Coral reefs are among the most diverse and productive communities on Earth. They are found in the warm, clear, shallow waters of tropical oceans worldwide. Reefs have functions ranging from providing food and shelter to fish and invertebrates to protecting the shore from erosion. Because many coral reef organisms can tolerate only a narrow range of environmental conditions, reefs are sensitive to damage from environmental changes. One of the greatest threats to coral reefs is human expansion and development.
 Where Do Coral Reefs Grow
Coral reefs grow in tropical seas in the Sunlit Zone, where there is mild wave action, not so strong to tear the reef apart yet strong enough to stir the water and deliver sufficient food and oxygen. Coral reefs also need nutrient-poor, clear, warm, shallow water to grow. Coral is built up from millions of skeletons from tiny animals called polyps, which are related to Sea Anemones.
 How Are Coral Reefs Formed
The building blocks of coral reefs are the "skeletons" of generations of reef-building algae, corals, and other organisms that are composed of calcium carbonate. Many other organisms living in the reef community contribute their skeletal calcium carbonate in the same manner. Coralline algae are actually the main contributors to the structure, at least in those parts of the reef subjected to the greatest forces by waves. These algae contribute to reef-building by depositing limestone in sheets over the surface of the reef and thereby contributing to the structural integrity of the reef.
 Where Are Corals Found
Reef-building or hermatypic corals are only found in the photic zone (above 50m depth), the depth to which sufficient sunlight penetrates the water for photosynthesis to occur. The coral polyps do not photosynthesize, but have a symbiotic relationship with single-celled algae called zooxanthellae; these algal cells within the tissues of the coral polyps carry out photosynthesis and produce excess organic nutrients that are then used by the coral polyps.
The more prolific growths of corals are to be found in water deeper than where the bottom is exposed at low tides: on the frontal reef slope (fore reef), in lagoons, and along reef channels that bisect the flat. Under conditions of clear, moving seawater, corals provide the bulk of the skeletal material comprising the reef and the structural complexity that results in a high diversity of reef associated fishes and invertebrates.
 Importance of Coral Reefs
Coral reefs support an extraordinary amount of biodiversity; although they are located in nutrient-poor tropical waters. This delicate community is not only composed of hard corals, but also of soft corals, sponges, fish, crustaceans, worms, snails, sea turtles, algae and many other organisms living in harmony with one another. The associated fauna shows an extraordinary diversity with many species of gastropods, particularly Cypraea, Conus, Triphora, Turbo and Trochus. Coral reefs are home to a variety of tropical or reef fishes, such as the colorful parrot fishes, angelfishes, damselfishes and butterfly fishes. Other fish groups found on coral reefs include groupers, snappers, grunts and wrasses. Over 4,000 species of fishes inhabit coral reefs.
Reefs are also home to a large variety of other organisms, including sponges, Cnidarians, worms, crustaceans including shrimp, spiny lobsters and crabs, mollusks, echinoderms including starfish, sea stars, sea urchins and sea cucumbers, sea squirts, sea turtles and sea snakes. Aside from humans, mammals are rare on coral reefs, with visiting cetaceans such as dolphins being the main group. A few of these varied species feed directly on corals, while others graze on algae on the reef and participate in complex food webs.
A number of invertebrates, collectively called cryptofauna, inhabit the coral rock substrate itself, either boring into the limestone surface or living in pre-existing voids and crevices. Those animals boring into the rock include sponges, bivalve mollusks, and Sipunculans. Those settling on the reef include many other species, particularly crustaceans and Polychaete worms. For centuries, reef resources have been utilized for food and building materials. Increased human activities in recent times have begun to degrade the quality of the reefs, particularly the nearshore habitats. The major uses of the reefs are extraction of living and dead coral for the lime industry, capture fisheries and the harvesting of exotic reef resources such as ornamental fish for export and for tourism related activities.
 Dangers To Coral Reefs
The environment that has surrounded coral reefs for hundreds of thousands of years is changing so fast that compensatory biological responses are lagging behind, putting at risk the marine ecosystem with the highest biodiversity on Earth.
Coral reefs are among the most diverse and biologically complex ecosystems on earth. These rainforest of the sea provide economic and environmental services to millions of people as areas of natural beauty and recreation, sources of food, jobs, chemicals, pharmaceuticals, and shoreline protection. Now under threat from multiple stresses that are overwhelming their natural resilience, coral reefs are deteriorating worldwide at alarming rates. An estimated 10% of the world’s reefs have already been lost and 60% are threatened by bleaching, disease and a variety of human activities including shoreline development, polluted runoff from agricultural and land-use practices, ship groundings, over-harvesting, destructive fishing, and global climate change. The trend in coral reef health is downward, and these ancient ecosystems are in peril.
Humans continue to represent the single greatest threat to coral reefs living in Earth's oceans. In particular, land-based pollution and over-fishing are the most serious threats to these ecosystems. Physical destruction of reefs due to boat and shipping traffic is also a problem. The live food fish trade has been implicated as a driver of decline due to the use of cyanide and other chemicals in the capture of small fishes.
 Carbon emissions
The rapid rises in carbon dioxide causes acidification, which adds a new threat: the inability of corals to create calcareous skeletons. Acidification actually threatens all marine animals and plants with calcareous skeletons, including corals, snails, clams and crabs. One study shows that levels of CO2 could become unsustainable for coral reefs in as little as five decades.
Finally, above normal water temperatures, due to climate phenomena such as El Nino and global warming, can cause coral bleaching. According to The Nature Conservancy, if destruction increases at the current rate, 70% of the world’s coral reefs will have disappeared within 50 years.
 Human expansion and development
Corals also face danger from human expansion and development. As development continues to alter the landscape, the amount of freshwater runoff increases. This terriginous runoff may carry large amounts of sediment from land-clearing areas, high levels of nutrients from agricultural areas or septic systems, as well as many pollutants such as petroleum products or insecticides. Whether it is direct sedimentation onto the reef or an increase in the turbidity of the water due to eutrophication, decreases in the amounts of light reaching corals may cause bleaching. In addition, increases in the amounts of nutrients enhance the growth of other reef organisms such as sponges which may outcompete the corals for space on crowded reefs.
 Did you know?
- The Caribbean region has an estimated 26,000 km2 of coral reef surface, possessing an estimated 7% of the world’s shallow coral reefs.
- Human activities threaten 2/3 of the Caribbean’s coral reefs, placing 1/3 at high risk.Human activities threaten 2/3 of the Caribbean’s coral reefs, placing 1/3 at high risk.
Gulf of Carpentaria Reefs
Geoscience Australia discovered a new coral reef province in the Gulf of Carpentaria during seabed mapping surveys carried out in 2003 and 2005 aboard the RV Southern Surveyor. The reefs are located along the southern part of the Gulf and are submerged 20-30 metres below sea level, making them invisible to satellites or air photography. Their existence was revealed using multibeam sonar mapping, underwater video and core sampling. Watch the movie to get the whole story.
- The Status of Coral Reefs of the World, 2004, 2002, AIMS
- Global Coral Reef Monitoring Network (GCRMN)
- Information and maps on coral reef monitoring programmes and activities around the world, in collaboration with GCRMN, Reef Check and others.
- National Institute of Oceanography, Goa
- CORDIO - Coral Reef Degradation in the Indian Ocean
- A global nonprofit membership organization dedicated to protecting coral reefs.
- Gulf of Carpentaria Reefs
 Resources, Online Help, Directories
- Online Resources
- Directory of Coral Reef Organizations Worldwide
 Additional Information
- Watch Canary in the Coal Mine, a short documentary by Chris Condayan of the American Society for Microbiology in cooperation with Craig Quirolo of Reef Relief
- Working together to Keep the Coral Reefs Alive
- [Watch The Carpentaria Reefs movie