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Scleractinia, also called stony corals or hard corals, are marine animals in the phylum Cnidaria that build themselves a hard skeleton. The individual animals are known as polyps and have a cylindrical body crowned by an oral disc in which a mouth is fringed with tentacles. Although some species are solitary, most are colonial. The founding polyp settles 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 asexually by budding, but remain attached to each other, forming a multi-polyp colony of cloning with a common skeleton, which may be up to several metres in diameter or height according to species.
The shape and appearance of each coral colony depends not only on the species, but also on its location, depth, the amount of water movement and other factors. Many shallow-water corals contain Symbiosis unicellular organisms known as zooxanthellae within their tissues. These give their colour to the coral which thus may vary in hue depending on what species of symbiont it contains. Stony corals are closely related to , and like them are armed with stinging cells known as . Corals reproduce both sexually and asexually. Most species release into the sea where fertilisation takes place, and the planula larvae drift as part of the plankton, but a few species brood their eggs. Asexual reproduction is mostly by fragmentation, when part of a colony becomes detached and reattaches elsewhere.
Stony corals occur in all the world's oceans. Much of the framework of modern coral reefs is formed by scleractinians. Reef-building or are mostly colonial; most of these are zooxanthellate and are found in the shallow waters into which sunlight penetrates. Other corals that do not form reefs may be solitary or colonial; some of these occur at Abyssal zone where no light reaches.
Stony corals first appeared in the Middle Triassic, but their relationship to the tabulate coral and of the Paleozoic is currently unresolved. In modern times stony corals numbers are expected to decline due to the effects of global warming and ocean acidification. Reef-Building Corals Lose Out to Softer Cousins Due To Global Warming March 24, 2013 Scientific American
The polyps are connected by horizontal sheets of tissue known as coenosarc extending over the outer surface of the skeleton and completely covering it. These sheets are continuous with the body wall of the polyps, and include extensions of the gastrovascular cavity, so that food and water can circulate between all the different members of the colony.
In colonial species, the repeated asexual division of the polyps causes the corallites to be interconnected, thus forming the colonies. Also, cases exist in which the adjacent colonies of the same species form a single colony by fusing. Most colonial species have very small polyps, ranging from in diameter, although some solitary species may be as large as . (2025). 9788131501047, Cengage Learning. ISBN 9788131501047
The septa are secreted by the mesenteries, and are therefore added in the same order as the mesenteries are. As a result, septa of different ages are adjacent to one another, and the symmetry of the scleractinian skeleton is radial or biradial. This pattern of septal insertion is termed "cyclic" by paleontologists. By contrast, in some fossil corals, adjacent septa lie in order of increasing age, a pattern termed serial and produces a bilateral symmetry. Scleractinians secrete a stony exoskeleton in which the septa are inserted between the mesenteries in multiples of six.
All modern scleractinian skeletons are composed of calcium carbonate in the form of crystals of aragonite, however, a prehistoric scleractinian ( Coelosimilia) had a non-aragonite skeletal structure which was composed of calcite. 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.
Extratentacular budding always results in separate polyps, each with its own corallite wall. In the case of bushy corals such as Acropora, lateral budding from axial polyps form the basis of the trunk and branches. The rate at which a stony coral colony lays down calcium carbonate depends on the species, but some of the branching species can increase in height or length by around a year (about the same rate as human hair grows). Other corals, like the dome and plate species, are more bulky and may only grow per year.Ross Piper (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press. 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.
Solitary corals do not bud. They gradually increase in size as they deposit more calcium carbonate and produce new whorls of septa. A large Ctenactis echinata for example normally has a single mouth, may be about long and have more than a thousand septa.
In reef-forming corals, the endodermal cells are usually replete with symbiotic unicellular 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.
Most stony corals extend their tentacles to feed on zooplankton, but those with larger polyps take correspondingly larger prey, including various and even small fish. In addition to capturing prey in this way, many stony corals also produce mucus films they can move over their bodies using cilia; these trap small organic particles which are then pulled towards and into the mouth. In a few stony corals, this is the primary method of feeding, and the tentacles are reduced or absent, an example being Acropora acuminata. Caribbean stony corals are generally nocturnal, with the polyps retracting into their skeletons during the day, thus maximising the exposure of the zooxanthellae to the light, but in the Indo-Pacific region, many species feed by day and night.
Non-zooxanthellate corals are usually not reef-formers; they can be found most abundantly beneath about of water. They thrive at much colder temperatures and can live in total darkness, deriving their energy from the capture of plankton and suspended organic particles. The growth rates of most species of non-zooxanthellate corals are significantly slower than those of their counterparts, and the typical structure for these corals is less calcified and more susceptible to mechanical damage than that of zooxanthellate corals.
Scleratinians were previously believed to be obligatory hosts of another group of barnacles, the pyrgomatids, but a recent study recorded evidence of living pyrgomatids in , casting doubt on this idea.
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.
The earliest scleractinians were not reef builders, but were small, phaceloid or solitary individuals. Scleractinian corals were probably at their greatest diversity in the Jurassic and all but disappeared in the mass extinction event at the end of the Cretaceous, about 18 out of 67 genera surviving. 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.
The fact that zooxanthellate coral make up only about half of the order is unusual, as symbiosis is almost always an all-or-nothing phenomenon. This symbiotic equilibrium suggests that there must be evolutionary processes simultaneously maintaining and limiting symbiotic relationships. This is likely because despite the energetic benefits it provides, photosymbiosis appears to be an evolutionary disadvantage during mass extinctions. 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.
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 . In 1943, the American zoologists Thomas Wayland Vaughan and John West Wells, and Wells again in 1956, used the patterns of the septal trabeculae to divide the group into five Taxonomic rank. In addition, they considered polypoid features such as the growth of the tentacles. They also distinguished families by wall type and type of budding.
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. In 1942, W.H. Bryan and Dorothy Hill stressed the importance of microstructural observations by proposing that stony corals begin skeletal growth by configuring calcification centers, which are genetically derived. Therefore, diverse patterns of calcification centers are vital to classification. Alloiteau later showed that established morphological classifications were unbalanced and that there were many examples of convergent evolution between fossils and recent Taxon.
The rise of molecular techniques at the end of the 20th century prompted new evolutionary hypotheses that were different from ones founded on skeletal data. Results of molecular studies explained a variety of aspects of the evolutionary biology of the Scleractinia, including connections between and within extant taxa, and supplied support for hypotheses about extant corals that are founded on the fossil record. The 1996 analysis of mitochondrial RNA undertaken by American zoologists Sandra Romano and Stephen Palumbi found that molecular data supported the assembling of species into the existing families, but not into the traditional suborders. For example, some genus affiliated with different suborders were now located on the same branch of a phylogenetic tree. In addition, there is no distinguishing morphological character that separates clades, only molecular differences.
The Australian zoologist John Veron and his co-workers analyzed ribosomal RNA in 1996 to obtain similar results to Romano and Palumbi, again concluding that the traditional families were plausible but that the suborders were incorrect. They also established that stony corals are monophyletic, including all the descendants of a Common descent, but that they are divided into two groups, the robust and complex clades. Veron suggested that both morphological and molecular systems be used in future classification schemes.
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