Any of a group of sedentary colonial marine invertebrates belonging to the class Anthozoa of the phylum Cnidaria. A coral colony consists of individual polyps within a protective skeleton that they secrete: this skeleton may be soft and jelly-like, horny, or stony. The horny skeleton secreted by corals of the genus Corallium, especially C. rubrum, constitutes the red, or precious, coral used as a gemstone. The skeleton of stony, or true, corals consists of almost pure calcium carbonate and forms the coral reefs common in shallow waters of tropical seas. Photosynthesizing dinomastigotes (dinoflagellates) live within the cells of the reef-forming corals, providing them with up to 80% of their energy. In return, the coral provides its endosymbiotic residents with nutrients and a place to live. Coral formations play a key role in marine ecosystems, e.g. as ‘nurseries’ for juvenile fish and substrates for shellfish. But the majority of coral reefs are being damaged by various factors, including overfishing and poor fishing practices, pollution, unregulated tourism, deposition of sediment, coral mining, and climate change. Corals are very sensitive to rising sea temperature, which leads them to expel the dinomastigotes, causing the corals to lose their colour (‘coral bleaching’) and impacting their ability to survive. Also, ocean acidification tends to dissolve the coral skeletons.