Welcome to You Ask Andy

Timmy Sears, age 10, of Oromocto, N. B., Canada, for his question:

How do sea creatures make their shells?

It's nice to have a mini art collection of seashells. Many varieties can be found for free, beside lakes or washed up on sandy beaches. Even an ordinary old clam shell may have a shiny lining, tinged with pale colors like a frozen moonbeam. The more treasured types look like graceful little carvings, glowing with glossy floral colors. Most of these mini masterpieces are imported from faraway tropical seas.

The conch builds a large roomy shell for himself, gracefully coiled and lined with pearly smooth walls. He is a sea snail called a univalve mollusk. The scallop builds a pretty pair of fluted, fan shaped shells, hinged together with a tough muscle. He is a bivalve, or two shelled mollusk. Altogether, there are about 100,000 species of shell building mollusks and most of them live in the sea.

Each species inherits the pattern for his shell and he builds it just like those that his ancestors built through millions and millions of years. The raw materials are minerals dissolved in the salty sea. The main ingredient is calcium carbonate, which the earth uses to make marble and limestone. Through the ages, streams and rivers dissolved this and other minerals from the land, washed them away and dumped them into the oceans.

The average shell builder feeds on scraps of plants and animals that have fed on the rich minerals of the sea. He has a soft little body, wrapped in a loose cloak of flesh called the mantle. The mantle contains the special glands that build the shell. It selects and mixes the right minerals in concentrated liquids. The liquids ooze from the glands and set to form hard shell material.

The shell is a cemented sandwich of three layers. The rough outer layer is tough enough to cope with tumbling sands and toothy enemies. The inside layer has a smooth surface to protect the little builder from scratches. The middle layer cements these two layers together.

Most of the mantle glands secrete a liquid mixture of calcium carbonate. Others secrete a hardening fluid that mixes with the main ingredient to make it set. The outer layer sets in hard rough prisms. The smooth lining is a series of thin layers, made from tiny prisms that bend the light to create pearly colors.

Many seashells also wear colorful exteriors. The colors are added by special glands in the mantle. The freckled cowrie has several pockets of coloring glands that work for a while and then rest. Others have three or four dye making pockets that add colored stripes to the shells as they grow.

 

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