Coral reef

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Coral reefs are diverse underwater ecosystems joined by calcium carbonate structures segregated by corals. Coral reefs are built by colonies of small animals found in marine waters that contain few nutrients. Most coral reefs are constructed from stony corals, which in turn consist of polyps that are grouped into groups. Polyps belong to a group of animals known as Cnidaria, which also includes sea anemones and jellyfish.

Often called "Jungles of the sea", shallow coral reefs form some of the most diverse ecosystems on Earth. They occupy less than 0.1% of the world's oceanic surface, about half of the region of France, but provide a home for at least 25% of all marine species, including fish, molluscs, worms, crustaceans, echinoderms, sponges, tunics and other cnidarians. Paradoxically, coral reefs flourish despite being surrounded by oceanic waters that provide few nutrients. They are most commonly found at shallow depths in tropical waters, but deep-sea and cold-water corals also exist on smaller scales in other areas.

comoHow it was formed?

Most of the coral reefs we can see today were formed after the last glacial period when the melted ice caused the sea level to rise and flood the continental platforms. This means that most modern coral reefs are less than 10,000 years old. As communities settled on the platforms, the reefs grew upward following the rise in sea levels. Reefs that climbed too slowly could become drowned reefs. They were covered by so much water that there was not enough light. Coral reefs can also be found in the deep sea away from continental platforms, around the Oceanic islands and as atolls. The vast majority of these islands are of volcanic origin. The few exceptions have tectonic origins where the plate's movements have raised the ocean floor to the surface.

The Great Barrier Reef is approximately 20,000 years old and offers an example of how coral reefs were formed on continental platforms. The sea level was then 120 m lower than in the 21st century. As the sea level rose, water and corals invaded what had been hills of the Australian coastal Plain. 13,000 years ago, sea level had risen to 60 m lower than today, and many coastal plain hills had become continental islands.

Corals could then surpass the hills, forming the current cays and reefs. The sea level in the Great Barrier Reef has not changed significantly over the last 6,000 years, and the age of the modern reef structure is estimated to be between 6,000 and 8,000 years. The development stopped at the Barrier Reef stage, since Australia is not about to submerge.



Coral reefs are estimated to cover 284,300 km2 just below 0.1% of the ocean surface. The Indo-Pacific region (including the Red Sea, the Indian Ocean, Southeast Asia, and the Pacific) account for 91.9% of this total. Southeast Asia accounts for 32.3% of that figure, while the Pacific, including Australia, represents 40.8%. Coral reefs in the Atlantic and Caribbean represent 7.6%.

Although there are corals in both temperate and tropical waters, shallow-water reefs are formed only in an area ranging from approximately 30 ° N to 3 ° S of the equator. Tropical corals do not grow at depths of more than 50 meters. The optimum temperature for most coral reefs is 26 – 27 °c), and few reefs exist in waters below 18 °c.

Deep-sea corals can exist at greater depths and colder temperatures at much higher latitudes, as far north as Norway. Although deep-sea corals can form the reefs, very little is known about them.

Coral reefs are rare along the western coasts of America and Africa, mainly due to strong coastal and cold currents that reduce water temperatures in these areas. Corals are seldom found along the South Asian coast — from the eastern tip of India (Chennai) to the borders of Bangladesh and Myanmar — as well as along the coasts of northeastern South America and Bangladesh, due to the release of freshwater Of the Amazon and Ganges rivers, respectively.


Coral reefs form some of the world's most productive ecosystems, providing complex and varied marine habitats that support a wide range of other organisms. Marginal reefs just below the low-tide level have a mutually beneficial relationship with high-tide mangrove forests and seagrass meadows: reefs protect mangroves and marine algae from strong currents and Waves that would damage or erode the sediments in which they are rooted, while mangroves and seagrass protect the coral from large influxes of silt, freshwater and contaminants. This level of variety in the environment benefits many coral reef animals, which, for example, can feed on the seagrass and use the reefs for protection or breeding.

Reefs are home to a wide variety of animals, including fish, seabirds, sponges, cnidarians (which includes some types of corals and jellyfish), worms, crustaceans (including shrimp, cleaning shrimp, lobsters and crabs), molluscs (including cephalopods), echinoderms (including starfish, sea urchins and cucumbers), sea jets, marine turtles and marine snakes. In addition to humans, mammals are rare in coral reefs, and visiting cetaceans, such as dolphins, are the main exception. Some of these various species feed directly from the corals, while others graze on algae on the reef. The biomass of the reef is positively related to the diversity of species.

The same hideouts on a reef can be inhabited regularly by different species at different times of the day. Nocturnal predators hide during the day, while other species hide from eels and sharks.




Coral reefs offer ecosystem services to tourism, fishing and coastal protection. The global economic value of coral reefs has been estimated between 290 and 375 billion dollars per year. Coral reefs protect the coasts by absorbing the energy of the waves, and many small islands would not exist without their reefs to protect them. About 6 million tons of fish are caught every year from coral reefs. Well-managed coral reefs have an annual yield of 15 tonnes of shellfish on average per square kilometre. Southeast Asian coral reef fisheries yield about three billion dollars annually of shellfish.


Coral reefs are dying around the world. In particular, coral mining, agricultural and urban runoff, pollution (organic and inorganic), overfishing, explosive fishing, disease and channel excavation, and access to islands and bays are localized threats to coral ecosystems. The broader threats are sea temperature rise, sea-level rise and pH changes in ocean acidification, all associated with greenhouse gas emissions. A 2014 study lists factors such as population explosion along coastlines, overfishing, coastal zone pollution, global warming, and invasive species among the main reasons that have put reefs at risk of extinction.

To find answers to these problems, researchers study the various factors that affect reefs. The list includes the role of the ocean as a carbon dioxide sink, atmospheric changes, ultraviolet light, ocean acidification, viruses, dust storm impacts that bring agents to distant reefs, contaminants, algae blooms, and others.


Marine protected Areas (AMP) have become increasingly important for reef management. MPAs promote responsible fisheries management and habitat protection. Like national parks and Wildlife refuges, and in varying degrees, AMP restricts potentially damaging activities. MPAs encompass both social and biological objectives, including reef restoration, aesthetics, biodiversity, and economic benefits. However, there are very few AMP that have really made a substantial difference.

To help combat ocean acidification, there are some laws to reduce greenhouse gases such as carbon dioxide. The Drinking Water Act puts pressure on state government agencies to monitor and limit the runoff of pollutants that may cause ocean acidification. The prevention of storm tides, as well as barriers between agricultural land and the coast, have also been launched. This action also ensures that the sensitive ecosystems of the watershed are intact, such as wetlands.

In Australia, the Great Barrier Reef is protected by the authority of the Great Barrier Reef Marine Park, and is subject to a lot of legislation, including a plan of action for biodiversity. They have compiled a plan of action for the resilience of coral reefs. This detailed action plan consists of numerous adaptive management strategies, including the reduction of our carbon footprint, which would ultimately reduce the amount of ocean acidification in the waters surrounding the Great Barrier Reef.

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