What Is Coral?
The hard, dry corals seen in stores and museums are made up the combined skeletons of tiny creatures which once lived joined together as a colony in the sea.
The coral animal, or polyp, measures only about a fifteenth of an inch in diameter. It is a primitive type of plant-like creature closely related to the sea anemone.
But, unlike the sea anemone, the polyp has a stony skeleton and, once it has made its home, it is unable to move. Each polyp is like a tiny tube, closed at one end and with a mouth, fringed with tentacles, at the other. The tentacles have a sting and capture minute sea creatures for food.
Although some corals can catch small fish and plankton, using stinging cells on their tentacles, most corals obtain the majority of their energy and nutrients from photosynthetic unicellular dinoflagellates in the genus Symbiodinium that live within their tissues.
These are commonly known as zooxanthellae and the corals that contain them are zooxanthellate corals. Such corals require sunlight and grow in clear, shallow water, typically at depths shallower than 60 metres (200 ft).
Coral animals inhabit shallow water in warm seas. In tropical waters colonies form reefs round islands which can be dangerous to ships. Eventually new islands are formed by the accumulated skeletons of billions of dead polyps.
But the romantic coral islands and reefs of the South Seas do not supply the precious material used for beads and brooches. These are made from the red and pink coral found in the Mediterranean off the coasts of Africa and Italy.
Corals are major contributors to the physical structure of the coral reefs that develop in tropical and subtropical waters, such as the enormous Great Barrier Reef off the coast of Queensland, Australia.
Other corals do not rely on zooxanthellae and can live in much deeper water, with the cold-water genus Lophelia surviving as deep as 3,000 metres (9,800 ft). Some have been found on the Darwin Mounds, north-west of Cape Wrath, Scotland. Corals have also been found as far north as off the coast of Washington State and the Aleutian Islands.