ASK A SCIENTIST
Question: How are seashells made?
Answer: Beautifully patterned colors and intricate architecture can make a seashell look like a work of art, but the value of the shell to its maker is the protection it gives against predators, the drying sun, or wave damage. Seashells are made by mollusks, but not all mollusks make shells. Squids, slugs and many other mollusks gained greater freedom of movement by giving up the shell-making ability of their ancestors.
Our mineral structures, bone and teeth, form internally. How does a mollusk produce a shell outside its body? The secret is a structure called the mantle. The surface cells of this flap of tissue produce a protein and carbohydrate coating in which dissolved minerals are concentrated and crystallized. The mineral part of seashells is calcium carbonate, and the chemicals needed to make it are abundant, in dissolved form, in seawater.
Bivalve mollusks, such as clams, have two shells attached by a hinge. The youngest part of the shell is the edge. As the animal grows, a shallow fold in the leading edge of the mantle spins a flexible protein-rich layer to the outside of the animal. Mantle cells behind the edge then deposit mineral on the inner surface of the protein layer. Layers of mineral differing in crystal type and orientation are added to the inner surface of the shell; this plywood-like construction greatly increases shell strength. The innermost layer of the shell is called nacre or mother-of-pearl; its shiny appearance results from the way the mineral scatters light. Damage to the shell causes additional nacre production but a grain of sand between the mantle and the inner shell surface also triggers this response. Normal movements of the animal turn the grain, so the nacre coats the grain evenly. The result is a free, spherical mass of nacre that we call a pearl.
A single, coiled shell is typical of gastropod mollusks. The youngest part of a shell is around the opening. New mineral is added where the mantle is in contact with the shell surface. The adult queen conch is able extend the mantle enough to add a large, flared lip not seen in juveniles. A cowry spreads its mantle over the entire shell and deposits a nacreous layer over the outer surface of the shell. A young cowry has a thin, obviously coiled shell with an opening to one side, but the thick nacre coating deposited on the outside of the adult shell hides the coiling and results in the opening being almost central.
The biology of the animals that produce seashells is fascinating. Unfortunately, seashells sold in gift shops and museum stores come from animals collected and killed to produce these souvenirs. Collecting empty shells on the beach does much less harm, but to avoid unpleasant surprises when you open your suitcase after returning from vacation, be sure that each shell is truly empty. The smell can be a lasting but unwelcome souvenir.
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