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Tri-layer shell protects from predators: golden-scale snail


Crysomallon squamiferum, vent snail / nders Warén.. / LicenseCC-by-nc - Attribution Non-commercial

The shell of the golden-scale snail protects from attack via a specialized tri- layered mechanical profile.

At the bottom of the Indian Ocean are large hydrothermal vents that spew hot water and minerals. They also provide an ecosystem for a variety of bizarre species adapted for living in harsh conditions. One such species is the golden scale snail (Crysomallon squamiferum, sometimes called the scaly-foot) which feeds off of the vent’s nutrients. Adhered to the vent, the snail is vulnerable to predators such as crabs and venomous snails that can puncture or crush the scaly-foot snail. To protect itself, the snail has developed a hard, armor-like shell comprised of a tri-layered composition. Each layer has a distinct chemical and physical profile that dissipates energy and predatory forces.
The outer layer is a thin organic shell reinforced by iron sulfide (fool’s gold) and greigite particles spewed out by the thermal vents. While most crustaceans build their shells from from inside out, the golden-scale snail does that in addition to using the thermal vent’s iron sulfide deposits. These metals not only reinforce the outer layer, but are used to repair it when it cracks. When the outer layer does crack, its microscopic wave texture cracks in a tortoise pattern (the hexagonal pattern on a tortoise shell) that dissipates energy horizontally rather than inward.
The middle layer is a thick, dense layer of organic material that is elastic in nature. This property allows this layer to act as a shock absorber, relieving the pressure of a crab’s grasp and protecting against a poisonous snail's smashing blow. It could be compared to a dense marshmallow underneath an eggshell. The outer layer and middle layer relieve most, if not all, of the shock. 
Any remaining energy reaches the inner layer, which is calcified and mineralized. It is the last layer of defense and if any forces are strong enough to impact it, they will permanently damage the snail. The inner layer is like a brick wall behind the marshmallow-egg shell complex.

This summary was contributed by Allison Miller.
"During the second ever expedition to hydrothermal vents in the Indian Ocean, biologists spotted a snail with a strange-looking foot. Many snails can close the opening to their shell with a flat, round bit of shell called an operculum. But this snail instead protects itself with scales, a feature seen before only in long extinct species, although the vent snail itself evolved recently. Even more unusually, the scales are reinforced with the iron sulphide minerals fool's gold and greigite, giving them a golden colour. No other multicellular animal is known to use these materials." (Schrope 2005:38)

"[T]he snail has evolved a tri-layered shell structure consisting of an outer layer embedded with iron sulfide granules, a thick organic middle layer, and a calcified inner layer. This creates a configuration in which the inner compliant layer is sandwiched between two rigid layers." (Trafton 2010)

"Ortiz and her colleagues, including MIT Dean of Engineering Subra Suresh, used nanoscale experiments and computer modeling to determine the shell's structure and mechanical properties. They found that the unique three-layer structure dissipates mechanical energy, which helps the snails fend off attacks from crabs that squeeze the shell with their claws in an attempt to fracture it. The shell of the scaly-foot snail possesses a number of additional energy dissipation mechanisms compared to typical mollusk shells that are primarily composed of calcium carbonate." (Trafton 2010)

"Many snails can close the opening to their shell with a flat, round bit of shell called an operculum. But this snail instead protects itself with scales…[T]he scales are reinforced with the iron sulphide minerals fool's gold and greigite, giving them a golden colour. No other multicellular animal is known to use these materials." (Schrope 2005:38)
“...through nanoscale experiments and computational simulations of a predatory attack that the specific combination of different materials, microstructures, interfacial geometries, gradation, and layering are advantageous for penetration resistance, energy dissipation, mitigation of fracture and crack arrest, reduction of back deflections, and resistance to bending and tensile loads. The structure-property-performance relationships described are expected to be of technological interest for a variety of civilian and defense applications.” (Yao et al. 2010: 987)
“The design space for synthetic multilayered structural composites for protective applications is enormous, with a large number of potential design parameters, e.g., layer thickness, geometry, gradation, number, and sequence, anisotropic elastic constants, plastic anisotropy, strain-rate dependence, strain hardening/softening, delamination criteria, crush strength, interphase properties, spatial dependence of mechanical properties such as gradation, etc.” (Yao et al. 2010: 991)
About the inspiring organism

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Organism/taxonomy data provided by:
Species 2000 & ITIS Catalogue of Life: 2008 Annual Checklist

Bioinspired products and application ideas

Application Ideas: Develop novel structural materials. Protective armor. Helmets. Vehicles.

Industrial Sector(s) interested in this strategy: Construction, materials science, engineering, military

Ortiz Laboratory
Christine Ortiz
Massachusetts Institute of Technology
Schrope, Mark. 2005. Deep sea special: The undiscovered oceans. New Scientist. 188(2525): 36-43.
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Trafton A. 2010. Iron-plated snail could inspire new armor. MIT News [Internet],
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Yao H; Dao M; Imholt T; Huang J; Wheeler K; Bonilla A; Suresh S; Ortiz C. 2010. Protection mechanisms of the iron-plated armor of a deep-sea hydrothermal vent gastropod. PNAS. 107(3): 987-992.
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Pickrell J. 2003. Armor-Plated Snail Discovered in Deep Sea. National Geographic [Internet], Accessed 11/7/2003.
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