Final answer:
Calcium is used for structural support in shells of marine organisms through the formation of calcium carbonate.
Step-by-step explanation:
Calcium is used for structural support in shells. The formation of shells in marine organisms involves the combination of calcium ions and carbonate ions in seawater to form calcium carbonate (CaCO3). This process occurs over geologic time and results in the formation of limestone, which is a major component of marine organism shells.