Coral
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{{Taxobox
| name = Coral
| image = PillarCoral.jpg
| image_width = 200px
| image_caption = [[Pillar coral]], ''Dendrogyra cylindricus''
| regnum = [[Animal]]ia
| phylum = [[Cnidaria]]
| classis = [[Anthozoa]]
| classis_authority = [[Christian Gottfried Ehrenberg|Ehrenberg]], 1831
| subdivision_ranks = Extant Subclasses and Orders
| subdivision =
[[Alcyonaria]]<br />
[[Alcyonacea]]<br />
[[Helioporacea]]<br />
[[Zoantharia]]<br />
[[Antipatharia]]<br />
[[Corallimorpharia]]<br />
[[Scleractinia]]<br />
[[Zoanthidea]]<br /><ref name=Daly /><ref name=McFadden /> ''See [[Anthozoa]] for details''
}}
{{otheruses}}
'''Corals''' are [[marine organism]]s from the [[class (biology)|class]] [[Anthozoa]] and exist as small [[sea anemone]]–like [[polyp]]s, typically in colonies of many identical individuals. The group includes the important [[Coral reef|reef]] builders that are found in tropical [[ocean]]s, which secrete [[calcium carbonate]] to form a hard skeleton.
A coral "head", commonly perceived to be a single organism, is formed from thousands of individual but genetically identical [[polyp]]s, each polyp only a few millimeters in diameter. Over thousands of generations, the polyps lay down a skeleton that is characteristic of their [[species]]. A head of coral grows by asexual reproduction of the individual polyps. Corals also breed sexually by spawning, with corals of the same species releasing [[gamete]]s simultaneously over a period of one to several nights around a full moon.
Although corals can catch [[plankton]] using [[nematocyst|stinging cells]] on their tentacles, these animals obtain most of their nutrients from [[symbiosis|symbiotic]] unicellular algae called [[zooxanthella]]e. Consequently, most corals depend on sunlight and grow in clear and shallow water, typically at depths shallower than 60 [[Metre|m]] (200 [[Foot (unit of length)|ft]]). These corals can be major contributors to the physical structure of the [[coral reef]]s that develop in tropical and subtropical waters, such as the enormous [[Great Barrier Reef]] off the coast of [[Queensland]], [[Australia]]. Other corals do not have associated algae and can live in much deeper water, such as in the [[Atlantic]], with the cold-water genus ''Lophelia'' surviving as deep as 3000 m.<ref name=Squires>
{{cite journal
| author = Squires, D.F.
| year = 1959
| month =
| title = Deep sea corals collected by the Lamont Geological Observatory. 1. Atlantic corals
| journal = American Museum Novitates
| volume = 1965
| issue =
| pages = 1–42
| id =
| url =
}}</ref> Examples of these can be found living on the [[Darwin Mounds]] located north-west of [[Cape Wrath]], [[Scotland]]. Corals have also been found off the coast of [[Washington]] State and the [[Aleutian Islands]] in [[Alaska]].
==Phylogeny==
[[Image:Muchroom coral.JPG|A [[mushroom coral]] in [[Papua New Guinea]]|thumb|right]]
{{main|Anthozoa}}
Corals belong to the [[class (biology)|class]] ''[[Anthozoa]]'' and are divided into two subclasses, depending on the number of tentacles or lines of symmetry, and a series of orders corresponding to their exoskeleton, nematocyst type and [[mitochondria]]l [[genetic analysis]].<ref name=Daly>
{{cite journal
| author = Daly, M., Fautin, D.G., and Cappola, V.A.
| year = 2003
| month = March
| title = Systematics of the Hexacorallia (Cnidaria: Anthozoa)
| journal = Zoological Journal of the Linnean Society
| volume = 139
| issue =
| pages = 419–437
| id =
| url = http://www.ingentaconnect.com/content/bsc/zoj/2003/00000139/00000003/art00003
| doi = 10.1046/j.1096-3642.2003.00084.x <!--Retrieved from CrossRef by DOI bot-->
}}</ref><ref name=McFadden>
{{cite journal
| author = McFadden, C.S., France, S.C., Sanchez, J.A., and Alderslade, P.
| year = 2006
| month = December
| title = A molecular phylogenetic analysis of the Octocorallia (Cnidaria: Anthozoa) based on mitochondrial protein-coding sequences.
| journal = Molecular Phylogenentics and Evolution
| volume = 41
| issue = 3
| pages = 413–527
| pmid = 12967605
| url =
}}</ref><ref name=France>
{{cite journal
| author = France, S. C., P. E. Rosel, J. E. Agenbroad, L. S. Mullineaux, and T. D. Kocher
| year = 1996
| month = March
| title = DNA sequence variation of mitochondrial large-subunit rRNA provides support for a two subclass organization of the Anthozoa (Cnidaria)
| journal = Molecular Marine Biology and Biotechnology
| volume = 5
| issue = 1
| pages = 15–28
| pmid = 8869515
| url =
}}</ref> Those with eight tentacles are called octocorallia or [[Alcyonaria]] and comprise [[Alcyonacea|soft corals]], [[sea fan]]s and [[sea pen]]s. Those with more than eight in a multiple of six are called hexacorallia or [[Zoantharia]]. This group includes reef-building corals ([[Scleractinia]]ns), [[sea anemones]] and [[zoanthid]]s.
== Anatomy ==
[[Image:Coral polyp.jpg|thumb|left|275px|Anatomy of a coral polyp]]
While a coral head appears to be a single organism, it is actually a head of many individual, yet [[Cloning|genetically identical]], [[polyp]]s. The polyps are multicellular organisms that feed on a variety of small organisms, from microscopic [[plankton]] to small fish.
Polyps are usually a few millimeters in diameter, and are formed by a layer of outer [[epithelium]] and inner jellylike tissue known as the [[mesoglea]]. They are radially symmetrical with tentacles surrounding a central mouth, the only opening to the stomach or coelenteron, through which both food is ingested and waste expelled.
The stomach closes at the base of the polyp, where the epithelium produces an [[exoskeleton]] called the basal plate or calicle ([[Latin|L.]] small cup). This is formed by a thickened calciferous ring (annular thickening) with six supporting radial ridges ([[Coral#Reproduction|as shown below]]). These structures grow vertically and project into the base of the polyp. When polyps are physically stressed, they contract into the calyx so that virtually no part is exposed above the skeletal platform. This protects the organism from predators and the elements (Barnes, R.D., 1987; Sumich, 1996).<ref name=Barnes87>
{{cite book
| author = Barnes, R.D.
| year = 1987
| title = Invertebrate Zoology; Fifth Edition
| pages = 149-163
| publisher = Harcourt Brace Jovanovich, Inc.
| location = Orlando, FL, USA
| id =
}}</ref><ref name=Sumich>
{{cite book
| author = Sumich, J. L.
| year = 1996
| title = An Introduction to the Biology of Marine Life; Sixth Edition
| pages = 255-269
| publisher = Wm. C. Brown
| location = Dubuque, IA, USA
| id =
}}</ref>
The polyp grows by extension of vertical calices which are occasionally septated to form a new, higher, basal plate. Over many generations this extension forms the large calciferous ([[Calcium]] containing) structures of corals and ultimately coral reefs.
Formation of the calciferous exoskeleton involves deposition of the mineral [[aragonite]] by the polyps from [[calcium]] ions they acquire from seawater. The rate of deposition, while varying greatly between species and environmental conditions, can be as much as 10 g / m² of polyp / day (0.3 ounce / sq yd / day). This is light dependent, with night-time production 90% lower than that during the middle of the day.<ref name=MarineReef>
{{cite web
| url = http://www.marinereef.org/reports.php?reportid=2
| title = Anatomy of Coral
| work = Marine Reef
| accessdate = 2006-03-31
}}</ref>
[[Image:Nematocyst discharge.png|thumb|right|300px|[[Nematocyst]] discharge: A dormant nematocyst discharges response to nearby prey touching the cnidocil, the operculum flap opens and its stinging apparatus fires the barb into the prey leaving a hollow filament through which poisons are injected to immobilise the prey, then the tentacles manoeuvre the prey to the mouth.]]
The polyp's tentacles trap prey using stinging cells called [[nematocysts]]. These are cells modified to capture and immobilize prey, such as plankton, by injecting poisons, firing very rapidly in response to contact. These poisons are usually weak but in [[fire coral]]s they are potent enough to harm humans. Nematocysts can also be found in [[jellyfish]] and [[sea anemone]]s. The toxins injected by nematocysts immobilize or kill prey, which can then be drawn into the polyp's stomach by the tentacles through a contractile band of epithelium called the [[pharynx]].
The polyps are interconnected by a complex and well developed system of [[gastrovascular]] canals allowing significant sharing of nutrients and symbiotes. In soft corals these range in size from 50-500 μm in diameter and to allow transport of both metabolites and cellular components.<ref name=Gateno>
{{cite journal
| author=D. Gateno, A. Israel, Y. Barki and B. Rinkevich
| title=Gastrovascular Circulation in an Octocoral: Evidence of Significant Transport of Coral and Symbiont Cells
| journal=The Biological Bulletin
| year=1998
| pages=178–186
| volume=194
| issue=2
| url=http://www.biolbull.org/cgi/reprint/194/2/178
| doi = 10.2307/1543048 <!--Retrieved from CrossRef by DOI bot-->
}}</ref>
[[Image:Montastrea cavernosa.jpg|thumb|left|Close-up of ''Montastrea cavernosa'' polyps. Tentacles are clearly visible.]]
Aside from feeding on plankton, many corals as well as other [[cnidarian]] groups such as [[sea anemones]] (e.g. [[Aiptasia]]), form a [[symbiotic]] relationship with a class of [[algae]], [[zooxanthellae]], of the genus ''[[Symbiodinium]]''. The sea anemone [[Aiptasia]], while considered a pest among coral reef aquarium hobbyists, has served as a valuable model organism in the scientific study of cnidarian-algal [[symbiosis]]. Typically a polyp will harbor one particular species of algae. Via photosynthesis, these provide energy for the coral, and aid in calcification.<ref name=MilneBay>
{{cite web
| author = Madl, P. and Yip, M.
| year = 2000
| url = http://www.sbg.ac.at/ipk/avstudio/pierofun/png/png3.htm
| title = Field Excursion to Milne Bay Province - Papua New Guinea
| format =
| work =
| publisher =
| accessdate = 2006-03-31
| accessyear =
}}</ref>
The algae benefit from a safe environment, and use the carbon dioxide and nitrogenous waste produced by the polyp. Due to the strain the algae can put on the polyp, stress on the coral often triggers ejection of the algae, known on a large scale as [[coral bleaching]], as it is the algae
that contribute to the brown coloration of corals; other colors, however, are due to host coral pigments, such as GFPs ([[green fluorescent protein]]). Ejecting the algae increases the polyps' chances of surviving stressful periods - they can regain the algae at a later time. If the stressful conditions persist, the polyps, and corals, will eventually die.<ref name=Toller>
{{cite journal
| author=W. W. Toller, R. Rowan and N. Knowlton
| title=Repopulation of Zooxanthellae in the Caribbean Corals ''Montastraea annularis'' and ''M. faveolata'' following Experimental and Disease-Associated Bleaching
| journal=The Biological Bulletin
| year=2001
| pages=360–373
| volume=201
| url=http://www.biolbull.org/cgi/content/full/201/3/360
| doi = 10.2307/1543614 <!--Retrieved from CrossRef by DOI bot-->
| pmid=11751248
}}</ref>
== Reproduction ==
Corals maintain a variety of ways to propagate and settle new areas, the two main methods being by sexual and asexual means. Corals can be both [[Gonochorism|gonochoristic]] and [[Hermaphroditism|hermaphroditic]], each of which can utilize sexual and asexual means of reproduction.
=== Sexual ===
[[Image:Coral Life Cycles ZP.svg|thumb|400px|Life cycles of broadcasters and brooders.]]
Corals predominantly reproduce [[sexual reproduction|sexually]], with 25% of [[hermatypic coral]]s (stony corals) forming single sex ([[gonochoristic]]) colonies, whilst the rest are [[hermaphroditic]].<ref name=Veron>
{{cite book
| author = Veron, J.E.N.
| year = 2000
| title = Corals of the World. Vol 3
| edition = 3rd
| pages =
| publisher = Australian Institute of Marine Sciences and CRR Qld Pty Ltd.
| location = Australia
| id = 0-64232-236-8
}}</ref> About 75% of all hermatypic corals "broadcast spawn" by releasing gametes - eggs and sperm - into the water to spread colonies over large distances. The gametes fuse during fertilisation to form a microscopic larvum called a [[planula]], typically pink and elliptical in shape; a moderately sized coral colony can form several thousands of these larvae per year to overcome the huge odds against formation of a new colony.<ref name=Barnes99>
{{cite book
| author = Barnes, R. and R. Hughes
| year = 1999
| title = An Introduction to Marine Ecology
| edition = 3rd
| pages = 117-141
| publisher = Blackwell Science, Inc.
| location = Malden, MA
| id = ISBN 0-86542-834-4
}}</ref>
The planula swims towards light, exhibiting positive [[phototaxis]], to surface waters where they drift and grow for a time before swimming back down to locate a surface on which it can attach and establish a new colony. At many stages of this process there are high failure rates, and even though millions of gametes are released by each colony very few new colonies are formed. The time from spawning to settling is usually 2 or 3 days, but can be up to 2 months.<ref name=Jones>
{{cite book
| author = Jones, O.A. and R. Endean.
| year = 1973
| title = Biology and Geology of Coral Reefs
| pages = 205-245
| publisher = Harcourt Brace Jovanovich
| location = New York, USA
| id = ISBN 0-12-389602-9
}}</ref> The larva grows into a coral polyp and eventually becomes a coral head by asexual budding and growth, creating new polyps.
[[Image:Stony coral spawning 2.jpg|thumb|left|200px|A male [[star coral]], ''Montastraea cavernosa'', releases sperm into the water.]]
Corals that do not broadcast spawn are called brooders, with most non-stony corals displaying this characteristic. These corals release sperm but harbour the eggs, allowing larger, negatively buoyant, planulae to form which are later released ready to settle.<ref name=MilneBay /> The larva grows into a coral polyp and eventually becomes a coral head by asexual budding and growth to create new polyps.
[[Reproductive synchrony|Synchronous spawning]] is very typical on a coral reef and often, even when there are multiple [[species]] present, all the corals on the reef release [[gametes]] during the same night. This synchrony is essential so that male and female gametes can meet and form planula. The cues that guide the release are complex, but over the short term involve lunar changes, sunset time, and possibly chemical signalling.<ref name=Veron /> Synchronous spawning may have the result of forming coral hybrids, perhaps involved in coral [[speciation]].<ref name=Hatta>
{{cite journal
| author=Hatta, M., Fukami, H., Wang, W., Omori, M., Shimoike, K., Hayashibara, T., Ina, Y., Sugiyama, T.
| title=Reproductive and genetic evidence for a reticulate evolutionary theory of mass spawning corals
| journal=Molecular Biology and Evolution
| year=1999
| pages=1607–1613
| volume=16
| issue=11
| pmid=8096089
}}</ref> In some places the coral spawn can be dramatic, usually occurring at night, where the usually clear water becomes cloudy with gametes.
Corals must rely on environmental cues, varying from species to species, to determine the proper time to release gametes into the water. There are two methods corals use for sexual reproduction which differ in whether the female gametes are released:
* '''Broadcasters''', the majority of which mass spawn, rely heavily on environmental cues, because in contrast to brooders they release both sperm and eggs into the water. The corals use long-term cues such as day length, water temperature, and/or rate of temperature change; and the short-term cue is most often the lunar cycle, with the sunset cuing the time of release.<ref name=Veron /> About 75% of coral species are broadcasters, the majority of which are hermatypic, or reef-building corals.<ref name=Veron /> The positively buoyant gametes float towards the surface where fertilization occurs to produce [[planula]] larvae. The [[planula]] larvae swim towards the surface light to enter into currents, where they remain usually for two days, but can be up to three weeks, and in one known case two months,<ref name=Jones /> after which they settle and metamorphose into polyps and form colonies.
* '''Brooders''' are most often ahermatypic (non-reef building), or some hermatypic corals which are in areas of high current or wave action. Brooders release only sperm, which is negatively buoyant, and can store unfertilized eggs for weeks, lowering the need for mass synchronous spawning events, but can still occur.<ref name=Veron /> After fertilization the corals release [[planula]] larvae which are ready to settle.
===Asexual===
[[Image:Orbicella annularis - calices.jpg|right|thumb|left|Calices (basal plates) of ''Orbicella annularis'' showing two methods of multiplication: gemmation (small central calicle) and division (large double calicle).]]
Within a head of coral the genetically identical polyps reproduce [[asexually]] to allow growth of the colony. This is achieved either through gemmation or budding or through division, both shown in the diagrams of ''Orbicella annularis''. Budding involves a new polyp growing from an adult, whereas division forms two polyps each as large as the original.<ref name=Barnes99 />
* '''Budding''' expands the size of a coral colony. It occurs when a new corallite grows out from the adult polyp. As the new polyp grows it produces a coelenteron (stomach), tentacles and a mouth. The distance between the new and adult polyps grows, and with it the coenosarc (the common body of the colony; see [http://en.wikipedia.org/wiki/Image:Coral_polyp.jpg coral anatomy]). Budding can occur by means of:
:* '''Longitudinal division''' begins with broadening of a polyp, which then divides the coelenteron. The mouth divides and new tentacles form. The difference with this is that each polyp must complete its missing parts of the body and exoskeleton.
:* '''Intra-tentacular budding''' forms from the oral discs of a polyp, meaning that both polyps are the same size and are within the same ring of tentacles.
:* '''Extra-tentacular budding''' forms from the base of a polyp, and the new polyp is smaller.
:* '''Transversal division''' occurs when polyps and the exoskeleton divide transversally into two parts. This means that one has the basal disc (bottom) and the other has the oral disc (top). The two new polyps must again complete the missing parts.
:* '''Fission''' occurs in some corals, especially among the family [[Fungiidae]], where the colony is able to split into two or more colonies during the early stages of their development.
Whole colonies can reproduce asexually through fragmentation or bailout, forming another individual colony with the same genome.
* '''Polyp bailout''' occurs when a single polyp abandons the colony and re-establishes on a new substrate to create a new adult colony.
*'''Fragmentation''', which can actually be included as a type of fission, involves individuals broken from the colony during storms, or other situations where breaking can occur. The separated individuals can start new coral colonies.
== Reefs ==
[[Image:Coral reef locations.jpg|thumb|380px|Locations of coral reefs]]
{{main|Coral reef}}
The hermatypic, stony corals are often found in [[coral reef]]s, large [[calcium carbonate]] structures generally found in shallow, [[tropical]] water. Reefs are built up from coral skeletons and held together by layers of calcium carbonate produced by [[coralline algae]]. Reefs are extremely diverse marine [[ecosystems]] being host to over 4,000 species of fish, massive numbers of cnidarians, [[mollusc]]s, [[crustacean]]s, and many other animals.<ref name=Spalding>{{cite book
| author = Spalding, Mark, Corinna Ravilious, and Edmund Green
| year = 2001
| title = World Atlas of Coral Reefs
| pages = 205-245
| publisher = University of California Press and UNEP/WCMC
| location = Berkeley, CA, USA
| id =
}}</ref>
==Geological history==
[[Image:Fossil Coral Heliophyllum.jpg|thumb|left|200px|The fossil coral ''Heliophyllum halli'' from the [[Devonian]] period, found in [[Canada]].]]
Although corals first appeared in the [[Cambrian]] period,<ref name=Pratt>
{{cite book
| last = Pratt
| first = B.R.
| coauthors = Spincer, B.R., R.A. Wood and A.Yu. Zhuravlev
| title = Ecology of the Cambrian Radiation
| year = 2001
| url =
| accessdate = 2007-April-06
| edition =
| publisher = Columbia University Press
| isbn = 0231106130
| pages = 259
| chapter = 12: Ecology and Evolution of Cambrian Reefs
| chapterurl = http://earthscape.org/r3/ES14785/ch12.pdf
}}</ref> some {{Ma|542}}, [[fossil]]s are extremely rare until the [[Ordovician]] period, 100 million years later, when [[Rugosa|Rugose]] and [[Tabulate coral]]s became widespread.
Tabulate corals occur in the [[limestone]]s and calcareous [[shale]]s of the Ordovician and [[Silurian]] periods, and often form low cushions or branching masses alongside Rugose corals. Their numbers began to decline during the middle of the Silurian period and they finally became extinct at the end of the [[Permian]] period, 250 million years ago. The skeletons of Tabulate corals are composed of a form of calcium carbonate known as [[calcite]].
Rugose corals became dominant by the middle of the Silurian period, and became extinct early in the [[Triassic]] period. The Rugose corals existed in solitary and colonial forms, and like the Tabulate corals their skeletons are also composed of calcite.
The [[Scleractinia]]n corals filled the niche vacated by the extinct Rugose and Tabulate corals. Their fossils may be found in small numbers in rocks from the Triassic period, and become relatively common in rocks from the [[Jurassic]] and later periods. The skeletons of Scleractinian corals are composed of a form of calcium carbonate known as [[aragonite]].<ref name=Ries>
{{cite journal
| author = Ries, J.B., Stanley, S.M., Hardie, L.A.
| year = 2006
| month = July
| title = Scleractinian corals produce calcite, and grow more slowly, in artificial Cretaceous seawater
| journal = Geology
| volume = 34
| issue =
| pages = 525–528
| id = 10.1130/G22600.1
| url =
| doi = 10.1130/G22600.1
}}</ref> Although they are geologically younger than the Tabulate and Rugose corals, their aragonitic skeleton is less readily preserved, and their fossil record is less complete.
{{Coral fossil record timeline}}
At certain times in the geological past corals were very abundant, just as modern corals are in the warm clear tropical waters of certain parts of the world today. Like modern corals their ancestors built reefs, some of which now lie as great structures in [[sedimentary rocks]].
These ancient reefs are not composed entirely of corals. Algae, sponges, and the remains of many [[Echinoderm|echinoids]], [[brachiopod]]s, [[bivalve]]s, [[gastropod]]s, and [[trilobite]]s that lived on the reefs are preserved within them. This makes some corals useful [[index fossil]]s, enabling geologists to date the age the rocks in which they are found.
Corals are not restricted to reefs, and many solitary corals may be found in rocks where reefs are not present, such as ''Cyclocyathus'' which occurs in [[England]]'s [[Gault clay]] formation.
== Environmental effects ==
[[Image:Reef0484.jpg|thumb|left|A healthy coral reef has a striking level of biodiversity in many forms of marine life.]]
Corals are highly sensitive to [[natural environment|environmental]] changes. Scientists have predicted that over 50% of the coral reefs in the world may be destroyed by the year 2030;<ref name=Norlander>
{{cite magazine
| author= Norlander
| title= Coral crisis! Humans are killing off these bustling underwater cities. Can coral reefs be saved? (Life science: corals)
| journal=Science World
| date=[[8 December]] [[2003]]
| url=http://www.highbeam.com/doc/1G1-112022348.html
}}</ref> as a result they are generally protected through environmental laws. A coral reef can easily be swamped in [[algae]] if there are too many [[nutrients]] in the water. Coral will also die if the water temperature changes by more than a degree or two beyond its normal range or if the [[salinity]] of the water drops. In an early symptom of environmental stress, corals expel their [[zooxanthellae]]; without their symbiotic unicellular algae, coral tissues become colorless as they reveal the white of their calcium carbonate skeletons, an event known as [[coral bleaching]].<ref name=Hoegh>
{{cite journal
| author=[[Ove Hoegh-Guldberg (biologist)|Hoegh-Guldberg, O.]]
| title=Climate change, coral bleaching and the future of the world's coral reefs
| journal=Marine and Freshwater Research
| year=1999
| pages=839–866
| volume=50
| issue=8
| url=http://scholar.google.com/url?sa=U&q=http://www.geo.cornell.edu/ocean/EAS350/Readings/Hoegh-Guldberg.pdf
| doi=10.1071/MF99078
}}</ref>
Many governments now prohibit removal of coral from reefs to reduce damage by [[Scuba diving|divers]]. However, damage is still caused by anchors dropped by dive boats or fishermen. In places where local fishing causes reef damage, education schemes have been run to inform the population about reef protection and ecology.
The narrow [[ecological niche|niche]] that coral occupies, and the [[scleractinia|stony corals]]' reliance on [[calcium carbonate]] deposition, means they are very susceptible to changes in water [[pH]]. [[Ocean acidification]], caused by dissolution of carbon dioxide in the water that lowers pH, is currently occurring in the surface waters of the world's oceans due to increasing atmospheric carbon dioxide. Lowered pH reduces the ability of corals to produce calcium carbonate skeletons, and at the extreme, results in the dissolution of those skeletons entirely. Without deep and early cuts in anthropogenic CO<sub>2</sub>, scientists fear that ocean acidification may inevitably result in the severe degradation or destruction of coral species and ecosystems.<ref name=Gattuso>{{cite journal
| author = Gattuso, J.P., Frankignoulle, M., Bourge, I., Romaine, S. and Buddemeier, R.W.
| title = Effect of calcium carbonate saturation of seawater on coral calcification
| journal = Global Planet Change
| year = 1998
| pages = 37–46
| volume = 18
| issue =
| url = http://www.obs-vlfr.fr/~gattuso/jpg_papers_list.php
| doi = 10.1016/S0921-8181(98)00035-6 <!--Retrieved from CrossRef by DOI bot-->
}}</ref>
[[Image:Coral stained hg.jpg|thumb|A section through a coral, dyed to determine growth rate]]
A combination of temperature changes, pollution, and overuse by divers and jewelry producers has led to the destruction of many coral reefs around the world. This has increased the importance of [[coral biology]] as a discipline. Climatic variations can cause temperature changes that destroy corals. For example, during the 1997-98 warming event all the [[hydrozoa]]n ''[[Millepora boschmai]]'' colonies near [[Panamá]] were bleached and died within six years - this species is now thought to be extinct.<ref name=Glynn>
{{cite journal
| author = Glynn, P.W.
| year = 2001
| month =
| title = History of significant coral bleaching events and insights regarding amelioration
| journal = Coral Bleaching and Marine Protected Areas: Proceedings of the Workshop on Mitigating Coral Bleaching Impact Through MPA Design. Bishop Museum, Honolulu, Hawaii, 29-31 May 2001
| volume =
| issue =
| pages = 36–39
| id =
| url = http://www.reefresilience.org/r2coral/coral_pdf/Glynn.pdf
}}</ref>
== Uses ==
=== Live corals ===
Local economies near major coral reefs benefit from an abundance of fish and octopus as a food source. Reefs also provide recreational [[scuba diving]] and [[snorkeling]] tourism. Unfortunately all these activities can also have deleterious effects, such as removal or accidental destruction of coral. Besides the recreational use, coral is also useful as a protection against hurricanes and other extreme weather.
===Coral as a gemstone===
See: [[Precious coral]].
Red shades of coral are sometimes used as a [[Precious coral|gemstone]], especially in [[Tibet]]. In vedic astrology, [[red coral]] represents Mars.
Intensely red coral is sometimes known as fire coral (but this is not at all the same thing as [[fire coral]]). This extremely red coral is very rare now because of overharvesting due to the great demand for perfect red coral in jewelry-making.
[[Image:Coral reefs papua.JPG|thumb|Live corals in [[Papua New Guinea]].]]
=== Ancient corals ===
Ancient coral reefs on land are often mined for lime or use as building blocks ("[[coral rag]]"). Coral rag is an important local building material in places such as the East African coast.
Some coral species exhibit banding in their skeletons resulting from [[Year|annual]] variations in their growth rate. In [[fossil]] and modern corals these bands allow [[geologist]]s to construct year-by-year chronologies, a form of [[incremental dating]], which can provide high-resolution records of past [[paleoclimatology|climatic]] and [[paleoenvironmental|environmental]] changes when combined with [[geochemistry|geochemical]] analysis of each band.<ref name=Schrag>
{{cite journal
| author=Schrag, D.P. and Linsley, B.K.
| title=Corals, Chemistry, and Climate
| journal=Science
| year=2002
| pages=277–278
| volume=296
| issue=8
| pmid=11951026
| doi= 10.1126/science.1071561
}}</ref>
Certain species of corals form communities called [[microatoll]]s. The vertical growth of microatolls is limited by average tidal height. By analyzing the various growth morphologies, microatolls can be used as a low resolution record of patterns of sea level change. Fossilized microatolls can also be dated using radioactive carbon dating to obtain a chronology of patterns of sea level change. Such methods have been used to used to reconstruct [[Holocene]] sea levels.<ref name=Smithers>
{{cite journal
| author = Smithers, S.G. and Woodroffe, C.D.
| year = 2000
| month = August
| title = Microatolls as sea-level indicators on a mid-ocean atoll.
| journal = Marine Geology
| volume = 168
| issue = 1-4
| pages = 61–78
| id =
| url = http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V6M-40WDSPX-4&_user=10&_coverDate=08%2F15%2F2000&_rdoc=1&_fmt=summary&_orig=browse&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=844934e86d603e4aa8f0c42faa6b42ef
| doi = 10.1016/S0025-3227(00)00043-8 <!--Retrieved from CrossRef by DOI bot-->
}}</ref>
== Gallery ==
{{cleanup-gallery}}
<gallery>
Image:Mushroom Coral (Fungia) Top Macro 91.JPG|Mushroom Coral skeleton
Image:Brain_coral.jpg|[[Brain coral]], ''Diploria labyrinthiformis''
Image:Eusmilia fastigiata large.jpg|Polyps of ''Eusmilia fastigiata''
Image:Staghorn coral closeup.jpg|Staghorn coral, ''[[Acropora]]''
Image:Orange cup coral (Balanophyllia elegans) 01.jpg|Orange cup coral, ''Balanophyllia elegans''
Image:Brain coral spawning.jpg|Brain coral spawning
Image:Stony coral spawning 3.jpg|Brain coral releasing eggs
Image:EilatFringingReef.jpg|Fringing [[coral reef]] off the coast of [[Eilat]], [[Israel]].
Image:Growing coral.JPG|Growing coral
</gallery>
== References ==
{{reflist|2}}
== External links ==
{{wikispecies|Anthozoa|Coral}}
{{Commonscat_show2|Coral|Anthozoa}}
* [[University of Southern Mississippi]] - [http://www.usm.edu/marineeducation/old/coralreef/index.html Coral Reef Resource Guide]
* [[NOAA]] Ocean Service Education - [http://www.oceanservice.noaa.gov/education/kits/corals/welcome.html Corals]
* [[NOAA]] CoRIS - [http://www.coris.noaa.gov/about/ Coral Reef Biology]
[[Category:Anthozoa]]
[[Category:Incremental dating]]
[[Category:Coral reefs]]
{{Link FA|he}}
[[bg:Корали]]
[[ca:Corall]]
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[[es:Coral (animal)]]
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[[fr:Corail]]
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[[nl:Koraal (zoölogie)]]
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[[ru:Коралл]]
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