What natural material forms the hard shells of most mollusks such as clams and oysters?

Calcium carbonate forms the hard shells of most shelled mollusks, including clams, oysters, snails, and many mussels.

Mollusks build shells by secreting minerals and organic molecules from a specialized tissue called the mantle. The calcium carbonate may occur mainly as aragonite, calcite, or layers of both crystal forms, depending on the species. A protein-rich organic matrix helps control crystal growth and gives the shell structure and toughness.

Shells protect soft bodies from predators, physical damage, and water loss in species that live on land. In marine mollusks, they also provide an important defense against crushing and drilling predators. Oysters and other bivalves add shell material as they grow, producing visible growth lines in many specimens.

Ocean acidification matters because seawater chemistry affects the availability of carbonate ions needed for shell formation. More acidic conditions can make calcification harder, especially for larvae and other organisms building very small shells. The shell material should not be confused with chitin, which forms the exoskeletons of arthropods such as crabs.