Photography provided by Brilliant Studios

How Coral Reefs Are Formed

Coral reef consists of thousands of small individual animals living together. There are many different kinds of corals, but the ones that build reefs are called “stony corals.” This is because they extract calcium and bicarbonate from sea water to form their calcium carbonate (also known as limestone) homes.

Many corals start our life as free-swimming larvae. Once the larvae finds a suitable surface upon which to attach itself it will establish its new home by secreting its limestone home underneath itself. Suitable surfaces are generally hard, algae free rocks, dead corals, and so on.

Once it is attached to a hard surface, the coral will eat by grabbing microscopic plants and animals from the water column. They also get a lot of their nutrition from symbiotic algae, called zooxanthallae that live inside the coral’s transparent body (the zooxanthellae are responsible for giving coral its color). Because the zooxanthellae photosynthetic, corals have to live in clear water where the algae can be exposed to lots of sunlight. So most corals grow best in less than 90 feet of saltwater. They also have a preferred temperature range of 70-85 degrees Fahrenheit (21-29 degrees Celsius). Water that is colder, corals won’t survive. Water that is too warm and the zooxanthellae leave and the coral starve to death since a great deal of their food comes from the algae.

Coral reefs take many years to form as most corals are relatively slow growing, adding only an inch or less to their size each year. There are some corals, however, that are much faster growing, such as staghorn coral (Acropora cervicornis) can grow up to 8 inches in a year, but this type of coral is near extinction due to a coral disease which wiped out most of the growths in the 1980s. Because of their slow growth rates, a large coral reef can be thousands of years old.