Coral reefs are masses of natural structures composed of skeletal material from by reef-building (or hard) corals, small organisms that grow in shallow water. Corals are found in both temperate and tropical waters. Reefs, however, can only form within about 30° of the Equator. They leave their exoskeletons when they die and the tide movement slowly layers them to form many different types of coral. The corals can only grow within 30 meters (100 feet) of the surface of the water where the temperature is above 16° .

Some people mistakenly call corals polyps. ‘Polyp’ describes a stage in the life of a coral, and is not specific to that species. Corals are hydroids, in a polyp form.

A healthy reef normally has a lot of algae, including turf algae, coralline algae, and macro algae. The coral itself does not actually produce the amazing pigments that make it so pleasing to look at. The colour comes from an algae which has a symbiotic relationship with the coral. A symbiotic relationship is when two plants and or animals live with each other and they both benefit from the relationship. The corals provide a safe place for the algae to live, and the algae give the corals food. The algae is called zooxanthellae. Coral reefs support a huge diversity of marine animal and plant life. They are, in fact, the most biodiverse type of marine ecosystem in the world. It takes many years to form a reef, (the average growth rate being a minute 1 mm per year) yet with the new modern threats such as dynamite fishing, it can take a few seconds to destroy one.

Coral reefs are threatened in many ways. Coral bleaching happens when the zooxanthellae in the coral is killed, usually by global warming or by a chemical change in the water. When coral is bleached it turns white and dies. Dynamite fishing, as mentioned before, consists of throwing explosives into a reef, liquifying the innards of small fish and causing them to float to the surface for easy collection. Cyanide fishing is somewhat similar—dumping poison on a reef to make the fish sluggish and easy to capture. People eat the poisoned fish without knowing it. Just plain overfishing kills coral reefs even if no incredibly destructive methods are used. Reefs are very valuable, even in non-environmental fields; medicine, tourism, and of course the fishing industry.

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