Candidiopotamon rathbanae phylogeography

Intraspecific genetic diversity of the endemic freshwater crab Candidiopotamon rathbunae (Decapoda, Brachyura, Potamidae) reflects five million years of the geological history of Taiwan

H.-T. Shih, H.-C. Hung, C. D. Schubart, C. A. Chen & H.-W. Chang

Aim Candidiopotamon rathbunae (Crustacea: Brachyura: Potamidae) is a freshwater crab endemic to Taiwan, with a distribution covering almost the entire island. Crab populations from different river systems cannot be distinguished morphologically. In this study, we investigate the phylogeography of C. rathbunae from 18 of the main river systems of Taiwan, in order to reveal geographical differentiation and cryptic endemism. We used specimens of two congeners (C. okinawense and C. kumejimense) and the closely related Amamiku amamensis from the Ryukyu Islands as outgroups.

Location Taiwan, with the Ryukyu Islands (Japan) as the outgroup locality.

Methods Mitochondrial DNA sequences encoding 553 basepairs of the large subunit rRNA (16S rRNA) gene were obtained from 96 specimens throughout Taiwan and the Ryukyus. We compared these sequences by means of phylogenetic analyses (minimum evolution, maximum likelihood, maximum parsimony, and a genealogical parsimony network) and molecular-clock time estimates.

Results The estimated timing of the corresponding separations clearly correlates with geological events during the Penglai Orogeny of Taiwan (c. 5 Ma) following the collision of the Philippine Sea plate with the Eurasian continental plate. A deep split of 6.29% sequence divergence was found between the eastern clade (S, SE, and E groups) and the western clade (NW, W, and SW groups) of C. rathbunae. Separation of these groups reflects the isolating effect of the uplift of the Taiwan Central Range (c. 5 Ma). The separation of the SW group from the W group, and of the E group from the SE group can also be explained by geological events during the Taiwanese orogeny, the estimated geological timing being in close accordance with our molecular dating.

Main conclusions According to the molecular results, the ancestral Candidiopotamon was among the earliest non-marine colonizers of Taiwan, settling on this island no later than the early Pliocene. Our molecular data reveal that C. rathbunae can be subdivided into six groups in accordance with geography.

Keywords 16S rRNA gene, allopatric divergence, Candidiopotamon molecular clock, mtDNA sequence, phylogeography, Potamidae, Taiwanese orogeny.

Figure 1 Collection sites (open circles) for Candidiopotamon rathbunae in Taiwan, and for the congeneric and closely related species from the Ryukyus. Dotted lines indicate the possible biogeographical boundaries among them.

Figure 2 A linearized minimum evolution tree of six megapopulations of Candidiopotamon rathbunae, and the congenetic and closely related species from the Ryukyus based on 553 basepairs of the 16S rRNA gene, with the estimated times of divergence based on molecularclock calibrations and the distribution of haplotypes in each region. Probability values at the nodes represent confidence after 2000 (ME, MP) and 100 (ML) bootstrap reiterations in the analysis of minimum evolution, maximum parsimony, and maximum likelihood, respectively. The main geological events in Taiwan are shown below.

Figure 3 A genealogical parsimony network for the 16S mtDNA haplotypes of the populations of Candidiopotamon rathbunae. Unlabelled nodes indicate inferred steps not found in the sampled populations, and the numbers beside nodes indicate the number of substitutional changes between haplotypes.

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