Based on current estimates, shallow water coral reefs occupy somewhere between 284,000 and 512,000 km2 of the planet (add cold-water deep corals, and it comes to even more area). If all the world’s shallow water coral reefs were crammed together, the space would equal somewhere between an area of land ranging from the country of Ecuador (the low estimate) to Spain (the higher estimate). This area represents less than 0.015 percent of the ocean -- yet coral reefs harbor more than one quarter of the ocean’s biodiversity. That’s an amazing statistic when you think about it: no other world ecosystem occupies such a limited area with so many life forms.
Coral reefs are found both in tropical and subtropical waters, in a zone extending from 30°N to 30°S of the equator. Reef-building corals do not grow at depths of over 30 m (100 ft) or where the water temperature falls below 16 °C (72 °F).3
It was Charles Darwin who originally classified coral reefs as to their structure and morphology, and described them as follows:
Fringing reefs lie near emergent land. They are fairly narrow and recently formed. They can be separated form the coast by a navigable channel (which is sometimes incorrectly termed a “lagoon ").
Barrier reefs are broader and lie farther away from the coast. They are separated from the coast by a stretch of water which can be up to several miles wide and several tens of meters deep. Sandy islands covered with a characteristic pattern of vegetation have sometimes formed on top of a barrier reef. The coastline of these islands is broken by passes, which have occupied the beds of former rivers.
Atolls are large, ring-shaped reefs lying off the coast, with a lagoon in their middle. The emergent part of the reef is often covered with accumulated sediments and the most characteristic vegetation growing on these reefs consists of coconut trees.
Darwin’s three
stages of atoll formation (source: NOAA)
