scleractinians

Scleractinia, commonly called stony corals or hard corals, are an order of marine cnidarians within the class Anthozoa. They secrete calcium carbonate skeletons that form the rigid framework of most modern reefs. Most species are colonial polyps, though some are solitary. The polyp sits in a calyx and bears a mouth surrounded by tentacles; the skeleton is strengthened by internal septa.

Many scleractinians host photosynthetic symbionts, dinoflagellates in the family Symbiodiniaceae, which provide energy and drive reef growth in shallow, sunlit waters. This symbiosis makes corals susceptible to bleaching under thermal stress, when symbionts are expelled. Some species are azooxanthellate and feed mainly on plankton.

Geographically, scleractinians occur worldwide but are most diverse in tropical and subtropical seas. Reef-building species dominate clear, nutrient-poor waters, while deep or cryptic species may lack symbionts. Reproduction is sexual, often via mass spawning or brooding, and asexual fragmentation can aid colony expansion and recovery after disturbance.

Taxonomically, Scleractinia is the order of hard corals within Hexacorallia. The fossil record reveals abundant skeletonized corals from the Mesozoic onward, and scleractinians are the principal architects of modern reefs. They face climate change, ocean acidification, pollution, and disease, prompting conservation and restoration efforts.