Scleractinian ("hard-rayed") corals first appeared in the Middle Triassic and refilled the ecological niche once held by tabulate and rugose corals. They are probably not closely related to the extinct tabulate or rugose corals, and probably arose independently from a sea anemone-like ancestor. Their pattern of septa differs markedly from that of the Rugosa, being basically six-rayed. For this reason, scleractinians are sometimes referred to as hexacorals.
Colonial scleractinians from modern tropical seas are now the world's primary reef formers. Colonial corals consist of large numbers of polyps, cemented together by the calcium carbonate that they secrete. In the brain coral shown above, the individual polyps are no longer visible, having fused into long, meandering rows that resemble the wrinkles of a human brain. Some scleractinians, however, do not form colonies, like the specimen of the tropical Pacific fungiid shown on the Introduction to Cnidaria page. Other solitary scleractinians live in temperate waters or below the photic zone, the uppermost ocean waters, into which light penetrates.
A slide show of coral reefs and the organisms associated with them is available from the One Tree Island Research Station, operated by the University of Sydney, Australia.
Sea World and Busch Gardens maintain the Animal Information Data Base, which includes information on corals and coral reefs. For information on conservation of reefs and other marine habitats, visit the home page of Ocean Voice International, or read some abstracts on coral health collected from various journals.