainage, the Yangtze River, can be subdivided into several zones corresponding to the upper, middle and lower reaches of the river basin. The freshwater crab fauna of continental China, the East Asian islands and Hainan Island were all different from each other, and this finding agreed with the findings of molecular phylogenetic studies. The distribution of freshwater crabs in the Yangtze River showed a similar pattern with other freshwater organisms in this drainage. The present biogeographical interpretation contrasts sharply with the traditional zoogeographical patterns proposed that are based on Chinese terrestrial vertebrates. The present analyses of crab distributions suggest that Yunnan is a possible centre of origin for the Potamidae of China and perhaps the whole of East Asia. The freshwater crab species richness in Yunnan is probably the result of its generally warmer climate, more diverse habitats and a long history of cladogenesis.