As the polyps of stony corals grow, they produce limestone for their skeletons. When they die, their skeletons are left behind and are used as foundations for new polyps, which build new skeletons over the old ones. An actual coral mound or tree is composed of layer upon layer of dead coral polyps covered by a thin layer of living polyps. Many types of algae, seaweed, sponge, sediment and even mollusks like giant clams and oysters, add to the architecture of a coral reef. When these organisms die, they also serve as foundations for new corals.
Coral reefs are one of the most ancient ecosystems on the planet.
The first stages of coral reef evolution began 350 million years ago. Modern coral reefs have been in existence for over 50 million years. Corals grow at different rates. The massive corals are the slowest species, growing anywhere between 5 and 25 millimeters per year. Branching and Staghorn corals can grow much faster than this, adding as much as 20 centimeters to their branches each year.