Shells

Every year, thousands of shells wash up on beaches around the world. Have you ever found a shell and wondered where it came from or what its function once was?

These shells come from a large phylum of animals known as mollusks which includes members such as snails, clams, oysters, scallops, squid, and octopuses. One of the prominent characteristics of most mollusks is a hard exterior shell, although some mollusks, including the squid and the sea hare, produce internal shells.

The bodies of mollusks are very soft. Their shells protect them from predators. The body of the mollusk is formed of a combined structure called a "head-foot" that is used for locomotion. If threatened, shelled mollusks can retreat the "head-foot" quickly inside their shells.

A thin, fleshy fold of tissue called the mantle covers the internal organs of most mollusks. The mantle, which is made up of specialized cells, secretes the substance that creates the shell. The shell is formed almost entirely of calcium carbonate crystals, which is the chemical base of rocks like limestone. The shell starts as a semi-liquid substance that hardens very rapidly as soon as it is exposed to water or air.

The shell begins to form as tiny crystals of calcium carbonate are exuded onto a matrix of conchiolin , a brownish, horn-like material composed of proteins and polysaccharides produced by the animal. This surface serves as a microscopic latticework for new crystals. Additional calcium carbonate crystals are laid down in thin layers as either calcite or aragonite .

Shells vary in structure and hardness depending upon their crystallization type. Six-sided calcium carbonate crystals may be prism-like, becoming calcite, or they may be very flat and thin, becoming aragonite.

Both calcite and aragonite are forms of calcium carbonate. Aragonite is heavier than calcite, and it has a mother-of-pearl appearance. The shells of freshwater clams and land snails are typically made of aragonite. In saltwater genera such as Nerita, layers of calcite and aragonite may alternate during shell formation. Whether shells are made of calcite or aragonite, as additional layers are added, the shells grow in thickness.

Many mollusks also have special glands called chromogenic glands located on the margin of the mantle. These glands secrete colored pigments that stain the calcium carbonate, which would otherwise create a white shell. When the glands are in a continuous series and are steadily active, the developing shell will be a solid color. If the glands secrete continuously but are some distance from each other, spiral lines will appear on the shell. The color will show as dotted lines, if the glands work intermittently. In some cases, groups of adjacent glands secrete different pigments, creating multicolored shell designs of infinite variety.

Shelled mollusks are linked directly to their shells; they ordinarily cannot live separately from the shell. When a mollusk dies and the shell is no longer attached to a living creature, other aquatic animals (i.e. crustaceans such as crabs) sometimes use the shell as temporary shelter, in the water or on the shore. Throughout human history, shells have been valued as decorative objects, tools, and mediums of economic exchange. They continue to be popular collectibles around the world.

see also Morphology.

Stephanie A. Lanoue

Bibliography

Saul, Mary. Shells. New York: Doubleday and Company, Inc., 1974.

Solem, Alan. The Shell Makers. New York: John Wiley and Sons, 1974.

Tucker, Robert. Kingdom of the Shell. New York: Crown Publisher, Inc., 1982.