Ryukyu Islands
Biogeographic boundaries

Watase's Line marks a major biogeographic boundary. The Ōsumi islands north of the line belong to the Palearctic realm while the Amami islands south of it are at the northern limit of the Indomalayan realm. A deep undersea canyon, the Tokara Gap, lies to the east of the Tokara islands, but where the line crosses the island chain is disputed. It has been "placed between Akusekijima and Kodakarajima islands of the Tokara archipelago ", but there is variation throughout the Tokaras, with multiple inter-island gaps having similar influence. Species diffusion among the Tokara islands is thought to have been over sea, not via land bridge, and their faunal composition also depends on other factors such as island size.

Miyake's line for insects lies between Kyūshū and the Ōsumi island groups, and Hachisuka's line for birds between the Okinawa and Miyako groups.
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