Abstract
The Austral Islands comprising five islands of volcanic origin situated in the south of Polynesia, harbour what is probably the most isolated fauna of hydrobioid gastropods. To date, only two species belonging to the genus Fluviopupa have been known. We describe an additional four species. Each species is exclusive to one island. Two species each occur on Rapa and Raivavae. Only on the western-most, oldest, and most degraded island, Rimatara, were no hydrobioids found. The closest relatives occur on Fiji. Since no hydrobioids are known from the island groups between Fiji and the Austral Islands, viz. Tonga, Samoa and the Cook Islands, we assume that the common ancestor has arrived directly from Fiji before these islands were formed in the late Miocene, possibly transported by a bird. Four of the Austral species meet the IUCN criteria for Vulnerable while F. jeanyvesi n. sp. and F. tubuaia n. sp. are considered Critically Endangered. Both are known from a single locality on Raivavae and Tubuai, respectively, and they are threatened by habitat loss.