The first traces of coral reefs known to man date back more than 500 million years. Their distribution is remarkably varied over time, responsive in turn to phases of great geographic expansion and contraction following climatic changes that occurred during the different geologic eras.

Today the distribution of reefs is limited to tropical seas, where reefs extend over a total area of nearly 600,000 square kilometers (232,000 square miles).

Coral formations develop primarily between the surface and a depth of 30 m (100 ft) but only in waters whose average winter temperature stays above 20°C (68°F). Other factors that limit the development of coral reefs are water salinity, which must be constant, and the intensity of ambient light. Why are these three factors—temperature, salinity and sunny exposure—so important? If one thinks carefully about the reef environment, one quickly realizes that the structural elements that underlie this ecosystem—those that help determine its appearance—are the so-called corals or, more correctly, madreporic colonies.

These are admirably complex structures whose imperceptible growth and extraordinary fragility represent the very foundation of the reef habitat.