Shell shock

Like Darwin's finches, Partula snails are essential to our understanding of evolution - yet they're almost extinct. Sanjida O'Connell reports on a plan to reintroduce them into the wild

Captain Cook first mentioned them. They became one of the most important proofs of evolution in action. But that was before they became the victims of a man-made conservation disaster. Partula, a group of Pacific tree-dwelling snails, have become the unlikely heroes of an ecological detective tale. "It's a horror story," says Professor Diarmaid O'Foighil, "but it has a silver lining." O'Foighil, an associate professor in the department of ecology and evolutionary biology at the University of Michigan in Ann Arbor, has discovered a hidden legacy that could help save the Partula tree snail from complete extinction.

Captain Cook first mentioned them. They became one of the most important proofs of evolution in action. But that was before they became the victims of a man-made conservation disaster. Partula, a group of Pacific tree-dwelling snails, have become the unlikely heroes of an ecological detective tale. "It's a horror story," says Professor Diarmaid O'Foighil, "but it has a silver lining." O'Foighil, an associate professor in the department of ecology and evolutionary biology at the University of Michigan in Ann Arbor, has discovered a hidden legacy that could help save the Partula tree snail from complete extinction.

The importance of the Partula was first recognised by Dr Henry Crampton, a biologist from the University of Columbia who wrote and illustrated three beautiful monographs on the snails in the early part of the 20th Century. The snails live only on high volcanic islands in the Pacific. It is likely that they were carried to the islands by birds. Once there, they evolved into separate species and thus have become important to zoologists trying to understand evolution.

They have become the Polynesian equivalent of the famous Galapagos finches, a group of birds that were central to Charles Darwin's theory of natural selection. Both the finches and the snails demonstrate how one species can evolve into another species.

However, Crampton believed that the snails illustrated the randomness of evolution. He could not see a difference in the snails' environment - either between or within the islands - which could account for the change from one species to another. He theorised that random mutations or "genetic drift" rather than directed selection, accounted for the differences between Partula species.

Thirty years after the publication of the third and final monograph in 1962, two young biologists decided to challenge Crampton's theory. Jim Murray and Bryan Clarke, both of whom had just completed their doctorates at Oxford and are now emeritus professors, visited the island of Moorea near Tahiti to search for Crampton's snails. The snails have become a lifelong obsession for the pair who, despite retirement, continue to study them.

Initially, they began with rudimentary surveys; now they analyse the snails' enzymes and DNA. One particularly laborious experiment showed that the snails - like the finches - occupy separate niches in their environment to which they have become adapted. Murray and Clarke measured out a 10-metre cube of space in the jungle and described every plant, snail and snail's location within it.

Using a powerful computer programme, they were able to show associations between the nine endemic species of snail and the part of the plant they were found on. For example, Partula suturalis is a striped snail - chocolate brown on a light brown background, or vice versa, which feeds on algae and fungi that grow on the trunk of hibiscus trees. In other words, because each species occupies such a specific niche, Clarke and Murray were right: each species looks different because each has adapted to its own space in the environment. Crampton was wrong - the differences were not due to random genetic variation.

Meanwhile, as they concentrated on Moorea, a young biologist, Jack Burch, from the University of Michigan, attempted to catalogue the snails on neighbouring Tahiti. He was working with a colleague from the University of Hawaii who died before their monograph could be completed and the research was abandoned. Nevertheless, Clarke and Murray had amassed so much information from Moorea alone that the Partula became a staple of biology text books. That is, until 1975.

That year a French custom's official in Tahiti decided to make a bit of money on the side by breeding African snails to sell to his countrymen for food. With shells 15cm long, these creatures are exceptionally meaty. However, he was an unsuccessful entrepreneur, and a short while later he tipped his snails over his garden wall. With no natural predators, these giant molluscan herbivores spread throughout Tahiti and the rest of the Pacific, decimating crops as they went.

The French government, thanks to a recommendation from the US Department of Agriculture, introduced a carnivorous snail, the rosy wolf snail, to eat the African one. Clarke warned them against such an action, as he said the carnivorous snail would not remove all of its intended prey, but would definitely find the delicate centimeter-long Partula species delicate morsels. Sadly, he was correct. "Over the whole of the Pacific, thousands of species became extinct," says Clarke. "These snails ate dead leaves. Not only have we lost them, but we have no idea how their absence will change the ecology of these islands."

In the islands round Tahiti alone, more than 100 species were driven to extinction within 10 years. As Clarke and Murray charted the demise of the Partula, they collected some snails and brought them back to Nottingham. These snails, from 22 species, formed the nucleus of an international breeding programme now taking place at a number of zoos, including those in Jersey, London and Detroit, which operates in collaboration with the local Polynesian community.

In 1994 the first trial reintroduction took place on Moorea. A 20-metre reserve was built for three species, bounded by corrugated metal, a moat filled with salt water and, in case the rosy wolf snail circumnavigated these two hurdles, an electric fence. The experiment showed that the snails could be successfully reintroduced. Dr Paul Pearce-Kelly, the curator of invertebrates at London Zoo and the co-ordinator for the International Partula Programme, says: "The beauty of snails is that you don't have to teach them anything."

Another ray of hope is that on the highest island of Tahiti snails have been spotted, and they could be Partula. "At least we know that something is alive up there," says Clarke.

The problem for the conservationists is knowing where to put which species, since the original ones are all extinct. Also, Clarke and Murray's work was only carried out on Moorea and, finally, Crampton did not have the advantage of today's technology, which allows scientists to show unequivocally whether a set of animals belong to different species, or are merely natural variations within a single species.

For example, in spite of differences in skin, hair, eye colour and physique, we humans are all part of one species, just as snails with different shell patterns could also be from the same species. This is why O'Foighil's recent finding is so important.

"Jack Burch mentioned he'd been to Tahiti, but I didn't pay attention as he's worked everywhere," says O'Foighil. Then one day Burch showed O'Foighil a tray in a freezer in his lab. "It was like finding the treasure of the Aztecs," says O'Foighil. The tray contained 600 Partula snails that Burch had collected 30 years ago before the rosy wolf snail had been introduced to Tahiti. O'Foighil studies marine invertebrates and has no background in this area, but realising the collection's importance, was compelled to study Partula.

Burch had shipped the snails back alive and freeze-dried them so their DNA was in immaculate condition in spite of the age of the specimens. "This will give us a complementary data set to Murray and Clarke's data from Moorea and will give us a fuller picture of how the species are related to each other," says O'Foighil. Not only do these species provide a unique snapshot of evolution in action, they are exactly what the conservationists needed.

"One of the dilemmas the reintroduction team has faced is how to prioritise, since they don't know what the original fauna were, what species were there, how they were related, what proportion of the original species we have and whether the populations we bred are truly distinct species or whether we have redundancies."

O'Foighil is feeding this information to Dr Trevor Coote from London Zoo, who is in Tahiti carrying out surveys, searching for surviving populations of the predator snail and any remnants of Partula, and trying to assess the best sites for the new reserves.

"We think of pandas and rhinos as being flagship species," says Pearce-Kelly, "but in this case a snail is helping focus all our conservation efforts in Polynesia."

O'Foighil adds: "These snails are beautiful; they were a cultural heritage, as the Polynesians used their shells as jewellery, and they are a biological treasure. The situation is horrific, but it can be rectified, and that is inspirational."