Scleractinia, also called stony corals or hard corals, are marine animals in the phylum Cnidaria that live on the seabed and build themselves a hard skeleton. Although some species are solitary, most are colonial. The founding polyp settles on the seabed and starts to secrete a calcium carbonate skeleton to protect its soft body. Solitary corals can be as much as 25 cm (10 in) across but in colonial species the polyps are usually only a few millimetres in diameter. These polyps reproduce by budding but remain attached to each other, forming a multi-polyp unit, which may be metres in diameter or height.
The shape and appearance of each coral colony depends not only on the species, but also on its location, depth, the amount of water movement and other factors. Many shallow-water corals contain symbiont unicellular organisms known as zooxanthellae within their tissues. These give their colour to the coral which thus may vary in hue depending on what species of symbiont it contains. Stony corals are closely-related to sea anemones, and like them are armed with stinging cells known as cnidocytes.
Stony corals first appeared in the Middle Triassic and their relationship to the tabulate and rugose corals of the Paleozoic is currently unresolved. Much of the framework of modern coral reefs is formed by scleractinians. Stony corals numbers are expected to decline due to the effects of global warming.