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 structures, 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 uses a hard, armor-like shell with a tri-layered composition. Each layer has distinct chemical and physical properties that enable them to play different roles in managing forces from predatory attacks.
The outer layer is a thin organic shell reinforced by greigite (iron sulfide) particles spewed out by the thermal vents. Most mollusks build their shells from inside out, and the golden-scale snail does that in addition to using the thermal vent’s iron sulfide deposits. When an intruding crab claw, for instance, does start cracking the outer layer, its particular microscopic structure localizes the damage as “sacrificial microcracks” around the iron sulfide particles. That is, many small, manageable cracks form right around the site of impact, instead of one large crack that could severely damage the whole shell.
The middle layer is a thick, dense layer of organic material that is pliant in nature, meaning it easily deforms. This property enables the middle 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 mechanical energy reaches the calcified inner layer. It is the last layer of defense and if any forces are strong enough to impact it, they could permanently damage the snail. The inner layer is like a brick wall behind the marshmallow-egg shell complex.
“…[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)
“We have determined 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)
“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)