Deep Sea Snail Shell Unique Armour Strategies

A deep-sea snail shell's ability to withstand heavy blows could inspire new generation of body armour.

Crysomallon squamiferum, commonly known as the scaly-foot gastropod, was discovered in 1999 in the Kairei "black smoker" field on the Central Indian Ridge, at a depth of 2420 metres.

Christine Ortiz at the Massachusetts Institute of Technology and her colleagues studied the snail's three-layered shell to find out how it defends itself from crab attacks.

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(Image: Dr. Anders Warén/Swedish Museum of Natural History, Stockholm, Sweden)

To assess the shell's strength and stiffness, they penetrated it with diamond-tipped probe – applying the same amount of force that an attacking crab's claws might use. They then used the data to model the shell's layers and launched a virtual crab attack on it.

Iron hard

It turns out that the snail employs some unique tricks to protect itself. For example, the shell's outermost layer consists of strong particles of iron sulphide created in the hydrothermal vents, each around 20 nanometres across, embedded in a soft organic matrix secreted by the snail. This structure is designed to crack when hit, but in a way that absorbs energy.

Cracks spread only by fanning out around the iron sulphide particles. This "microcracking" not only absorbs energy, it also ensures that larger cracks do not form. What's more, the particles of iron sulphide may blunt and deform intruding claws, the study suggests.

A thick, spongy middle layer acts as padding to dissipate further the energy of the blow. This makes it less likely that the mollusc's brittle inner shell, which is made of calcium carbonate, will crack.

The middle layer may be an important adaptation to life at a deep-sea hydrothermal vent, suggests Cortiz: the acidic water near black smokers dissolves calcium carbonate and so can quickly enlarge fractures.

The three-layer design could be used to improve body armour "without the addition of excessive weight", says Ortiz.

Crack to survive

The idea of coating armour in iron-based nanoparticles that dissipate the energy of a blow by generating microcracks is "largely unexplored in synthetic systems" and particularly promising, says Cortiz.

Helmets, motorbikes and Arctic pipelines that collide with icebergs, leading to costly oil spills, could also benefit, says Cortiz, who is also exploring the armour systems deployed by the marine molluscs known as chitons, sea urchins, beetles and a fish known as the Senegal bichir.

"It is the first step in understanding how we could create an engineered synthetic mimic of a protective structure," she says.

Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0912988107