Coral polyps are basically colorless. One type of algae is especially important to coral growth. Living a symbiotic existence within the actual tissue of the polyps are zooxanthellae. These microscopic plant cells convert sunlight into energy, giving off oxygen that nourishes their coral hosts. The polyps, in turn, provide protection and nutrition to support the zooxanthallae.
All corals are fragile, and require a very specific aquatic environment in which to live.
There must be the right balance of warm water, sunlight, salinity, nutrients and gentle wave action in order for them to thrive. It is important for divers to realize that only the very surface of the coral heads are alive, and that they are very vulnerable. Their mucous and mineral coverings offer little protection against physical contact, and they are extremely susceptible to stress from disease and pollution. The islands of the Florida Keys were part of a living reef system 125,000 years ago. The present reef tract is approximately 6 thousand years old.