What are two kinds of solid structures made by marine from minerals they extract from seawater?

Two kinds of solid structures made by marine organisms from minerals extracted from seawater are coral reefs and mollusk shells. Coral reefs are built from calcium carbonate secreted by coral polyps, forming complex ecosystems that support diverse marine life. Mollusks, such as clams and oysters, create hard shells also composed primarily of calcium carbonate, which serve as protective exoskeletons for these animals.