Karyology and biogeography of Dugesia japonica and Dugesia ryukyuensis in Kyûshû, southern Japan

Abstract

In southern Japan, two dugesiid species of freshwater planarians are known: Dugesia japonica Ichikawa et Kawakatsu, 1964, and Dugesia ryukyuensis Kawakatsu, 1976. D. japonica is a common and polymorphic species widely distributed in the Far East (karyotypes: n=8, 2x=16, 3x=24). D. ryukyuensis is a species recorded only from the Southwest Islands of Japan (Nansei Shoto) (karyotypes: n=7, 2x=14, 3x=21). Recently, small populations of D. ryukyuensis were found in the lowland areas in Kyushu on the East China Sea (the Nishisonogi Peninsula, the Shimabara Peninsula, the Goto Islands, the Satsuma Peninsula, and the Osumi Peninsula). The current geographical distribution of D. japonica and D. ryukyuensis in southern Japan can be explained from geological and faunal viewpoints, as follows: (1) two separate inversions by the ancestor of D. japonica, one in the Miocene and one after early Quaternary; (2) only one expansion of its domain by the ancestor of D. ryukyuensis in the Miocene.