How Reefs Are Made
Coral reefs are built by coral polyps as they secrete layers of calcium carbonate beneath their bodies.
The corals that build reefs are known as “hard” or “reef-building” corals. Soft corals, such as sea fans and sea whips, do not produce reefs. The calcium carbonate that is created by hard corals provides a foundation for baby corals to settle upon, and if the local threats to coral reefs are limited, then the corals will build up over time and create a healthy, vibrant coral reef.
Other types of animals and plants also contribute to the structure of coral reefs. Many types of algae, seaweed, sponges, sediment, and even mollusks like giant clams and oysters add to the architecture of coral reefs. When these organisms die, they also serve as foundations for new corals.