The biogeographical history of giant earthworms of the Metaphire formosae species group (Clitellata: Megascolecidae) in Taiwan and the Ryukyu Archipelago, with the description of a new species from Yonagunijima, Southern Ryukyus

摘要

The non-volant terrestrial fauna of the Ryukyu Archipelago is characterized by high endemism with island groups forming distinct biogeographical regions separated by straits. This endemism has resulted from complex histories of insularization caused by geological events and dispersal through land bridges or across the sea during a timeframe of several million years. However, little is known about the zoogeography of terrestrial invertebrates in this region. We presented the first study looking into the earthworm fauna between Taiwan and the Ryukyu Archipelago under a phylogeographical context, and investigated the systematics and phylogeography of the giant earthworms in the Metaphire formosae species group endemic to this region. We highlighted strong evidence supporting that giant earthworms collected from Yonagunijima, the westernmost island of the Ryukyus, represent a new member of the species group, and further described it as a new species, Metaphire dunan sp. nov. Phylogenetic analyses and divergence time estimation rejected the hypothesis that the separation between M. dunan and its Taiwanese sister species, M. formosae, took place 8 Ma in late Miocene, when Taiwan and the Ryukyu region were supposedly first separated from each other and from the Eurasian continent. Instead, our results suggested that the speciation within the M. formosae species group occurred during the arc-continent collision in Taiwan after 5 Ma and M. dunan diverged from its sister species soon after the diversification of the species group had started. In Taiwan, the divergence of mountain-dwelling species followed a north-to-south pattern, consistent with the southward-propagating arc-continent collision on the island.