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. The individual animals are known as
polyps and have a cylindrical body crowned by an oral disc with a mouth and a fringe of tentacles. Although some species are solitary, most are
colonial. The founding polyp settles on the seabed and starts to secrete
calcium carbonate to protect its soft body. Solitary corals can be as much as 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 colony with a common skeleton, which may be up to several metres in diameter or height according to species.