Systematic and biogeographical aspects of the first Antarctic discovery of Orthoplana bregazzii Karling, 1973 (Proseriata, Otoplanidae)

ABSTRACT Three essential questions remain unresolved about the present Antarctic biota: its antiquity, origin, and ubiquity. Some benthic animals are capable of active self-dispersal following shallow corridors between islands. Identifying and characterizing the biota in such corridors is fundamental for biogeographic studies and the delimitation of priority sites to conserve biodiversity. The Scotia Arc provides an example of such passages, being a system of islands and underwater ridges connecting the Magellan Region to the Antarctic Peninsula and South Shetland Islands (Maritime Antarctic). Specimens of the free-living flatworm Orthoplana bregazzii Karling 1973 (Proseriata, Otoplanidae) were obtained in King George Island (South Shetland Islands). This is the first report of the family Otoplanidae from Antarctica, and the first rediscovery of the species 50 years after its original description. The present article provides a complete description of the Antarctic material of O. bregazzii, complements its original description, and provides an identification key to the species of Orthoplana. We analyze the distribution of the genus and species and argue that the range expansion reported here could be the result of a recent, successful colonization event. This work, therefore, gives insights into the systematics and biogeography of one of the lesser-known groups in the Antarctic fauna.