Free-swimming coral larvae attach themselves to submerged rocks or other hard surfaces at the edges of islands or continents to begin the process of forming coral reefs. The coral polyps then secrete skeletons from the underside of their skin. These skeletons, made from calcium carbonate, protect the coral animals from predators and also offer a substrate on which new coral polyps can attach themselves. The process of growing the skeleton consumes a lot of energy, which is conveniently provided by the algae living in the corals’ tissues.
Different species of coral 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.
Coral reefs grow best in warm water (70–85° F or 21–29° C). Corals prefer clear and shallow water, where lots of sunlight filters through to their symbiotic algae. It is possible to find corals at depths of up to 300 feet (91 meters), but reef-building corals grow poorly below 60–90 feet (18–27 meters). Corals need salt water to survive, so they grow poorly near river openings or coastal areas with excessive runoff.
The geological record indicates that ancestors of modern coral reef ecosystems were formed at least 240 million years ago. Most established coral reefs are between 5,000 and 10,000 years old. Although size sometimes indicates the age of a coral reef, this is not always true. Corals form many different types of reef structures.