Underwater structures of calcium carbonate built over thousands of years by colonies of tiny animals called coral polyps, which live in partnership with photosynthetic algae and support some of the richest marine life on Earth.
A coral polyp is a soft-bodied animal only a few millimetres across, related to sea anemones and jellyfish. Reef-building corals take calcium and carbonate from seawater and lay down a hard cup-shaped skeleton beneath themselves. Generation after generation, colonies grow on top of the skeletons of their ancestors, and the resulting limestone frameworks can stretch for hundreds of kilometres. The Great Barrier Reef off north-eastern Australia is the largest such system and can be seen from space.
The secret of corals' success in nutrient-poor tropical seas is a partnership. Microscopic algae called zooxanthellae live inside the polyps' tissues, photosynthesise, and pass most of the sugars they make to their host. In return they get shelter and nutrients from the coral's waste. The algae also give corals much of their colour. Because the algae need light, reef-building corals are confined to shallow, clear water, typically less than about 50 metres deep.
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