Why are many mollusk shells made of calcium carbonate?

Mollusk shells are primarily composed of calcium carbonate because the animal's mantle is biologically specialized to secrete this mineral to form a protective structure. Through a process known as biomineralization, the mantle's epithelial cells deposit calcium carbonate—often in the form of calcite or aragonite—alongside organic proteins like conchiolin to create a stiff, durable shell.1234

This shell serves as a vital defensive mechanism, allowing the animal to withdraw its body for protection against predators and environmental stressors. The shell grows through the continuous secretion of these minerals at the mantle's edge, resulting in a layered structure that provides both strength and structural integrity. Because calcium carbonate is abundant and stable in many marine environments, it serves as an efficient and effective material for mollusks to construct their protective skeletons.5267