A deep-sea snail wears a multilayered suit of armor, complete with iron, new research shows. Dissecting details of the shell's structure could inspire tough new materials for use in everything from body armor to scratch-free paint.

"If you look at the individual properties of the bits and pieces that go into making this shell, they're not very impressive," comments Robert Ritchie of the University of California, Berkeley. "But the overall thing is."

The snail, called the scaly-foot gastropod, was discovered nearly a decade ago living in a hydrothermal vent field in the Indian Ocean. In its daily life, the snail encounters extreme temperatures, high pressures and high acidity levels that threaten to dissolve its protective shell. Worse, it is hunted by crabs that try to crush the mollusk between strong claws.

To understand how the valiant gastropod holds up to these trials, Christine Ortiz of MIT and her colleagues used nanoscale experiments and computer simulations to dig in to the shell's structure. Many other species' shells exhibit what Ortiz calls "mechanical property amplification," in which the whole material is hundreds of times stronger than the sum of its parts.

The scaly-foot snail's shell employs a structure "unlike any other known mollusk or any other known natural armor," the researchers report January 19 in Proceedings of the National Academy of Sciences. Ortiz and her colleagues found that the shell consists of a 250-micrometer-thick inner layer of aragonite, a common shell material, sheathed in a 150-micrometer-thick layer of squishy organic materials. The organic layer is encased in a thin, stiff outer layer (about 30 micrometers thick) made of hard iron sulfide–based scales. The gastropod wears larger versions of the scales on its exposed foot.

"Most mollusks only have a relatively thin outer organic layer followed by inner calcified layers," Ortiz says. But the snail's organic layer is surprisingly thick, and no other gastropod has ever been shown to use iron sulfide in its shell.

Each of the shell's layers plays a unique role in protecting the snail from crab attacks, Ortiz found. The researchers measured material properties like stiffness and fracture resistance, and fed them into a computational model of a predator penetrating the armor.

The model showed that the outer layer, the shell's "first line of defense," sacrificed itself by cracking slightly under pressure. But the cracks were branched and jagged, dissipating energy widely through the shell and keeping any one crack from spreading too far. The iron-based scales could shift and roughen the shell's surface during a crab attack, which in turn would grind down the attacking claw, the researchers suggest.

The soft organic middle layer changed shape in response to pressure, keeping the brittle inner layer from feeling too much of the pinch. Organic material could also insert itself in any cracks that formed in either sandwiching layer and keep the crack from spreading. Plus, the middle layer together with the outer layer protects against acidic waters and may also help shield the snail from high temperatures.

The shell's curvature also helped reduce stress on the calcified inner layer. The inner layer's rigidity provided structural support, to keep the whole shell from caving in.

"It shows that by changing the geometry of the materials … you can improve their properties quite significantly," comments Markus Buehler of MIT, who was not involved in the research.

Ortiz hopes that studying the snail's shell could one day lead to improved materials for armor or helmets for people. Studying organisms that have been optimized for extreme environments through millions of years of evolution could offer ideas that engineers would never think of on their own, she says.

But it will probably be a while, Ritchie cautions. His lab built a ceramic material based on mother-of-pearl in 2008.

"I'm a great fan of this kind of research, but the next step is the critical one. Can you actually harness that information and make a synthetic structure in its image which has the same properties?" he asks. "That's the most difficult step."

Images: 1) Anders Warén, Swedish Museum of Natural History, Stockholm, Sweden. 2) Zina Deretsky, National Science Foundation.

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