The distribution is unusual for several reasons. Most species of Partula are restricted to single islands — they're not good dispersers — yet this one has clearly got around. But if it dispersed from Tahiti under its own volition how (or why) did it skip nearby spots to get to the distant Cook Islands? And to make the story more intriguing why did only one of the multiple colour forms go roaming? So many questions, so few snails.
Diarmaid Ó Foighil of the University of Michigan suggests that the snails were transported around the South Pacific by prehistoric travellers who prized the white shells for their beauty.
The combination of aesthetic preference and fashionability made the unlikely introduction possible.
Read the fascinating story of Partula, pulchritude and Polynesian sea passages at the University of Michigan's web site and in the links kindly provided by Mo (which include the abstract of the paper in Proceedings of the Royal Society B). (Thanks, Mo!)