Distribution pattern of Armadillidae species (Crustacea: Isopoda: Oniscidea) on the Izu Islands, Japan

Faunal studies of oceanic islands provide unique opportunities to study ecology and evolution. Four Armadillidae taxa were recorded from the Izu Islands, a group of oceanic islands off the coast of Japan. The distributions of these taxa and a phylogenetic tree are based on molecular data for 249 specimens from 88 sites. The Spherillo russoi (Arcangeli, 1927) species complex is distributed widely throughout these islands, and dispersal may have occurred via oceanic currents. On the nearby Izuoshima and Toshima Islands, Spherillo dorsalis (Iwamoto, 1943) may have recently migrated from the Izu Peninsula either via currents or been transported by human activity. In the middle islands, Shikinejima, Kozushima, Miyakejima, and Mikurajima, Venezillo hasegawai (Nunomura, 1991) was found, which may be derived from Spherillo obscurus (Budde-Lund, 1885) distributed in the Kanto region on the mainland. On the most southern and distant Hachijojima Island, Spherillo punctatus Nunomura, 2007 was found, and it does not differ genetically from specimens from Izenajima and Tokunoshima Islands in the Nansei Island Group, despite an approximate 1,000-km distance between them; this species may have been transferred by human activity. Our study showed that the Armadillidae assemblages in the Izu Islands were formed by several migration routes.