What material makes up the shells of the animals that starfish eat?

The shells of the animals that starfish eat, such as clams and mussels, are composed of calcium carbonate. Calcium carbonate is a mineral that provides a protective layer to the animal's delicate soft tissues while also helping to prevent dehydration. To form a robust and durable structure, the calcium carbonate in the shells is often combined with other materials such as proteins. Some molluscs, such as clams and mussels, also use their shells to anchor themselves to rocks and other ocean substrates. Despite the hardness of their shells, many mollusc species are still vulnerable to predation from starfish and other shell-crushing predators and have evolved a variety of defensive strategies to avoid being eaten. Therefore, the correct answer is "Calcium carbonate."