Taiwan & SE China Somanniathelphusa

Phylogeny of the freshwater crabs genus Somanniathelphusa Bott (Decapoda: Parathelphusidae) from Taiwan and the coastal regions of China, with notes on their biogeography

Hsi-Te Shih, Shao-Hua Fang and Peter K. L. Ng

It is generally accepted that the southern part of mainland China was connected to, and then separated from, the island of Taiwan many times as a result of successive glaciation events, the most recent being 15000 years ago. On the basis of this, many biogeographical hypotheses have been proposed to explain the origins and relationships of the flora and fauna of Taiwan and mainland China. However, no clear genetic evidence from the flora and fauna of Taiwan has been forthcoming to support the geological history or biogeographical hypotheses. Land-locked freshwater crabs of the genus Somanniathelphusa Bott, 1968 (Crustacea: Brachyura: Parathelphusidae) inhabit the lowland coastal plains of East Asia and also Taiwan Island, which is their only known locality outside of the main continent. Of the eight species of Somanniathelphusa known from Taiwan and the coastal regions of China (Fujian, Guangdong and Hong Kong), five are studied here: S. amoyensis Naiyanetr & Dai, 1997; S. taiwanensis Bott, 1968; S. zanklon Ng & Dudgeon, 1992; S. zhangpuensis Naiyanetr & Dai, 1997; and S. zhapoensis Naiyanetr & Dai, 1997. Somanniathelphusa taiwanensis has a restricted range in west-central Taiwan, which has been explained by the invasion of species from the mainland via the landbridge of Taiwan Strait during glaciation. This is confirmed by a molecular phylogenetic analysis of the species and its congeners on the mainland. Based on the comparisons of DNA sequences encoding part of the mitochondrial large subunit (16S) rRNA gene and cytochrome oxidase I (COI), two major groups are discernible. Somanniathelphusa zhapoensis from western Guangdong belongs to its own distinct group. The remaining species belong to the second group, but two constituent clades can be recognised, here referred to as the ‘S. taiwanensis’ and ‘S. zanklon’ clades, respectively. These two clades are currently isolated by a major mountain range between Fujian and Guangdong. The molecular evidence also strongly supports the recent invasion of Taiwan by the genus Somanniathelphusa from Fujian during the last glacial event.

Key words: cytochrome oxidase I, glaciations, mitochondrial 16S rRNA, phylogeography, Somanniathelphusa

Fig. 1. Collection sites for Somanniathelphusa species from Taiwan, Hong Kong, Hainan Island and the Chinese coastal provinces (Guangdong and Fujian) used in this study. The numbers beside the grey circles represent the collection sites: 1, Shihguei R., Dounan, Yunlin, Taiwan; 2, Wushulin, Baihe, Tainan, Taiwan; 3, Xiamen, Fujian, China; 4, Zhangpu, Fujian, China; 5, Dongguan City, Guangdong, China; 6, Nanhai City, Guangdong, China; 7, Zhapo, Yanjiang, Guangdong, China; 8, Su Kwun, New Territories, Hong Kong; 9, Qiongshan, Hainan, China.

Fig. 2. A neighbour-joining (NJ) tree of the Somanniathelphusa species from Taiwan and the Chinese coastal regions based on 1212 base pairs of the combined 16S rRNA and cytochrome oxidase I genes. Probability values at the nodes represent bootstrap values for NJ and maximum parsimony (MP), and posterior probability for Bayesian inference (BI). The species name(s) beside the haplotype was the species identified based on morphological traits. For haplotype abbreviations see Table 1.

Fig. 3. A postulated paleo-drainage system on the Taiwan continental shelf during glaciations in Late Pleistocene (~15000 years ago). Sea level is assumed to be 140 m below the present level (modified from Boggs et al. 1979). The broken line in west-central Taiwan indicates the present distribution of Somanniathelphusa taiwanensis. The arrow from China to Taiwan indicates the possible dispersal pathway of the ‘S. taiwanensis’ clade during glaciations.

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