Triple Layered Armored Shell Protects From Predators

photograph of black snail against black background

Biological Strategy

Scaly-foot snail

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Image: Dr. Chong Chen / Squamiferum / Some rights reserved

Prevent Fracture/Rupture

High force impact or stress can cause materials that comprise living systems to separate into two or more pieces (called fracturing) or to break or burst suddenly (called rupturing). For example, a scallop prevents structural failure from fracture because its shell is comprised of two materials of varying stiffness. When a crack moves from the scallop’s stiff material to the less stiff one, the latter reduces the force at the tip of the crack, thereby stopping it from spreading farther.

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Manage Compression

When a living system is under compression, there is a force pushing on it, like a chair with a person sitting on it. When evenly applied to all sides of a living system, compression results in decreased volume. When applied on two sides, it results in deformation, such as when pushing on two sides of a balloon. This deformation can be temporary or permanent. Because living systems must retain their most efficient form, they must ensure that any deformation is temporary. Managing compression also provides an opportunity to lessen the effects of other forces. Living systems have strategies to help prevent compression or recover from it, while maintaining function. For example, African elephant adults weigh from 4,700 to 6,048 kilograms. Because they must hold all of that weight on their four feet, the tissues of their feet have features that enable compression to absorb and distribute forces.

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Manage Impact

An impact is a high force or mechanical shock that happens over a short period of time, such as a hammer hitting a nail rather than a hand pushing slowly against a wall. Because of their speed and force, impacts don’t allow materials to slowly adjust to the force, which can lead to cracks, ruptures, and complete breakage. Therefore, living systems have strategies that can absorb, dissipate, or otherwise survive that force without the need to add large amounts of material. For example, the Toco toucan’s large beak is very lightweight, yet can withstand impacts because it’s made of a composite material with rigid foam inside and layers of a hard, fibrous material outside.

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Protect From Animals

Animals–organisms that range from microscopic to larger than a bus–embody a wide variety of harms to living systems, including other animals. They threaten through predation, herbivory, defense, and parasitism, and they compete for resources such as water, nutrients, and space. Any given living organism commonly faces threats from a variety of animals, requiring strategies that effectively defend from each. Trout and other bony fish, for example, escape predators by having scales made of very thin, flake-like pieces of bone covered with slippery mucus. They also have behavioral strategies such as camouflage, fast swimming, and twisting and turning to achieve release from a predator’s grip.

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Distribute Energy

Many forms of energy are naturally available, including kinetic, potential, thermal, elastic, radiant, chemical, and more. All living systems require energy to carry out their many activities. Energy isn’t a physical object that can be held, so it also can’t be pushed, pulled, or carried. However, it can be transmitted or transformed. For example, the golden-scale snail has a tri-layered shell that dissipates, or transfers, forces from a predator biting down on it, thereby minimizing damage to its soft body inside the shell.

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Gastropods

Class Gastropoda (“stomach-foot”): Snails and slugs

There are at least 40,000 known species of gastropod, with many more yet to be discovered, making it the largest class of mollusks. These animals exhibit torsion, meaning their internal organs are twisted 180° during development. The majority of gastropods have shells, and most open up on the right side. The largest gastropods are the size of a small child (3 feet, or 1 meter) while the smallest are barely visible to the naked eye.

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The shell of the golden-scale snail protects from attack with a specialized tri-layered composition

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 (Chrysomallon 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.

This summary was contributed by Allison Miller.

Image: Haimin Yao et al. / CC BY - Creative Commons Attribution alone

Schematic of the multilayered design principles of the shell of C. squamiferum. Each material layer serves distinct functional roles leading to many advantages with regards to mechanical protection and penetration resistance.

Last Updated July 2, 2020

References

Journal article

Three-dimensional viewing in the deep sea

Proceedings of the National Academy of Sciences | 21/05/2013 | M. Schrope

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Reference

“…[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)

Book

Iron-plated snail could inspire new armor

27/01/2010 | Anne Trafton

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Reference

“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)

Journal article

Protection mechanisms of the iron-plated armor of a deep-sea hydrothermal vent gastropod

Proceedings of the National Academy of Sciences | 20/01/2010 | H. Yao, M. Dao, T. Imholt, J. Huang, K. Wheeler, A. Bonilla, S. Suresh, C. Ortiz

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Reference

Book

Armor-Plated Snail Discovered in Deep Sea

07/11/2003 | John Pickrell

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Reference

“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)

Journal article

Deep sea special: The undiscovered oceans

09/11/2005 | Schrope, Mark

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