Scleractinia

Scleractinian corals may be solitary or colonial. Colonies can reach considerable size, consisting of a large number of individual polyps. Soft parts Stony corals are members of the class Anthozoa and like other members of the group, do not have a medusa stage in their life cycle. The individual animals are known as polyps and have a cylindrical body crowned by an oral disc surrounded by a ring of tentacles. The base of the polyp secretes the stony material from which the coral skeleton is formed. The body wall of the polyp consists of mesoglea sandwiched between two layers of epidermis. The mouth is at the centre of the oral disc and leads into a tubular pharynx which descends for some distance into the body before opening into the gastrovascular cavity that fills the interior of the body and tentacles. Unlike other cnidarians however, the cavity is subdivided by a number of radiating partitions, thin sheets of living tissue, known as mesenteries. The gonads are also located within the cavity walls. The polyp is retractable into the corallite, the stony cup in which it sits, being pulled back by sheet-like retractor muscles. Skeleton '', a brain coral The skeleton of an individual scleractinian polyp is known as a corallite. It is secreted by the epidermis of the lower part of the body, and initially forms a cup surrounding this part of the polyp. The interior of the cup contains radially aligned plates, or septa, projecting upwards from the base. Each of these plates is flanked by a pair of mesenteries. The structure of both simple and compound scleractinians is light and porous, rather than solid as is the case in the prehistoric order Rugosa. Scleractinians are also distinguished from rugosans by their pattern of septal insertion. Growth In colonial corals, growth results from the budding of new polyps. There are two types of budding, intratentacular and extratentacular. In intratentacular budding, a new polyp develops on the oral disc, inside the ring of tentacles. This can form individual, separate polyps or a row of partially separated polyps sharing an elongate oral disc with a series of mouths. Tentacles grow around the margin of this elongated oral disc and not around the individual mouths. This is surrounded by a single corallite wall, as is the case in the meandroid corallites of brain corals. The rate of aragonite deposition varies diurnally and seasonally. Examination of cross sections of coral can show bands of deposition indicating annual growth. Like tree rings, these can be used to estimate the age of the coral. == Distribution ==

Stony corals occur in all the world's oceans. There are two main ecological groups. Hermatypic corals are mostly colonial corals which tend to live in clear, oligotrophic, shallow tropical waters; they are the world's primary reef-builders. Ahermatypic corals are either colonial or solitary and are found in all regions of the ocean and do not build reefs. Some live in tropical waters but some inhabit temperate seas, polar waters, or live at great depths, from the photic zone down to about . == Ecology ==

'' extends its polyps at night to feed. Scleractinians fall into one of two main categories: • Reef-forming or hermatypic corals, which mostly contain zooxanthellae; • Non-reef-forming or ahermatypic corals, which mostly do not contain zooxanthellae In reef-forming corals, the endodermal cells are usually replete with symbiotic unicellular dinoflagellates known as zooxanthellae. There are sometimes as many as five million cells of these per of coral tissue. Up to 50% of organic compounds produced by symbionts are used as food by polyps. The oxygen byproduct of photosynthesis and the additional energy derived from sugars produced by zooxanthellae enable these corals to grow at a rate up to three times faster than similar species without symbionts. These corals typically grow in shallow, well-lit, warm water with moderate to brisk turbulence and abundant oxygen, and prefer firm, non-muddy surfaces on which to settle. == Life cycle ==

Stony corals have a great range of reproductive strategies and can reproduce both sexually and asexually. Many species have separate sexes, the whole colony being either male or female, but others are hermaphroditic, with individual polyps having both male and female gonads. Some species brood their eggs but in most species, sexual reproduction results in the production of a free-swimming planula larva that eventually settles on the seabed to undergo metamorphosis into a polyp. In colonial species, this initial polyp then repeatedly divides asexually, to give rise to the entire colony. Under adverse conditions, certain species of coral resort to another type of asexual reproduction in the form of "polyp bail-out", which may allow polyps to survive even though the parent colony dies. It involves the growth of the coenosarc to seal off the polyps, detachment of the polyps and their settlement on the seabed to initiate new colonies. In other species, small balls of tissue detach themselves from the coenosarc, differentiate into polyps and start secreting calcium carbonate to form new colonies, and in Pocillopora damicornis, unfertilised eggs can develop into viable larvae. In temperate regions, the usual pattern is synchronized release of eggs and sperm into the water during brief spawning events, often related to the phases of the moon. In tropical regions, reproduction may occur throughout the year. In many cases, as in the genus Acropora, the eggs and sperm are released in buoyant bundles which rise to the surface. This increases the concentration of sperm and eggs and thus the likelihood of fertilization, and reduces the risk of self-fertilization. Immediately after spawning, the eggs are delayed in their capability for fertilization until after the release of polar bodies. This delay, and possibly some degree of self-incompatibility, likely increases the chance of cross-fertilization. A study of four species of Scleractinia found that cross-fertilization was actually the dominant mating pattern, although three of the species were also capable of self-fertilization to varying extents. == Evolutionary history ==

rugose corals with calcite skeletons, like this Grewingkia canadensis, are doubtfully ancestors of the Scleractinia. There is little evidence on which to base a hypothesis about the origin of the scleractinians; plenty is known about modern species but very little about fossil specimens, which first appeared in the record in the Middle Triassic (). It was not until 25 million years later that they became important reef builders, their success likely a result of teaming up with symbiotic algae. Nine of the sub-orders were in existence by the end of the Triassic and three more had appeared by the Jurassic (200 million years ago), with a further suborder appearing in the Middle Cretaceous (100 million years ago). Recently discovered Paleozoic corals with aragonitic skeletons and cyclic septal insertion – two features that characterize Scleractinia – have strengthened the hypothesis for an independent origin of the Scleractinia. Whether the early scleractinian corals were zooxanthellate is an open question. The phenomenon seems to have evolved independently on numerous occasions during the Tertiary, and the genera Astrangia, Madracis, Cladocora and Oculina, all in different families, each have both zooxanthellate and non-zooxanthellate members. Traits that generally enable corals to survive mass extinction include deep-water or large habitat range, non-symbiotic, solitary or small colonies, and bleaching resistance, all of which tend to characterize azooxanthellate (non-symbiotic) corals. Endosymbionts, on the other hand, which rely on specialized conditions and access to light to survive, are especially vulnerable to prolonged darkness, temperature change, and eutrophication, all of which have been hallmarks of past mass extinctions. This makes zooxanthellate coral especially vulnerable to unstable conditions. Therefore, it is possible that coral and zooxanthellate coevolved loosely, with the relationship dissolving when advantages decreased, then reforming when conditions stabilized. == Classification ==

The taxonomy of Scleractinia is particularly challenging. Many species were described before the advent of scuba diving, with little realisation by the authors that coral species could have varying morphologies in different habitats. Collectors were mostly limited to observing corals on reef flats, and were unable to observe the changes in morphology that occurred in more turbid, deeper-water conditions. More than 2,000 nominal species were described in this era, and by the rules of nomenclature, the name given to the first described species has precedence over the rest, even when that description is poor, and the environment and even sometimes the country of the type specimen is unknown. Even the concept of "the species" is suspect, with regard to corals which have large geographical ranges with a number of sub-populations; their geographic boundaries merge with those of other species; their morphological boundaries merge with those of other species; and there are no definite distinctions between species and subspecies. The evolutionary relationships among stony corals were first examined in the 19th and early 20th centuries. The two most advanced 19th century classifications both used complex skeletal characters; The 1857 classification of the French zoologists Henri Milne-Edwards and Jules Haime's was based on macroscopic skeletal characters, while Francis Grant Ogilvie's 1897 scheme was developed using observations of skeletal microstructures, with particular attention to the structure and pattern of the septal trabeculae. The 1952 classification by French zoologist J. Alloiteau was built on these earlier systems but included more microstructural observations and did not involve the anatomical characters of the polyp. Alloiteau recognized eight suborders. • Acroporidae • Agariciidae • †Agathiphylliidae • Anthemiphylliidae • Astrocoeniidae • Caryophylliidae • Coscinaraeidae • Deltocyathidae • Dendrophylliidae • Diploastreidae • Euphylliidae • Flabellidae • Fungiacyathidae • Fungiidae • Gardineriidae • Guyniidae • Lobophylliidae • Meandrinidae • Merulinidae • Micrabaciidae • Montastraeidae • Mussidae • Oculinidae • †Oulastreidae • Plesiastreidae • Pocilloporidae • Poritidae • Psammocoridae • Rhizangiidae • Schizocyathidae • Siderastreidae • Stenocyathidae • †Stylinidae • Turbinoliidae • Mussidae accepted as Faviidae ==Conservation==

All Scleractinian corals (excluding fossils) are listed under Appendix II of the Convention on International Trade in Endangered Species (CITES) meaning that their international trade (including in parts and derivatives) is regulated. == See also ==