While corals may look like rocks and share several characteristics of plants, they are in fact animals. To be exact they are aquatic marine invertebrates (known as polyps) that live in the warm shallows of the clear coastal waters located around the world. A huge number of marine organisms make their home among the corals, making reefs some of the most abundant and varied habitats on Earth. Because the nutrients on which plankton need to feed dissolve better in deeper, cooler water, the warmer layers become a less attractive spot for the huge numbers of floating plankton to occupy. Therefore, the upper shallows remain
warm and clear – the ideal living conditions for microscopic algae, which use sunlight to combine carbon dioxide and water to create their own food source, which they share with their coral.
Corals live in partnership with single-celled zooxanthellae algae,
which are also responsible for the bright colours. If the algae die the coral will turn white, a damaging effect known as coral bleaching. Like jellyfish, corals are cnidarians, except they are rooted to the spot by a tube attached to a surface (usually rock), rather than floating freely like jellyfish. Cnidarians consist of a simple body, featuring a central mouth opening that is surrounded by stinging tentacles. The coral polyp is
the soft individual organism that forms from a single-celled alga and lives within a larger community of similar polyps called a colony.
They use calcium and a variety of other minerals in the seawater – together with the food waste they produce – to construct their own protective calcium carbonate skeleton shelters in which to live. When coral dies,
the hard, chalky skeletal remains are left behind and new polyps will grow on top of these. Sedimentary limestone rock is formed when the coral skeletons are compacted over many thousands of years.
Over hundreds of thousands of years, a colony of polyps can grow big enough to link up with other colonies to form a large coral reef.
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