BioEd Online

For an animal with one foot, it certainly gets around. Even across oceans.

Scientists have shown that the land snail Balea perversa has somehow managed to travel from Europe to the Azores, then right down the length of the Atlantic Ocean to a remote set of isles between South Africa and South America - a 9,000 kilometre trip that seems unlikely for an animal that doesn't even know how to swim. The snails also seem to have made the return trip back to Europe.

The Tristan da Cunha islands, the furthest point from Europe where the snails have been found, are "among the most remote islands in the world", says Richard Preece of the University of Cambridge. Preece and his colleagues confirm, through a genetic study published in Nature, that the snails in both places are of the same genus¹.

Getting around

Balea perversa snails are a fairly distinctive bunch, typically with a 7 millimetre-long shell that has an unusual counter-clockwise swirl to their shells. The snails were thought to live only in the Palaearctic region, including parts of Eurasia north of the Himalayas and northern Africa.

In 1824 two new species of snail were described in the Tristan islands; they were very similar looking to B. perversa, just slightly larger. It was assumed that these snails couldn't possibly have travelled all the way from Europe, and so the snails, of which there are now eight known species, were given a genus name of their own: Tristania.

A few decades later, Darwin became fascinated with these creatures and how very similar snails had been found on diverse land masses. He determined that they could not survive in sea water for longer than about two weeks, and so hypothesized that humans or birds were carrying the snails around.

Now Preece and other snail experts think that Darwin was on to something.

Spot the difference

Preece and his colleagues established a few years ago that despite their size difference, anatomically both Tristania and Balea were the same². Now they have studied the mitochondrial DNA of the snails to confirm that they are indeed of the same genus. They add that one of the eight Tristan species seems to have made its way back to Europe, since its DNA is similar to that of modern European cousins¹.

Plants and sedentary animals have long travelled great distances thanks to water, wind, or hitching a ride on other, more mobile animals. But the enormous distances travelled by these snails more commonly occurs in the plant than in the animal kingdom.

Preece notes that the snails have an unusually sticky slime, and previous research has shown that the snails can stick to birds' feet. He says that it would be implausible for humans to have transported them, since Tristan was not permanently settled until 1816. That leaves too little time for the snails to have evolved into eight separate species, he says.

Preece is now trying to collect molecular data as further evidence of globetrotting in these snails, beyond the Atlantic region. He points out that past research has also shown the creatures doing some mountaineering: they were found on a remote mountain peak on Madeira Island, Portugal, back in 1921. "They get around the world," says Preece.

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