A coral reef provides one of the most important natural habitats in the world, sheltering enormous amounts of biodiversity with its solid calcium carbonate skeleton. Shallow-water coral formations provide the perfect place for many underwater species to breed and spawn -- in fact, more than 25 percent of all fish biodiversity on the planet Earth is associated with the coral reef ecosystem [source: NOAA]. For this reason, coral reefs are often known as the rainforests of the undersea world.
Despite their incalculable value to marine ecosystems, coral reefs require fairly specific environmental conditions in order to thrive. Coral reefs and the zooxanthellae algae living within the cell walls of coral polyps must have adequate sunlight and warm salt water -- water that gets no colder than 64.4 degrees Fahrenheit (18 degrees Celsius). Coral reefs survive best in the waters between 30 degrees north and 30 degrees south of the equator, mostly in the Pacific and Indian oceans. They also thrive in clear waters with little or no floating particles that can filter out or block direct sunlight. Optimal conditions for coral growth require water temperatures between 73.4 and 84.2 degrees Fahrenheit (23 to 29 degrees Celsius). Shallow-water coral grows approximately 3.9 inches (9.9 centimeters) per year, given these optimal conditions [source: NOAA].
While tropical shallow-water coral reefs have been studied much more extensively, there are also species of coral that live in deep-water conditions, from about 50 to 2,000 meters (about 165 to 6,560 feet) below the surface, where the water is cold, the pressure is high and the local fauna is quite different [source: NOAA].
The world's shallow-water coral reefs are threatened by the manifold effects of climate change, destructive fishing methods and other hazards. Conservationists have suggested coral reefs can be protected if we take measures to limit greenhouse gas emissions and establish Marine Protected Areas (MPAs) to shield existing reefs from destructive human activities.
Coral reefs, indicated by red dots, are found predominantly in tropical waters 30 degrees north and south of the equator. (Photo courtesy NOAA)
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