How Do Seashells Form?
Quick Answer
Seashells are built by molluscs such as snails and clams, which secrete calcium carbonate from a body layer called the mantle, adding to the shell as they grow.
The Full Story
A mollusc's mantle extracts calcium and carbonate ions from seawater and food. It secretes them together with proteins to build layers of calcium carbonate, usually as the minerals calcite or aragonite. New material is added at the shell's edge, so the shell grows outward, often leaving visible growth lines. Many shells have an inner layer of nacre, or mother-of-pearl, formed of tiny aragonite plates. Spiral shells often follow a logarithmic spiral, keeping the same shape as they grow larger.
Key Facts
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