Scleractinia

From Palaeos

All Mesozoic and Cenozoic reef-building corals are members of this order (sometimes, subclass). Reef-building corals are also known as "hard corals" or "stony corals." They are similar to and closely related to sea anemones but unlike those soft-bodied forms they secrete an aragonitic (calcium carbonate) skeleton. It is this skeleton that forms coral reefs.

Some forms, like the Indo-Pacific Fungia, are solitary, with single polyps reaching 25 cm in diameter. The majority however are colonial, with very small polyps (about 1 to 3 mm), although the colony as a whole can grow very large.

As with all the Hexacorallia, septal insertion occurs in multiples of six, and many scleractinial corals have 6-fold symmetry.

Over 60 genera of Scleractinia have a symbiotic relationship with a type of microscopic algae, called zooxanthellae, living within the coral polyp's tissue (within the gastrodermal cells). Although deep-water and some cold-water corals lack zooxanthellae, virtually all reef-building species possess them. These corals are generally found in clear water at depths of less than 50 meters, the zone where sunlight penetrates. The algae not only provide food but help polyp calcification, and may account for up to 50% of the protein nitrogen of the coral, as well as giving then a yellow-brown or dark brown colour. These so-called hermatypic corals can lay down massive amounts of limestone in the photic zone of shallow tropic seas.

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