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Corals are small cnidarians, most of which form colonies bound together by secreted calcium carbonate. They include the most important reef builders, found in tropical seas. They obtain much of their energy from symbiotic algae called zooxanthellae, and so are dependent on sunlight. As a result, they are usually found at or near the surface, though occurring to a depth of at least 60 m. There are two main forms of coral, hard and soft.
Corals are slow growing, and as a result are generally protected through environmental laws. Coral breed by spawning, with all corals in a region spawning on a few nights around full moon.
From southern California
Coral reefs grow as atolls, barrier reefs or fringing reefs also called shore reefs. Coral reefs were first described in 1842 in the book Coral Reefs by Charles Darwin.
Normally coral reefs live in tropical waters, but coral exist even in cold waters of the coast of Norway.
The largest collection of coral reefs is of the coast of Queensland, Australia. These reefs are known collectively as the Great Barrier Reef.
Dead coral reefs on land are often mined for marble.
Reddish coral is sometimes used as a gemstone especially in Tibet. Pure red coral is known as 'fire coral' and it is very rare because of the demand for perfect fire coral for jewellery-making purposes.
Coral is very sensitive to environmental changes. A coral reef can easily be swamped in algae if there is too much nitrogen in the water. Coral will also die if the water temperature changes by more than a degree or two and becomes too hot or too cold or if the salinity of the water drops. A combination of temperature changes, pollution, and overuse has led to the destruction of many coral reefs around the world.
Coral is also a color, a pinkish orange, named after the above cnidarians. On a browser that supports visual formatting in Cascading Style Sheets, the following box should appear in this color: