Scleractinia

Scleractinia, the stony or hard corals, is an order of marine animals in the phylum Cnidaria that build a hard calcium carbonate skeleton. The individual animals are polyps, each with a cylindrical body topped by an oral disc whose mouth is fringed with tentacles. Most species are colonial, budding asexually into colonies of clones that share one skeleton and can reach several metres across, though some are solitary and up to about 25 centimetres wide. Many shallow-water species host symbiotic zooxanthellae that colour them. Stony corals occur in all the oceans, much of the framework of modern coral reefs is built by them, and some live at abyssal depths without light. They first appeared in the Middle Triassic, belong to the class Anthozoa, and are expected to decline with global warming and ocean acidification.