Carabid beetles (Carabus blaptoides) from Nii-jima and O-shima isles, Izu-Bonin oceanic islands: Dispersion by Kuroshio current and the origin of the insular populations

Carabus blaptoides is endemic on the Japanese islands. Two populations of this species were recently found on the oceanic islets of O-shima and Nii-jima, offshore of central Japan. We evaluate the origin of these populations based on morphological and molecular phylogenetic analyses. Molecular analyses show that the O-shima population is closely related to the Boso peninsula population of Honshu, and the Nii-jima population is closely related to the northern Kyushu population. We suggest that the Boso population migrated relatively recently from central Japan, apparently by way of a land bridge formed by an emergent forearc high, followed by further migration to O-shima. In contrast, the Nii-jima population migrated very recently from northern Kyushu, probably by rafting on the Kuroshio current aboard driftwood swept into the sea from floods that occurred in the Ariake bay area in 2012. Similar Kuroshio-driven migration is also expected for the ancient Izu-Bonin oceanic islands.