Construction
Coral reefs are complex, multistory structures with holes and crevices shared by various creatures. If a coral reef is likened to a bustling city, then a coral colony is like a single apartment building with many rooms and hallways that house different marine species.
Not all coral species build reefs. The actual architects of coral reefs are hard or stony corals, which are referred to as hermatypic, or reef-building corals. As the polyps of stony corals grow, they produce limestone skeletons. When they die, their skeletons are left behind and used as foundations for new polyps, which build new skeletons over the old ones.
An actual coral branch, or mound, is composed of layer upon layer of skeletons covered by a thin layer of living polyps. Other types of animals and plants also contribute to the structure of coral reefs. Many types of algae, seaweed, sponge, 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.
Corals grow at different rates depending on water temperature, salinity, turbulence, and the availability of food. The massive corals are the slowest growing species, adding between 5 and 25 millimeters (0.2–1 inch) per year to their length. Branching and Staghorn corals can grow much faster, adding as much as 20 centimeters (8 inches) to their branches each year.
Source: Coral Reef Alliance