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What are sea shells made of?

June 13, 2005



Shells are made of calcium carbonate, a substance produced by a sea animal’s fleshy mantle. As the animal grows, so does its shell.


Generally speaking, sea animals that produce shells are called mollusks. Mollusks are a diverse group of invertebrates – animals without backbones. They include one-shell animals (univalves), like whelks, olives, and moon snails, and two-shelled animals (bivalves), like clams, oysters and scallops. Oddly enough, squid and octopus, which have no visible shells, are categorized as mollusks, but that’s another column.


In general, scientists generally recognize seven different classes of mollusks, consisting of some 130,000 species. This easily makes them the second largest group of animals in the world, outnumbered only by insects, and explains the thousands upon thousands of shells you find on the beach.


The body of a typical mollusk is pretty basic: a fleshy mass that contains the digestive, excretory and reproductive organs, covered by the fleshy mantle. Some mollusks, however, have well-developed heads, complete with eyes and a mouth, and a foot for getting around.


No other animal group can compare with mollusks for form, texture and pattern.

Their kaleidoscopic colors and endless variety of sizes and shapes continue to amaze us whenever we walk the beach.

A whelk on the beach

Whelks, often called conchs, are single-shelled snails that can grow very large. Their empty shells are common on North Carolina’s barrier island beaches. (Photo by John Mauser)


Shells have been used by many cultures for many different purposes; jewelry, tools, in ceremonial rites, as corporate logos, in home décor. Companies have been named after them, science has derived medicines from them, and industry has used them for road building and landscaping.


Early people used shells to make utensils, ornaments, and even as currency. American Indians cut and polished shells to make necklaces, or wove them into belts called wampum, which they used for trading. The most valued wampum came from the deep purple shells of the northern quahog clam, one of North Carolina’s most common clams and a popular seafood.