Prehistoric aesthetics explains snail biogeography puzzle

The answer to a mystery that long has puzzled biologists may lie in prehistoric Polynesians' penchant for pretty white shells, a research team headed by University mollusk expert Diarmaid Ó Foighil has found.

(Photo by Olivier Gargominy)

The team's findings, published online Sept. 12 in the British biological research journal Proceedings of the Royal Society B, have implications for conservation efforts aimed at rescuing nearly extinct Tahitian tree snails.

The study focused on a tree snail species, Partula hyalina, found on the island of Tahiti — where it has been nearly wiped out by a predatory snail introduced in the 1970s — and also on the Austral and Southern Cook Islands. The snail's multiarchipelago distribution is unique in the partulid tree snail family; most are restricted to single islands.

But even more intriguing is the observation that while this snail exhibits a range of shell colors, including white, on Tahiti, only white-shelled variants are found in the Austral and Southern Cook Islands. What's more, P. hyaline — white-shelled or otherwise —isn't found at all on Tahiti's nearest neighbors, Moorea and the other islands in the Society archipelago.

The odd distribution pattern has had biologists scratching their heads since at least the 1880s. Over the years they've come up with a variety of possible explanations, suggesting for example that the white-shelled forms are actually all distinct species that independently evolved on different islands.

But Ó Foighil and coworkers, who knew from previous genetic research that P. hyalina originated on Tahiti, thought a more likely explanation was that the snails were introduced to the other islands. The question was, how?

"Land snails are known to have been introduced to many Pacific islands by Polynesians but all the other cases were inadvertent introductions involving tiny snails of continental origin associated with food crops; the introduced snails were not endemic to the islands," says Ó Foighil, who is an associate professor of ecology and evolutionary biology and a curator at the Museum of Zoology.

"We know that Polynesians used these shells ornamentally. We think the fact that the white shells were aesthetically valuable within this regional trading network explains their unusual distribution," Ó Foighil says.

In addition to providing insights into early Polynesians' taste in jewelry, and into prehistoric trading links among these archipelagoes, the research suggests solutions to conservation quandaries.

Since the 1980s, researchers and conservationists have been setting up captive populations of Tahitian tree snails in European and North American zoos, including the Detroit Zoo.

Ó Foighil, Lee and Burch collaborated on the work with Trevor Coote of the International Partulid Conservation Programme in Tahiti, Benoít Fontaine and Olivier Gargominy of the Muséum National d'Histoire Naturelle in Paris and Paul Pearce-Kelly of the Zoological Society of London.