Shell Protects, Supports, and Allows for Growth

close up photograph of a yellow and brown shell against a black background

Biological Strategy

Mollusks (Snails, Octopuses)

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Image: Sam Droege/USGS / Flickr / Public Domain - No restrictions

Modify Size/Shape/Mass/Volume

Many living systems alter their physical properties, such as size, shape, mass, or volume. These modifications occur in response to the living system’s needs and/or changing environmental conditions. For example, they may do this to move more efficiently, escape predators, recover from damage, or for many other reasons. These modifications require appropriate response rates and levels. Modifying any of these properties requires materials to enable such changes, cues to make the changes, and mechanisms to control them. An example is the porcupine fish, which protects itself from predators by taking sips of water or air to inflate its body and to erect spines embedded in its skin.

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Optimize Shape/Materials

Resources are limited and the simple act of retaining them requires resources, especially energy. Living systems must constantly balance the value of resources obtained with the costs of resources expended; failure to do so can result in death or prevent reproduction. Living systems therefore optimize, rather than maximize, resource use. Optimizing shape ultimately optimizes materials and energy. An example of such optimization can be seen in the dolphin’s body shape. It’s streamlined to reduce drag in the water due to an optimal ratio of length to diameter, as well as features on its surface that lie flat, reducing turbulence.

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Physically Assemble Structure

Living systems use physical materials to create structures to serve as protection, insulation, and other purposes. These structures can be internal (within or attached to the system itself), such as cell membranes, shells, and fur. They can also be external (detached), such as nests, burrows, cocoons, or webs. Because physical materials are limited and the energy required to gather and create new structures is costly, living systems must use both conservatively. Therefore, they optimize the structures’ size, weight, and density. For example, weaver birds use two types of vegetation to create their nests: strong, a few stiff fibers and numerous thin fibers. Combined, they make a strong, yet flexible, nest. An example of an internal structure is a bird’s bone. The bone is comprised of a mineral matrix assembled to create strong cross-supports and a tubular outer surface filled with air to minimize weight.

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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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Mollusks (Snails, Octopuses)

Phylum Mollusca (“soft”): Snails, clams, octopus, oysters, slugs

There are 50,000 known species of mollusk, and Members of this phylum have a mantle, which is a thick layer of tissue that contains the internal organs and can secrete a calcium carbonate shell. Mollusks also have radula, which is a circular band of teeth made of chitin. The muscle on the underside of mollusks forms a foot that can be used to move or attach to surfaces. The animals across the seven classes vary greatly in body shape, size, and life history.

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Nature’s Innovations: Animals as Engineers

This lesson, with eight short videos, examines how animals have solved engineering problems and how humans have mimicked those solutions. This lesson addresses middle and high school Next Generation Science Standards in Life Sciences and Engineering Design

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The shells of many mollusks provide protection and support while accomodating growth due to their conical structure.

Image: Peter MacMurchy /

Many shells have conical shapes.

Image: H. Zell / Wikimedia Commons /

Mikadotrochus hirasei Pilsbry, 1903. The Emperors Slit Shell; Diameter 5,6 cm; Originating Eastern Chinese Sea, trawled in deep water (200m), 2005; Shell of own collection, therefore not geocoded.

Image: H. Zell / Wikimedia Commons /

Chicoreus aculeatus (Lamarck, 1822). Pendant Murex, Lenght 4,4 cm; Originating from Panglao Island, Bohol, Philippines; Shell of own collection, therefore not geocoded. Dorsal, lateral (right side), ventral, back, and front view.

Image: H. Zell / Wikimedia Commons /

Cypraecassis rufa, Cassidae, Red Helmet; Length 11 cm; Originating from the Indopacific; Shell of own collection, therefore not geocoded. Dorsal, lateral (right side), ventral, back, and front view.

Image: H. Zell / Wikimedia Commons /

Chicoreus erythrostomus, Pink Mouthed Murex; Length 9,5 cm; Originating from the East Pacific (Region from California to Peru); Shell of own collection, therefore not geocoded. Dorsal, lateral (right side), ventral, back, and front view.

Image: H. Zell / Wikimedia Commons /

Thatcheria mirabilis Angas, 1877. Japanese Wonder Shell ; Lenght 7,5 cm; Originating from Japan; Shell of own collection, therefore not geocoded. Dorsal, lateral (right side), ventral, back, and front view

“Consider shapes that satisfy the following set of conditions. To provide both support and protection for the organism, the shape must be a hollow one, but an opening must exist somewhere. Growth can occur only by addition to the inner surface or the free edge. And the shape should change only minimally as it grows. A cubic shell with an open face won’t work: addition to walls will give more shell relative to its contained volume, and addition to cylinder doesn’t meet the conditions–addition to the edge will move it from short and fat to long and (relatively) thin. What will work are cones, whether circular or elliptical. Add to the edge and thicken the walls and one gets a bigger cone, isometric with the original.

With only slight variations of the condition of isometry, all sorts of wild derivatives of cones are possible–and these latter are the shapes in which shelled mollusks occur.” (Vogel 2003:88-89)

Mollusk Shell Growth and Repair

This video and animation from Shape of Life illustrates the way mollusks grow their shells in an ever enlarging conic spiral. How this growth process and the materials used enables crack repair is also addressed.

Last Updated October 26, 2016

References

With only slight variations of the condition of isometry, all sorts of wild derivatives of cones are possible–and these latter are the shapes in which shelled mollusks occur.” (Vogel 2003:88-89)

Book

Comparative Biomechanics: Life's Physical World, Second Edition

Princeton University Press | 17/06/2013 | Steven Vogel

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