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How Are Shells Formed?

| Updated September 26, 2017

Seashell Basics

Seashells are the protective exoskeletons of mollusks. They shelter the body from predators and other potential dangers. Seashells are made almost entirely of calcium, but also have a small amount of protein. Because they are calcified, they take much longer to break down than the creatures they protect, which is why empty seashells wash up so frequently on beaches.

Secreting a Shell

The seashell is created by the mantle; part of the outer body of mollusks. It is created in two or three layers, depending on the particular creature. The edge of the mantle secretes the outer layer, while the harder, inner layers are secreted by the rest of the mantle. The outer edge of the mantle lengthens the shell, while the rest of the mantle continually thickens it.


As mollusks grow, their shells have to increase in size. The shell is elongated, broadened and thickened in periodic spurts. In many species, this growth results in interesting, beautiful spirals and other patterns. Seashells can give information about the lives of the creatures that inhabited them. The diet of the mollusk can affect the color of the seashell, and injuries or periods of poor diet can cause irregularities in shell geometry.