Published online 8 January 2008 | Nature | doi:10.1038/news.2008.421

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Snail shell takes a weird turn

The snail that twists again. And again. And again.

A funny turn: this shell has four axes of rotation.Royal Society

A newly discovered Malaysian snail has defied the established rules of growth and form by creating a home that twists in four independent directions. No one knows how or why it does it.

"It is sort of biological madness," says Jaap Jan Vermeulen, a taxonomist from the Nationaal Herbarium of the Netherlands in Leiden and a member of the team that discovered the snails.

Most land snails' shells have a single coiling axis, making for a simple spiral. Some have two axes, often starting out coiling one way during the early stages of growth, and then flipping by 90 or 180 degrees. Most of the hundreds of species of Opisthostoma snails have three axes of coiling, with a final twist at the end of their growth.

Now researchers searching the soils of Malaysia to catalogue the country's molluscs have found two species with even more twisting. They report the first of these, which they name Opisthostoma vermiculum or 'little worm', in Biology Letters this week1.

Open-mouthed

The super-curvy specimens, found in soil from a single limestone site, were obviously unusual. "We had our mouths agape for a good few seconds," says lead researcher Reuben Clements, species conservation manager for the environmental group WWF in Selangor, Malaysia.

"It could have been a mutant — we thought so when we found one," says Vermeulen. "But now we have so many, and they are all exactly the same," Their paper describes 38 identically curvy shells.

“It is strange indeed. We know of something like 100,000 coiled molluscs, and this is the only one I know with four axes,” says Bernard Tursch, a biologist at the Free University of Brussels in Belgium, who specializes in marine shells.

There’s no explanation as yet for the shape. It should leave the shell vulnerable to damage, as some of the twists sit high in the air without any obvious support.

Without reason

Marine molluscs often have complicated shells, which sometimes grow wildly without any clear coiling axes. But that does not go for land dwellers and the curvy shells of Opisthostoma vermiculum are remarkably consistent. "There must be some internal mechanism governing this. We have no idea why. How they do it is also a mystery," says Vermeulen.

To work out how these animals live or what advantage the shell might confer the researchers need to find living samples. This will be difficult because the snails are just a few millimetres high. To make matters worse, similar creatures are known to camouflage themselves with bits of mud, Vermeulen says. The team hopes that night-time field trips might snag some living snails.

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The group intends to complete a field guide to the land snails of the Malaysian mainland in the next few years. So far, they say, two months of sampling have yielded more than 190 morphologically different species from 16 areas, with hundreds more yet to sample.

Documenting novel species will help protect them, says Clements. "Escalating rates of limestone quarrying in the region are likely to cause the extinction of many more new species before they are discovered," he says. 

  • References

    1. Clements, R., Liew, T.-S., Vermeulen, J. J. & Schilthuizen, M. Biol. Lett. doi:10.1098/rsbl.2007.0602 (2008)
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