Hydrobiidae of Lord Howe Island (Mollusca : Gastropoda : Prosobranchia)

Abstract

The Hydrobiidae of Lord Howe Island consist of three genera and nine species. Two species can be divided into two subspecies and one species into five subspecies. The total number of species-group taxa is 15. It is suggested that this fauna is the result of three separate colonizations during the late Tertiary or Pleistocene. Most of the species live in streams, one is adapted to living on waterfalls, and another is subterranean in habit.

Hemistomia Crosse contains six species. One, Bythinella whiteleggei Brazier, is probably extinct and two others are restricted to the summit of Mt Cower. A fourth species can be separated into two subspecies, both previously undescribed. One new species is subterranean in habit. Fluviopupa Pilsbry contains two species, one with two subspecies and the other with five, three of the subspecies being new. The remaining genus group, tentatively referred to Potamopyrgus Stimpson, contains only one species [P. oscitans (Iredale)] which is found living on waterfalls.

There are seven species pairs on the island, which show a size ratio of 1.3-1.8, suggesting that character displacement (size) may have occurred.