View source for Coral

{{Short description|Marine invertebrates of the subphylum Anthozoa}}
{{Other uses}}
{{distinguish|Corral}}
{{Paraphyletic group
| auto = yes
| name = Corals
| fossil_range = {{Geological range|535|0}}
| image = Coral Outcrop Flynn Reef.jpg
| image_caption = A coral outcrop on the [[Great Barrier Reef]], Australia
| taxon = Anthozoa
| authority = [[Christian Gottfried Ehrenberg|Ehrenberg]], 1834
| subdivision_ranks = Subdivisions
| subdivision = * [[Octocorallia]]
** [[Malacalcyonacea]]
** [[Scleralcyonacea]]
* [[Hexacorallia]]
** [[Scleractinia]]
** [[Antipatharia]]
** {{Extinct}}[[Rugosa]]
** {{Extinct}}[[Tabulata]]
}}

'''Corals''' are colonial [[marine invertebrates]] within the [[subphylum]] [[Anthozoa]] of the [[phylum]] [[Cnidaria]]. They typically form compact [[Colony (biology)|colonies]] of many identical individual [[polyp (zoology)|polyp]]s. Coral species include the important [[Coral reef|reef]] builders that inhabit tropical oceans and secrete [[calcium carbonate]] to form a hard skeleton.

A coral "group" is a colony of very many [[cloning|genetically identical]] polyps. Each polyp is a sac-like animal typically only a few millimeters in diameter and a few centimeters in height. A set of [[tentacle]]s surround a central mouth opening. Each polyp excretes an [[exoskeleton]] near the base. Over many generations, the colony thus creates a skeleton characteristic of the species which can measure up to several meters in size. Individual colonies grow by [[asexual reproduction]] of polyps. Corals also breed sexually by [[spawning]]: polyps of the same species release [[gamete]]s simultaneously overnight, often around a [[full moon]]. Fertilized eggs form planulae, a mobile early form of the coral polyp which, when mature, settles to form a new colony.

Although some corals are able to catch [[plankton]] and small [[fish]] using [[Cnidocyte|stinging cells]] on their tentacles, most corals obtain the majority of their energy and nutrients from [[photosynthetic]] [[unicellular]] [[dinoflagellate]]s of the genus ''[[Symbiodinium]]'' that live within their tissues. These are commonly known as [[zooxanthellae]] and give the coral color. Such corals require sunlight and grow in clear, shallow water, typically at depths less than {{convert|60|m|ft fathom|abbr=off}}, but corals in the genus ''[[Leptoseris]]'' have been found as deep as {{convert|172|m|ft fathom|abbr=off}}.<ref>{{Cite journal|title=Symbiotic associations of the deepest recorded photosynthetic scleractinian coral (172 m depth)|first1=Héloïse|last1=Rouzé|first2=Pierre E.|last2=Galand|first3=Mónica|last3=Medina|first4=Pim|last4=Bongaerts|first5=Michel|last5=Pichon|first6=Gonzalo|last6=Pérez-Rosales|first7=Gergely|last7=Torda|first8=Aurelie|last8=Moya|first9=Jean-Baptiste|last9=Raina|first10=Laetitia|last10=Hédouin|date=May 9, 2021|journal=The ISME Journal|volume=15|issue=5|pages=1564–1568|doi=10.1038/s41396-020-00857-y|doi-access=free|pmid=33452473 |pmc=8115523|bibcode=2021ISMEJ..15.1564R }}</ref> Corals are major contributors to the physical structure of the [[coral reef]]s that develop in tropical and subtropical waters, such as the [[Great Barrier Reef]] off the coast of [[Australia]]. These corals are increasingly at risk of [[coral bleaching|bleaching]] events where polyps expel the zooxanthellae in response to stress such as high water temperature or toxins.

Other corals do not rely on zooxanthellae and can live globally in much deeper water, such as the cold-water [[genus]] ''[[Lophelia]]'' which can survive as deep as {{convert|3300|m|ft fathom|abbr=off}}.<ref name=Squires>{{cite journal | author=Squires, D.F. | year=1959 | title=Deep sea corals collected by the Lamont Geological Observatory. 1. Atlantic corals | url = http://digitallibrary.amnh.org/bitstream/handle/2246/2502/N1965.pdf?sequence=1 | journal = American Museum Novitates | issue=1965 | page=23}}</ref> Some have been found as far north as the [[Darwin Mounds]], northwest of [[Cape Wrath]], Scotland, and others off the coast of [[Washington state]] and the [[Aleutian Islands]].

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==Taxonomy==
The classification of corals has been discussed for millennia, owing to having similarities to both plants and animals. [[Aristotle]]'s pupil [[Theophrastus]] described the [[red coral]], ''korallion'', in his book on stones, implying it was a mineral, but he described it as a deep-sea plant in his ''Enquiries on Plants'', where he also mentions large stony plants that reveal bright flowers when under water in the [[Red Sea|Gulf of Heroes]].<ref>{{cite book |last=Leroi |first=Armand Marie |author-link=Armand Marie Leroi |title=The Lagoon: How Aristotle Invented Science |title-link=Aristotle's Lagoon |publisher=Bloomsbury |date=2014 |isbn=978-1-4088-3622-4 |page=271}}</ref> [[Pliny the Elder]] stated boldly that several sea creatures including [[sea nettle]]s and sponges "are neither animals nor plants, but are possessed of a third nature (''tertia natura'')".<ref name=Bowen2015/> [[Petrus Gyllius]] copied Pliny, introducing the term ''zoophyta'' for this third group in his 1535 book ''On the French and Latin Names of the Fishes of the Marseilles Region''; it is popularly but wrongly supposed that Aristotle created the term.<ref name=Bowen2015/> Gyllius further noted, following Aristotle, how hard it was to define what was a plant and what was an animal.<ref name=Bowen2015>{{cite book |last=Bowen |first=James |title=The Coral Reef Era: From Discovery to Decline: A history of scientific investigation from 1600 to the Anthropocene Epoch |url=https://books.google.com/books?id=r-kXBgAAQBAJ&pg=PA5 |year=2015 |publisher=Springer |isbn=978-3-319-07479-5 |pages=5–7}}</ref> The [[Babylonian Talmud]] refers to coral among a list of types of trees, and the 11th-century French commentator [[Rashi]] describes it as "a type of tree (מין עץ) that grows underwater that goes by the (French) name 'coral'."<ref>Babylonian Talmud, Rosh Hashana 23a, and commentary of Rashi (24th narrow line)</ref>

The Persian polymath [[Al-Biruni]] (d.1048) classified sponges and corals as animals, arguing that they respond to touch.<ref name="Egerton">{{cite book |last=Egerton |first=Frank N. |title=Roots of Ecology: Antiquity to Hackel |year=2012 |publisher=University of California Press |isbn=978-0-520-95363-5 |page=24}}</ref> Nevertheless, people believed corals to be plants until the eighteenth century when [[William Herschel]] used a microscope to establish that coral had the characteristic thin cell membranes of an [[animal]].<ref>{{cite book |last1=Swett |first1=C. |title=Corals: Secrets of Their Reef-Making Colonies |date=5 March 2020 |publisher=Capstone Global Library Ltd |isbn=978-1-4747-7100-9}}</ref>

Presently, corals are classified as species of animals within the sub-classes [[Hexacorallia]] and [[Octocorallia]] of the [[class (biology)|class]] [[Anthozoa]] in the [[phylum]] [[Cnidaria]].<ref>{{cite WoRMS |author= Hoeksema, Bert |year=2015 |title=Anthozoa |id=1292 |access-date=2015-04-24}}</ref> Hexacorallia includes the stony corals and these groups have [[polyp (zoology)|polyp]]s that generally have a 6-fold symmetry. Octocorallia includes [[blue coral]] and soft corals. Species of Octocorallia have polyps with an eightfold symmetry, with each polyp having eight tentacles and eight [[Mesentery (zoology)|mesenteries]]. The group of corals is [[paraphyletic]] because the [[sea anemone]]s are also in the sub-class Hexacorallia.

== Systematics ==
{{External links|section|date=August 2023}}
The delineation of coral species is challenging as hypotheses based on morphological traits contradict hypotheses formed via molecular tree-based processes.<ref name=":1">{{Cite journal |last1=Ramírez-Portilla |first1=Catalina |last2=Baird |first2=Andrew H |last3=Cowman |first3=Peter F |last4=Quattrini |first4=Andrea M |last5=Harii |first5=Saki |last6=Sinniger |first6=Frederic |last7=Flot |first7=Jean-François |date=2022-03-01 |title=Solving the Coral Species Delimitation Conundrum |journal=Systematic Biology |volume=71 |issue=2 |pages=461–475 |doi=10.1093/sysbio/syab077 |pmid=34542634 |issn=1063-5157|doi-access=free |hdl=2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/337043 |hdl-access=free }}</ref> As of 2020, there are 2175 identified separate coral species, 237 of which are currently endangered,<ref>{{Cite journal |last1=Ritchie |first1=Hannah |author1-link=Hannah Ritchie |last2=Roser |first2=Max |author2-link=Max Roser |date=2021-04-15 |title=Biodiversity |url=https://ourworldindata.org/coral-reefs |journal=Our World in Data}}</ref> making distinguishing corals to be the utmost of importance in efforts to curb extinction.<ref name=":1" /> [[Adaptation]] and delineation continues to occur in species of coral<ref>{{Cite journal |last1=Hume |first1=Benjamin C. C. |last2=D'Angelo |first2=Cecilia |last3=Burt |first3=John A. |last4=Wiedenmann |first4=Jörg |date=2018 |title=Fine-Scale Biogeographical Boundary Delineation and Sub-population Resolution in the Symbiodinium thermophilum Coral Symbiont Group From the Persian/Arabian Gulf and Gulf of Oman |journal=Frontiers in Marine Science |volume=5 |article-number=138 |doi=10.3389/fmars.2018.00138 |bibcode=2018FrMaS...5..138H |issn=2296-7745|doi-access=free |hdl=10754/627647 |hdl-access=free }}</ref> in order to combat the dangers posed by the climate crisis. Corals are [[Colony (biology)|colonial modular organisms]] formed by [[Asexual reproduction|asexually]] produced and genetically identical modules called polyps. Polyps are connected by living tissue to produce the full organism.<ref name=":2">{{Cite journal |last1=Hemond |first1=Elizabeth M |last2=Kaluziak |first2=Stefan T |last3=Vollmer |first3=Steven V |date=2014-12-17 |title=The genetics of colony form and function in Caribbean Acropora corals |journal=BMC Genomics |volume=15 |page=1133 |doi=10.1186/1471-2164-15-1133 |issn=1471-2164 |pmc=4320547 |pmid=25519925 |doi-access=free }}</ref>&nbsp;The living tissue allows for inter module communication (interaction between each polyp),<ref name=":2" /> which appears in colony [[Morphology (biology)|morphologies]] produced by corals, and is one of the main identifying characteristics for&nbsp;a species of coral.<ref name=":2" />

There are two main classifications for corals: hard coral (scleractinian and stony coral)<ref name=":3">{{Cite web |last=US Department of Commerce |first=National Oceanic and Atmospheric Administration |title=NOAA's Coral Reef Conservation Program (CRCP) – Coral Facts |url=https://coralreef.noaa.gov/education/ |access-date=2022-04-26 |website=coralreef.noaa.gov |language=EN-US}}</ref> which form reefs by a calcium carbonate base, with polyps that bear six stiff tentacles,<ref name=":4">{{Cite web |title=Soft Corals: They Look Like Plants But Are Actually Animals |url=https://www.thoughtco.com/soft-corals-octocorals-2291391 |access-date=2022-04-26 |website=ThoughtCo |language=en}}</ref> and soft coral (Alcyonacea and ahermatypic coral)<ref name=":3" /> which are pliable and formed by a colony of polyps with eight feather-like tentacles.<ref name=":4" />&nbsp;These two classifications arose from [https://www.ncbi.nlm.nih.gov/books/NBK10061/ differentiation in gene expressions] in their branch tips<ref name=":2" /> and bases that arose through developmental signaling pathways such as [[Hox gene|Hox]], [[Hedgehog signaling pathway|Hedgehog]], [[Wnt signaling pathway|Wnt]], and BMP.

Scientists typically select ''Acropora'' as research models since they are the most diverse genus of hard coral, having over 120 species.<ref name=":2" />&nbsp;Most species within this genus have polyps which are dimorphic:<ref>{{Cite book |last1=DeVictor |first1=Susan T. |title=Guide to the Shallow-Water (0-200m) Octocorals of the South Atlantic Bight |last2=Morton |first2=Steve L. |publisher=Magnolia Press |year=2010 |isbn=978-1-86977-584-1 |location=Auckland, NZ |chapter=Octocoral Morphology |chapter-url=https://www.dnr.sc.gov/marine/sertc/octocoral%20guide/octomorph.htm}}</ref> axial polyps grow rapidly and have lighter coloration, while radial polyps are small and are darker in coloration.<ref name=":2" /><ref>{{Cite book |last=Gilbert |first=Scott F |title=Developmental Biology |publisher=Sinauer Associates |year=2000 |isbn=0-87893-243-7 |edition=6th |location=Sunderland, MA |chapter=Embryonic Development |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK10120/}}</ref> In the ''Acropora'' genus, [[Gamete|gamete synthesis]] and [[photosynthesis]] occur at the basal<ref>{{Cite journal |last1=Eric M |first1=Engstrom |last2=Izhaki |first2=Anat |last3=Bowman |first3=John L |date=July 2004 |title=Promoter Bashing, microRNAs, and Knox Genes. New Insights, Regulators, and Targets-of-Regulation in the Establishment of Lateral Organ Polarity in Arabidopsis |url=https://www.researchgate.net/figure/Four-axes-of-development-in-seed-plants-The-apical-basal-axis-1-of-the-plant_fig1_8500553 |journal=[[Plant Physiology (journal)|Plant Physiology]] |volume=135 |issue=2 |pages=685–694 |doi=10.1104/pp.104.040394 |pmid=15208415 |pmc=514105 |via=ResearchGate}}</ref> polyps, growth occurs mainly at the radial polyps. Growth at the site of the radial polyps encompasses two processes: [[asexual reproduction]] via [https://www.ncbi.nlm.nih.gov/books/NBK12640/ mitotic cell proliferation],<ref name=":2" /> and skeleton deposition of the calcium carbonate via [[extracellular matrix]] (ECM) proteins acting as differentially expressed (DE) signaling genes<ref name=":2" /> between both branch tips and bases. These processes lead to colony differentiation, which is the most accurate distinguisher between coral species.<ref name=":1" /> In the Acropora genus, colony differentiation through [[up-regulation and down-regulation]] of DEs.<ref name=":2" />

Systematic studies of soft coral species have faced challenges due to a lack of [[taxonomic]] knowledge.<ref name=":1" />&nbsp;Researchers have not found enough variability within the genus to confidently delineate similar species, due to a low rate in mutation of [[mitochondrial DNA]].<ref>{{Cite journal |last1=Stemmer |first1=Kristina |last2=Burghardt |first2=Ingo |last3=Mayer |first3=Christoph |last4=Reinicke |first4=Götz B. |last5=Wägele |first5=Heike |last6=Tollrian |first6=Ralph |last7=Leese |first7=Florian |date=June 2013 |title=Morphological and genetic analyses of xeniid soft coral diversity (Octocorallia; Alcyonacea) |url=http://link.springer.com/10.1007/s13127-012-0119-x |journal=Organisms Diversity & Evolution |language=en |volume=13 |issue=2 |pages=135–150 |doi=10.1007/s13127-012-0119-x |bibcode=2013ODivE..13..135S |s2cid=17511020 |issn=1439-6092}}</ref>

Environmental factors, such as the rise of temperatures and acid levels in our oceans account for some [[speciation]] of corals in the form of [[Species loss|species lost]].<ref name=":2" />&nbsp;Various coral species have [[heat shock protein]]s (HSP) that are also in the category of DE across species.<ref name=":2" />&nbsp;These HSPs help corals combat the increased temperatures they are facing which lead to protein denaturing, growth loss, and eventually coral death.<ref name=":2" />&nbsp;Approximately 33% of coral species are on the International Union for Conservation of Nature's endangered species list and at risk of species loss.<ref>{{Cite web |title=Coral Reefs |url=https://www.iucn.org/theme/marine-and-polar/get-involved/coral-reefs |access-date=2022-04-27 |website=IUCN |language=en |archive-url=https://web.archive.org/web/20220427041939/https://www.iucn.org/theme/marine-and-polar/get-involved/coral-reefs |archive-date=2022-04-27 }}</ref>&nbsp;[[Ocean acidification]] (falling pH levels in the oceans) is threatening the continued species growth and differentiation of corals.<ref name=":2" />&nbsp;Mutation rates of ''[[Vibrio]] shilonii'', the reef [[pathogen]] responsible for [[coral bleaching]], heavily outweigh the&nbsp;typical reproduction rates of coral colonies when pH levels fall.<ref name=":6">{{Cite journal |last1=Strauss |first1=Chloe |last2=Long |first2=Hongan |last3=Patterson |first3=Caitlyn E. |last4=Te |first4=Ronald |last5=Lynch |first5=Michael |date=2017-09-06 |editor-last=Moran |editor-first=Nancy A. |title=Genome-Wide Mutation Rate Response to pH Change in the Coral Reef Pathogen ''Vibrio shilonii'' AK1 |journal=mBio |language=en |volume=8 |issue=4 |pages=e01021–17 |doi=10.1128/mBio.01021-17 |doi-access=free |issn=2161-2129 |pmc=5565966 |pmid=28830944}}</ref> Thus, corals are unable to mutate their HSPs and other climate change preventative genes to combat the increase in temperature and decrease in pH at a competitive rate to these pathogens responsible for coral bleaching,<ref name=":6" /> resulting in species loss.

==Anatomy==
[[File:Coral polyp.jpg|thumb|Anatomy of a stony coral polyp]]
For most of their life corals are [[Sessility (motility)|sessile]] animals of [[Colony (biology)|colonies]] of genetically identical [[polyp (zoology)|polyp]]s. Each polyp varies from millimeters to centimeters in diameter, and colonies can be formed from many millions of individual polyps. Stony coral (also known as hard coral) polyps produce a skeleton composed of [[calcium carbonate]] to strengthen and protect the organism. This is deposited by the polyps and by the [[coenosarc]], the living tissue that connects them. The polyps sit in cup-shaped depressions in the skeleton known as [[corallite]]s. Colonies of stony coral are markedly variable in appearance; a single species may adopt an encrusting, plate-like, bushy, columnar or massive solid structure, the various forms often being linked to different types of habitat, with variations in light level and water movement being significant.<ref name=Ruppert>{{cite book |title=Invertebrate Zoology, 7th edition |last1=Ruppert |first1=Edward E. |last2=Fox |first2=Richard S. |last3=Barnes |first3=Robert D. |year=2004 |publisher=Cengage Learning |isbn=978-81-315-0104-7 |pages=132–48 }}</ref>

The body of the polyp may be roughly compared in a structure to a [[wikt:sac|sac]], the wall of which is composed of two layers of [[cell (biology)|cell]]s. The outer layer is known technically as the [[germ layer|ectoderm]], the inner layer as the [[germ layer|endoderm]]. Between ectoderm and endoderm is a supporting layer of gelatinous substance termed [[mesoglea]], secreted by the cell layers of the body wall.<ref name=EB1911>{{EB1911 |wstitle=Polyp |volume=22 |page=37 |first=Edward Alfred |last=Minchin |inline=1}}</ref> The mesoglea can contain [[endoskeleton|skeletal]] elements derived from cells [[cell migration|migrated]] from the ectoderm.

The sac-like body built up in this way is attached to a hard surface, which in hard corals are cup-shaped depressions in the skeleton known as [[corallite]]s. At the center of the upper end of the sac lies the only opening called the mouth, surrounded by a circle of [[tentacle]]s which resemble glove fingers. The tentacles are [[organ (anatomy)|organ]]s which serve both for tactile sense and for the capture of food.<ref name=EB1911/> Polyps extend their tentacles, particularly at night, often containing coiled stinging cells ([[cnidocyte]]s) which pierce, poison and firmly hold living prey paralyzing or killing them. Polyp prey includes plankton such as [[copepods]] and fish larvae. Longitudinal muscular fibers formed from the cells of the ectoderm allow tentacles to contract to convey the food to the mouth. Similarly, circularly disposed muscular fibers formed from the endoderm permit tentacles to be protracted or thrust out once they are contracted.<ref name=EB1911/> In both stony and soft corals, the polyps can be retracted by contracting muscle fibers, with stony corals relying on their hard skeleton and cnidocytes for defense. Soft corals generally secrete [[terpenoid]] toxins to ward off predators.<ref name="Ruppert" />

In most corals, the tentacles are retracted by day and spread out at night to catch plankton and other small organisms. Shallow-water species of both stony and soft corals can be [[zooxanthella]]te, the corals supplementing their plankton diet with the products of photosynthesis produced by these [[symbionts]].<ref name=Ruppert/> The polyps interconnect by a complex and well-developed system of [[gastrovascular]] canals, allowing significant sharing of nutrients and symbionts.<ref name=Gateno>{{cite journal |author1=D. Gateno |author2=A. Israel |author3=Y. Barki |author4=B. Rinkevich | title=Gastrovascular Circulation in an Octocoral: Evidence of Significant Transport of Coral and Symbiont Cells | journal=The Biological Bulletin | year=1998 | pages=178–86 | volume=194 | issue=2 | url=http://www.biolbull.org/cgi/reprint/194/2/178 | doi=10.2307/1543048 |pmid=28570841 | jstor=1543048 |s2cid=19530967 }}</ref>

The external form of the polyp varies greatly. The column may be long and slender, or may be so short in the axial direction that the body becomes disk-like. The tentacles may number many hundreds or may be very few, in rare cases only one or two. They may be simple and unbranched, or feathery in pattern. The mouth may be level with the surface of the peristome, or may be projecting and trumpet-shaped.<ref name=EB1911/>

===Soft corals===
{{see also|Octocorallia}} 
Soft corals have no solid exoskeleton as such. However, their tissues are often reinforced by small supportive elements known as [[sclerites]] made of calcium carbonate. The polyps of soft corals have eight-fold symmetry, which is reflected in the ''Octo'' in Octocorallia.<ref>{{cite journal|doi=10.1017/jpa.2017.49|title=A problematic cnidarian (Cambroctoconus; Octocorallia?) from the Cambrian (Series 2–3) of Laurentia |year=2017 |last1=Peel |first1=John S. |journal=Journal of Paleontology |volume=91 |issue=5 |pages=871–882 |bibcode=2017JPal...91..871P |s2cid=134826884 |doi-access=free }}</ref>

Soft corals vary considerably in form, and most are colonial. A few soft corals are [[stolon]]ate, but the polyps of most are connected by sheets of tissue called coenosarc, and in some species these sheets are thick and the polyps deeply embedded in them. Some soft corals encrust other sea objects or form lobes. Others are tree-like or whip-like and have a central axial skeleton embedded at their base in the matrix of the supporting branch.<ref>{{cite web|url=https://coralreef.noaa.gov/aboutcorals/coral101/anatomy/|title=existing and potential value of coral ecosystems with respect to income and other economic values|last=Administration|first=US Department of Commerce, National Oceanic and Atmospheric|website=coralreef.noaa.gov|language=EN-US|access-date=2018-02-04|archive-date=2018-02-05|archive-url=https://web.archive.org/web/20180205184335/https://coralreef.noaa.gov/aboutcorals/coral101/anatomy/}}</ref> These branches are composed of a fibrous protein called [[gorgonin]] or of a calcified material.

===Stony corals===
{{main|Scleractinia}}{{see also|Hexacorallia}} 
[[File:Montastrea cavernosa.jpg|thumb|''[[Montastraea cavernosa]]'' polyps with tentacles extended]]
The polyps of stony corals have six-fold symmetry. In stony corals, the tentacles are cylindrical and taper to a point, but in soft corals they are pinnate with side branches known as pinnules. In some tropical species, these are reduced to mere stubs and in some, they are fused to give a paddle-like appearance.<ref name=Sprung>{{cite book |title=Corals: A quick reference guide |last=Sprung |first=Julian |year=1999 |publisher=Ricordea Publishing |isbn=978-1-883693-09-1 |page=145 }}</ref>

Coral skeletons are biocomposites (mineral + organics) of calcium carbonate, in the form of calcite or aragonite. In scleractinian corals, "centers of calcification" and fibers are clearly distinct structures differing with respect to both morphology and chemical compositions of the crystalline units.<ref>{{Cite journal|last1=Cuif|first1=J.P.|last2=Dauphin|first2=Y.|date=1998|title=Microstructural and physico-chemical characterization of 'centers of calcification' in septa of some Recent scleractinian corals|journal=Paläontologische Zeitschrift|volume=72|issue=3–4|pages=257–269|doi=10.1007/bf02988357|bibcode=1998PalZ...72..257C |s2cid=129021387|issn=0031-0220}}</ref><ref>{{Cite journal|last1=Cuif|first1=J.P.|last2=Dauphin|first2=Y.|last3=Doucet|first3=J.|last4=Salomé|first4=M.|last5=Susini|first5=J.|date=2003|title=XANES mapping of organic sulfate in three scleractinian coral skeletons|journal=Geochimica et Cosmochimica Acta|volume=67|issue=1|pages=75–83|doi=10.1016/s0016-7037(02)01041-4|issn=0016-7037|bibcode=2003GeCoA..67...75C}}</ref> The organic matrices extracted from diverse species are acidic, and comprise proteins, sulphated sugars and lipids; they are species specific.<ref>{{Cite journal|last1=Dauphin|first1=Y.|last2=Cuif|first2=J.P.|last3=Williams|first3=C. T.|date=2008|title=Soluble organic matrices of aragonitic skeletons of Merulinidae (Cnidaria, Anthozoa)|journal=Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology|volume=150|issue=1|pages=10–22|doi=10.1016/j.cbpb.2008.01.002|pmid=18325807|issn=1096-4959}}</ref> The soluble organic matrices of the skeletons allow to differentiate [[zooxanthellae]] and non-zooxanthellae specimens.<ref>{{Cite journal|last1=Cuif|first1=J.P.|last2=Dauphin|first2=Y.|last3=Freiwald|first3=A.|last4=Gautret|first4=P.|last5=Zibrowius|first5=H.|date=1999|title=Biochemical markers of zooxanthellae symbiosis in soluble matrices of skeleton of 24 Scleractinia species|journal=Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology|volume=123|issue=3|pages=269–278|doi=10.1016/s1095-6433(99)00059-8|issn=1095-6433}}</ref>

[[File:The general organization of the scleractinian coral soft tissue and underlying skeleton.png|thumb|The general organization of the scleractinian coral soft tissue and underlying skeleton]]
Specifically, calcification in corals takes place within specialized vesicles inside cells or at the interface between the calicoblastic ectoderm and the growing skeleton. This process is precisely regulated to control the formation and orientation of calcium carbonate crystals. The Skeletal Organic Matrix (SOM), primarily composed of proteins secreted by the calicoblastic ectoderm, plays a central role in this regulation. Among these proteins, CARPs (coral acid-rich proteins) and SAPs (skeletal aspartic acid-rich proteins) facilitate the transport of calcium ions ({{chem2|Ca(2+)}}) toward the calcifying space. Importantly, carbonate precipitation in the calicoblastic space is not solely governed by the aragonite saturation state (<math>\Omega_{arag}</math>); rather, it depends on the orchestrated activity of SOM components. The distinct spatial distribution of these proteins—observed both on coral skeletons and in coral cell cultures—underscores their specific and diverse roles in directing crystal nucleation, growth, and overall skeletal architecture.<ref name="How corals made rocks through the a">{{cite journal
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|title=How corals made rocks through the ages
|journal=Global Change Biology
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}}</ref>

[[File:Coral skeleton formation.jpg|thumb|Coral skeleton formation]]
The enzyme '''α-carbonic anhydrase (CA)''' plays a vital role in regulating inorganic carbon balance within coral cells. It catalyzes two reversible reactions: the '''hydration''' of carbon dioxide ({{chem2|CO2}}) into bicarbonate ({{chem2|HCO3-}}) and a proton ({{chem2|H+}}), and the '''dehydration''' of bicarbonate back into {{chem2|CO2}} and a proton. Through these interconversions, α-carbonic anhydrase facilitates the efficient transport and supply of dissolved inorganic carbon for both photosynthesis and calcification, maintaining the delicate equilibrium between {{chem2|CO2}} and bicarbonate required for coral metabolic and skeletal processes.<ref name="How corals made rocks through the a"/>

Ocean acidification poses a major threat to coral calcification by reducing the availability of carbonate ions ({{chem2|CO3(2-)}}), which are essential for forming calcium carbonate ({{chem2|CaCO3}}) skeletons. Under increasingly acidified conditions, corals must expend more energy to pump protons ({{chem2|H+}}) out of the calcifying space in order to maintain favorable conditions for mineral deposition. This heightened energetic demand compromises coral growth and skeletal density, resulting in weaker and more brittle structures. Consequently, coral reefs become more susceptible to physical damage from waves and storms, as well as less resilient to other environmental stressors.

==Ecology==
[[File:nematocyst discharge.png|thumb|Discharge mechanism of a stinging cell (nematocyst)]]

===Feeding===
Polyps feed on a variety of small organisms, from microscopic zooplankton to small fish. The polyp's tentacles immobilize or kill prey using stinging cells called [[cnidocytes]], commonly called nematocysts. These cells carry [[venom]] which they rapidly release in response to contact with another organism. A dormant nematocyst discharges in response to nearby prey touching the trigger. A stiff flap called an [[Operculum (animal)|operculum]] opens and its stinging apparatus fires the barb into the prey. The venom is injected through the hollow filament to immobilise the prey; the tentacles then manoeuvre the prey into the stomach. Once the prey is digested the stomach reopens allowing the elimination of waste products and the beginning of the next hunting cycle.<ref name=murph/>{{rp|24}}

===Intracellular symbionts===
Many corals, as well as other [[cnidaria]]n groups such as [[sea anemone]]s form a [[symbiotic]] relationship with a class of [[dinoflagellate]] [[algae]], [[zooxanthellae]] of the genus ''[[Symbiodinium]]'', which can form as much as 30% of the tissue of a polyp.<ref name=murph>{{cite book |title=Coral Reefs: Cities Under The Seas |last=Murphy |first=Richard C. |year=2002 |isbn=978-0-87850-138-0 |publisher=The Darwin Press}}</ref>{{rp|23–24}} Typically, each polyp harbors one species of alga, and coral species show a preference for ''[[Symbiodinium]]''.<ref>{{cite journal|last1=Yuyama|first1=Ikuko|title=Comparing the Effects of Symbiotic Algae (Symbiodinium) Clades C1 and D on Early Growth Stages of Acropora tenuis|journal=PLOS ONE|date=2014|volume=9|issue=6|article-number=e98999|doi=10.1371/journal.pone.0098999|pmid=24914677|pmc=4051649|bibcode=2014PLoSO...998999Y|doi-access=free}}</ref> Young corals are not born with zooxanthellae, but acquire the algae from the surrounding environment, including the water column and local sediment.<ref>{{cite journal|last1=Yamashita|first1=Hiroshi|title=Establishment of Coral–Algal Symbiosis Requires Attraction and Selection|journal=PLOS ONE|date=2014|volume=9|issue=5|article-number=e97003|doi=10.1371/journal.pone.0097003|pmid=24824794|pmc=4019531|bibcode=2014PLoSO...997003Y|doi-access=free}}</ref> The main benefit of the zooxanthellae is their ability to photosynthesize which supplies corals with the products of photosynthesis, including glucose, glycerol, also amino acids, which the corals can use for energy.<ref>{{cite web|title=Zooxanthellae...What's That?|url=https://oceanservice.noaa.gov/education/kits/corals/coral02_zooxanthellae.html|website=NOAA Ocean Service Education|publisher=National Oceanic and Atmospheric Administration|access-date=1 December 2017|archive-date=13 April 2020|archive-url=https://web.archive.org/web/20200413062742/https://oceanservice.noaa.gov/education/kits/corals/coral02_zooxanthellae.html}}</ref> Zooxanthellae also benefit corals by aiding in [[calcification]], for the coral skeleton, and waste removal.<ref name=MilneBay>{{cite web |author1=Madl, P. |author2=Yip, M. |year=2000 |url=http://biophysics.sbg.ac.at/png/png3.htm |title=Field Excursion to Milne Bay Province – Papua New Guinea |access-date=2006-03-31 |archive-date=2020-05-11 |archive-url=https://web.archive.org/web/20200511062206/http://biophysics.sbg.ac.at/png/png3.htm }}</ref><ref>{{cite book|last1=van de Plaasche|first1=Orson|title=Sea-level research: a manual for the collection and evaluation of data|date=1986|publisher=Geo Books|location=Norwich, UK|isbn=978-94-010-8370-6|page=196}}</ref> In addition to the soft tissue, [[microbiome]]s are also found in the coral's mucus and (in stony corals) the skeleton, with the latter showing the greatest microbial richness.<ref>{{Cite web|url=https://www.psu.edu/news/research/story/corals-and-their-microbiomes-evolved-together/|title=Corals and their microbiomes evolved together &#124; Penn State University|website=psu.edu}}</ref>

The zooxanthellae benefit from a safe place to live and consume the polyp's [[carbon dioxide]], phosphate and nitrogenous waste. Stressed corals will eject their zooxanthellae, a process that is becoming increasingly common due to strain placed on coral by rising ocean temperatures. Mass ejections are known as [[coral bleaching]] because the algae contribute to coral coloration; some colors, however, are due to host coral pigments, such as [[green fluorescent protein]]s (GFPs). Ejection increases the polyp's chance of surviving short-term stress and if the stress subsides they can regain their algae. Sometimes, instead of regaining the same algal species, they will switch out their heat intolerant microalgal symbiont for a more heat tolerant species as a response to rising ocean temperatures. Unfortunately, heat-stress tolerant algae may not always be available or accepted by the coral host.<ref>{{cite web |first1=Monica D. | last1 = Schul | first2 = Ashley R. | last2 = Smyth | first3 = Joshua T. |last3 = Patterson | first4 = Ana N. | last4 = Zangroniz | first5 = Shelly L. | last5 = Krueger | first6 = Julie | last6 = Meyer |title=The Coral Holobiont: A Brief Overview of Corals and Their Microbiome |url=https://edis.ifas.ufl.edu/publication/SS733 |website=Askifas: powered by EDIS |publisher=University of Florida |access-date=26 October 2025}}</ref> If the stressful conditions persist, the polyp eventually dies.<ref name=Toller>{{cite journal | author1=W. W. Toller | author2=R. Rowan | author3=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=360–73 | volume=201 | url=http://www.biolbull.org/cgi/content/full/201/3/360 | doi=10.2307/1543614 | pmid=11751248 | issue=3 | jstor=1543614 | s2cid=7765487 | access-date=2006-03-30 | archive-url=https://web.archive.org/web/20060225043356/http://www.biolbull.org/cgi/content/full/201/3/360 | archive-date=2006-02-25 }}</ref> Zooxanthellae are located within the coral cytoplasm and due to the algae's photosynthetic activity the internal pH of the coral can be raised; this behavior indicates that the zooxanthellae are responsible to some extent for the metabolism of their host corals.<ref>{{cite journal|last1=Brownlee|first1=Colin|title=pH regulation in symbiotic anemones and corals: A delicate balancing act|journal=Proceedings of the National Academy of Sciences of the United States of America|date=2009|volume=106|issue=39|pages=16541–16542|doi=10.1073/pnas.0909140106|pmid=19805333|pmc=2757837|bibcode=2009PNAS..10616541B|doi-access=free}}</ref> Stony Coral Tissue Loss Disease has been associated with the breakdown of host-zooxanthellae physiology.<ref>Landsberg et al., "Stony Coral Tissue Loss Disease in Florida Is Associated With Disruption of Host–Zooxanthellae Physiology".</ref> Moreover, Vibrio bacterium are known to have virulence traits used for host coral tissue damage and photoinhibition of algal symbionts.<ref>de O Santos et al., "Genomic and Proteomic Analyses of the Coral Pathogen Vibrio Coralliilyticus Reveal a Diverse Virulence Repertoire".</ref> Therefore, both coral and their symbiotic microorganisms could have evolved to harbour traits resistant to disease and transmission.

==Reproduction==
Corals can be both [[gonochoristic]] (unisexual) and [[hermaphroditic]], each of which can reproduce sexually and asexually. Reproduction also allows coral to settle in new areas. Reproduction is coordinated by chemical communication.{{Clarify span||date=September 2021}}

===Sexual===
[[File:Coral Life Cycles ZP.svg|thumb|upright=1.75|Life cycles of broadcasters and brooders]]
Corals predominantly reproduce [[sexual reproduction|sexually]]. About 25% of [[hermatypic coral]]s (reef-building stony corals) form single-sex ([[gonochoristic]]) colonies, while the rest are [[hermaphroditic]].<ref>{{Cite web |date=2014-09-02 |title=Biology of coral,beauty of sea. |url=https://www.slideshare.net/slideshow/biology-of-coralbeauty-of-sea/38590013 |access-date=2025-06-09 |website=SlideShare |language=en}}</ref> It is estimated more than 67% of coral are [[Simultaneous hermaphroditism|simultaneous hermaphrodites]].<ref>{{Cite book|last=Avise|first=John C.|url=https://books.google.com/books?id=jqiR8C0lEckC&q=most+coral+are+hermaphrodites&pg=PA83|title=Hermaphroditism: A Primer on the Biology, Ecology, and Evolution of Dual Sexuality|date=2011-03-18|publisher=Columbia University Press|isbn=978-0-231-52715-6|page=83|language=en}}</ref>

====Broadcasters====
{{external media | width = 210px | float = right | headerimage= [[File:Brain coral spawning.jpg |210px]] | video1 = [https://www.youtube.com/watch?v=PR4bpVKVeQ4 " Out of Tune – Breakdown of Coral Spawning Synchrony"], Tom Shlesinger, Sep 5, 2019}}

About 75% of all hermatypic corals "broadcast spawn"<ref>{{Cite web |title=Coral Basics {{!}} Flower Garden Banks National Marine Sanctuary |url=https://flowergarden.noaa.gov/education/coralbasics.html |access-date=2025-06-09 |website=flowergarden.noaa.gov}}</ref> by releasing [[gamete]]s—[[egg (biology)|eggs]] and [[sperm]]—into the water where they meet and fertilize to spread offspring. Corals often synchronize their time of spawning. This [[reproductive synchrony]] is essential so that male and female gametes can meet.<ref>{{Cite journal |last=Ricardo |first=Gerard F. |last2=Doropoulos |first2=Christopher |last3=Babcock |first3=Russell C. |last4=Adam |first4=Arne A. S. |last5=Buccheri |first5=Elizabeth |last6=Robledo |first6=Natalia |last7=Uribe-Palomino |first7=Julian |last8=Mumby |first8=Peter J. |date=2025 |title=Spawning Asynchrony and Mixed Reproductive Strategies in a Common Mass Spawning Coral |url=https://onlinelibrary.wiley.com/doi/abs/10.1002/ece3.71654 |journal=Ecology and Evolution |language=en |volume=15 |issue=7 |article-number=e71654 |doi=10.1002/ece3.71654 |issn=2045-7758 |pmc=12202782 |pmid=40584676}}</ref><ref>{{Cite journal |last=Ricardo |first=Gerard |last2=Doropoulos |first2=Christopher |last3=Babcock |first3=Russell C. |last4=Buccheri |first4=Elizabeth |last5=Khalil |first5=Andrew |last6=Mumby |first6=Peter J. |date=2025-09-02 |title=Critical thresholds of adult patch density and spacing during coral fertilization |url=https://www.nature.com/articles/s41559-025-02844-y |journal=Nature Ecology & Evolution |language=en |pages=1–11 |doi=10.1038/s41559-025-02844-y |issn=2397-334X|url-access=subscription }}</ref> Spawning frequently takes place in the evening or at night, and can occur as infrequently as once a year, and within a window of 10–30 minutes.<ref name="Markandeya"/><ref name="Häfker"/>
Synchronous spawning is very typical on the coral reef, and often, all corals spawn on the same night even when multiple [[species]] are present.<ref name="Veron"/> Synchronous spawning may form hybrids and is perhaps involved in coral [[speciation]].<ref name=Hatta>{{cite journal |author1=Hatta, M. |author2=Fukami, H. |author3=Wang, W. |author4=Omori, M. |author5=Shimoike, K. |author6=Hayashibara, T. |author7=Ina, Y. |author8=Sugiyama, T. | title=Reproductive and genetic evidence for a reticulate evolutionary theory of mass spawning corals | journal=Molecular Biology and Evolution | year=1999 | pages=1607–13 | volume=16 | issue=11 | pmid=10555292 | doi=10.1093/oxfordjournals.molbev.a026073| doi-access=free }}</ref>

[[File:Stony coral spawning 2.jpg|thumb|A male [[great star coral]], ''Montastraea cavernosa'', releasing sperm into the water]]
Environmental cues that influence the release of gametes into the water vary from species to species. The cues involve temperature change, [[lunar cycle]], [[day length]], and possibly chemical signalling.<ref name="Veron">{{cite book|author=Veron, J.E.N.|title=Corals of the World. Vol 3|publisher=Australian Institute of Marine Sciences and CRR Qld|year=2000|isbn=978-0-642-32236-4|edition=3rd|location=Australia}}</ref> 
Other factors that affect the rhythmicity of organisms in marine habitats include salinity, mechanical forces, and pressure or magnetic field changes.<ref name="Häfker"/>

Mass coral spawning often occurs at night on days following a full moon.<ref name="Markandeya"/><ref name="Lin"/> A full moon is equivalent to four to six hours of continuous dim light exposure, which can cause [[light-dependent reactions]] in protein.<ref name="Markandeya"/><ref name="Häfker"/> Corals contain light-sensitive [[cryptochromes]], proteins whose light-absorbing flavin structures are sensitive to different types of light. This allows corals such as ''[[Dipsastraea speciosa]]'' to detect and respond to changes in sunlight and moonlight.<ref name="Markandeya"/><ref name="Häfker">{{cite journal |last1=Häfker |first1=N. Sören |last2=Andreatta |first2=Gabriele |last3=Manzotti |first3=Alessandro |last4=Falciatore |first4=Angela |last5=Raible |first5=Florian |last6=Tessmar-Raible |first6=Kristin |title=Rhythms and Clocks in Marine Organisms |journal=Annual Review of Marine Science |date=16 January 2023 |volume=15 |issue=1 |pages=509–538 |doi=10.1146/annurev-marine-030422-113038 |pmid=36028229 |bibcode=2023ARMS...15..509H |s2cid=251865474 |language=en |issn=1941-1405|doi-access=free }}</ref><ref name="Jabr">{{cite news |last1=Jabr |first1=Ferris |title=The Lunar Sea |url=https://hakaimagazine.com/features/lunar-sea/ |access-date=6 March 2023 |work=Hakai Magazine |date=June 13, 2017 |language=en}}</ref>

Moonlight itself may actually suppress coral spawning. The most immediate cue to cause spawning appears to be the dark portion of the night between sunset and moonrise. 
Over the lunar cycle, moonrise shifts progressively later, occurring after sunset on the day of the full moon. The resulting dark period between day-light and night-light removes the suppressive effect of moonlight and enables coral to spawn.<ref name="Markandeya"/><ref name="Lin">{{cite journal |last1=Lin |first1=Che-Hung |last2=Takahashi |first2=Shunichi |last3=Mulla |first3=Aziz J. |last4=Nozawa |first4=Yoko |title=Moonrise timing is key for synchronized spawning in coral Dipsastraea speciosa |journal=Proceedings of the National Academy of Sciences |date=24 August 2021 |volume=118 |issue=34 |article-number=e2101985118 |doi=10.1073/pnas.2101985118 |pmid=34373318 |pmc=8403928 |bibcode=2021PNAS..11801985L |language=en |issn=0027-8424 |doi-access=free }}</ref>

The spawning event can be visually dramatic, clouding the usually clear water with gametes. Once released, gametes fertilize at the water's surface and form a microscopic [[larva]] called a [[planula]], typically pink and elliptical in shape. A typical coral colony needs to release several thousand larvae per year to overcome the odds against formation of a new colony.<ref name=Barnes99>{{cite book | last1= Barnes |first1=R. and |first2=R. |last2=Hughes | year = 1999 | title = An Introduction to Marine Ecology | edition = 3rd | pages = 117–41 | publisher = Blackwell | location = Malden, MA | isbn = 978-0-86542-834-8}}</ref><ref name="Cameron">{{cite journal |last1=Cameron |first1=Kerry A. |last2=Harrison |first2=Peter L. |title=Density of coral larvae can influence settlement, post-settlement colony abundance and coral cover in larval restoration |journal=Scientific Reports |date=26 March 2020 |volume=10 |issue=1 |page=5488 |doi=10.1038/s41598-020-62366-4 |pmid=32218470 |pmc=7099096 |bibcode=2020NatSR..10.5488C |language=en |issn=2045-2322}}</ref>

Studies suggest that light pollution desynchronizes spawning in some coral species.
In areas such as the [[Red Sea]], as many as 10 out of 50 species may be showing spawning asynchrony, compared to 30 years ago. The establishment of new corals in the area has decreased and in some cases ceased. The area was previously considered a refuge for corals because mass bleaching events due to climate change had not been observed there.<ref name="Markandeya">{{cite journal |last1=Markandeya |first1=Virat |title=How lunar cycles guide the spawning of corals, worms and more |journal=Knowable Magazine |publisher= Annual Reviews |date=22 February 2023 |doi=10.1146/knowable-022223-2 |s2cid=257126558 |doi-access=free |url=https://knowablemagazine.org/article/living-world/2023/lunar-cycles-guide-spawning |access-date=6 March 2023 |language=en|url-access=subscription }}</ref><ref name="Ayalon">{{cite journal |last1=Ayalon |first1=Inbal |last2=Rosenberg |first2=Yaeli |last3=Benichou |first3=Jennifer I. C. |last4=Campos |first4=Celine Luisa D. |last5=Sayco |first5=Sherry Lyn G. |last6=Nada |first6=Michael Angelou L. |last7=Baquiran |first7=Jake Ivan P. |last8=Ligson |first8=Charlon A. |last9=Avisar |first9=Dror |last10=Conaco |first10=Cecilia |last11=Kuechly |first11=Helga U. |last12=Kyba |first12=Christopher C. M. |last13=Cabaitan |first13=Patrick C. |last14=Levy |first14=Oren |title=Coral Gametogenesis Collapse under Artificial Light Pollution |journal=Current Biology |date=25 January 2021 |volume=31 |issue=2 |pages=413–419.e3 |doi=10.1016/j.cub.2020.10.039 |pmid=33157030 |s2cid=226257589 |language=English |issn=0960-9822|doi-access=free |bibcode=2021CBio...31E.413A }}</ref> Coral restoration techniques for coral reef management are being developed to increase fertilization rates, larval development, and settlement of new corals.<ref name="Suzuki">{{cite journal |last1=Suzuki |first1=Go |last2=Okada |first2=Wataru |last3=Yasutake |first3=Yoko |last4=Yamamoto |first4=Hidekazu |last5=Tanita |first5=Iwao |last6=Yamashita |first6=Hiroshi |last7=Hayashibara |first7=Takeshi |last8=Komatsu |first8=Toshiaki |last9=Kanyama |first9=Toru |last10=Inoue |first10=Masahito |last11=Yamazaki |first11=Masashi |title=Enhancing coral larval supply and seedling production using a special bundle collection system "coral larval cradle" for large-scale coral restoration |journal=Restoration Ecology |date=September 2020 |volume=28 |issue=5 |pages=1172–1182 |doi=10.1111/rec.13178 |s2cid=218796945 |language=en |issn=1061-2971|doi-access=free |bibcode=2020ResEc..28.1172S }}</ref>

====Brooders====
Brooding species are most often ahermatypic (not reef-building) in areas of high current or wave action. Brooders release only sperm, which is negatively buoyant, sinking onto the waiting egg carriers that harbor unfertilized eggs for weeks. Synchronous spawning events sometimes occur even with these species.<ref name=Veron /> After fertilization, the corals release planula that are ready to settle.<ref name=MilneBay />

[[File:Life Cycle of Corals.svg|thumb|left|upright=1.2|Generalized life cycle of corals via sexual reproduction: Colonies release gametes in clusters (1) which float to the surface (2) then disperse and fertilize eggs (3). Embryos become planulae (4) and can settle onto a surface (5). They then metamorphose into a juvenile polyp (6) which then matures and reproduces asexually to form a colony (7, 8).]]

====Planulae====
The time from spawning to larval settlement is usually two to three days but can occur immediately or up to two months.<ref name=Jones>{{cite book |author1=Jones, O.A. |author2=Endean, R. | year = 1973 | title = Biology and Geology of Coral Reefs | pages = 205–45 | publisher = Harcourt Brace Jovanovich | location = New York, US | isbn = 978-0-12-389602-5}}</ref> Broadcast-spawned [[planula]] larvae develop at the water's surface before descending to seek a hard surface on the benthos to which they can attach and begin a new colony.<ref>{{Cite journal|last1=HARRISON|first1=P. L|last2=WALLACE|first2=C. C.|date=1990|title=Reproduction, dispersal and recruitment of scleractinian corals|url=https://pascal-francis.inist.fr/vibad/index.php?action=getRecordDetail&idt=19736342|journal=Reproduction, Dispersal and Recruitment of Scleractinian Corals|volume=25|pages=133–207|issn=0167-4579}}</ref> The larvae often need a biological cue to induce settlement such as specific crustose [[coralline algae]] species or microbial biofilms.<ref>{{Cite journal|last1=Morse|first1=Daniel E.|last2=Hooker|first2=Neal|last3=Morse|first3=Aileen N. C.|last4=Jensen|first4=Rebecca A.|date=1988-05-24|title=Control of larval metamorphosis and recruitment in sympatric agariciid corals|journal=Journal of Experimental Marine Biology and Ecology|language=en|volume=116|issue=3|pages=193–217|doi=10.1016/0022-0981(88)90027-5|bibcode=1988JEMBE.116..193M |issn=0022-0981}}</ref><ref>{{Cite journal|last1=Webster|first1=Nicole S.|last2=Smith|first2=Luke D.|last3=Heyward|first3=Andrew J.|last4=Watts|first4=Joy E. M.|last5=Webb|first5=Richard I.|last6=Blackall|first6=Linda L.|author6-link= Linda Blackall |last7=Negri|first7=Andrew P.|date=2004-02-01|title=Metamorphosis of a Scleractinian Coral in Response to Microbial Biofilms|journal=Applied and Environmental Microbiology|language=en|volume=70|issue=2|pages=1213–1221|doi=10.1128/AEM.70.2.1213-1221.2004|issn=0099-2240|pmid=14766608|pmc=348907|bibcode=2004ApEnM..70.1213W}}</ref> High failure rates afflict many stages of this process, and even though thousands of eggs are released by each colony, few new colonies form.<!--thousands of gametes or millions?--> During settlement, larvae are inhibited by physical barriers such as sediment,<ref>{{Cite journal|date=2017-12-31|title=Settlement patterns of the coral Acropora millepora on sediment-laden surfaces|journal=Science of the Total Environment|language=en|volume=609|pages=277–288|doi=10.1016/j.scitotenv.2017.07.153|issn=0048-9697|last1=Ricardo|first1=Gerard F.|last2=Jones|first2=Ross J.|last3=Nordborg|first3=Mikaela|last4=Negri|first4=Andrew P.|pmid=28750231|bibcode=2017ScTEn.609..277R|doi-access=free}}</ref> as well as chemical (allelopathic) barriers.<ref>{{Cite journal|last1=Birrell|first1=CL|last2=McCook|first2=LJ|last3=Willis|first3=BL|last4=Harrington|first4=L|date=2008-06-30|title=Chemical effects of macroalgae on larval settlement of the broadcast spawning coral Acropora millepora|url=https://www.int-res.com/abstracts/meps/v362/p129-137/|journal=Marine Ecology Progress Series|language=en|volume=362|pages=129–137|doi=10.3354/meps07524|bibcode=2008MEPS..362..129B|issn=0171-8630|doi-access=free}}</ref> The larvae metamorphose into a single polyp and eventually develops into a juvenile and then adult by asexual budding and growth.

===Asexual===
[[File:Orbicella annularis - calices.jpg|thumb|right|Basal plates (calices) of ''[[Orbicella annularis]]'' showing multiplication by budding (small central plate) and division (large double plate)]]

Within a coral head, the genetically identical polyps reproduce [[asexual reproduction|asexually]], either by [[budding]] (gemmation) or by dividing, whether longitudinally or transversely.

Budding involves splitting a smaller polyp from an adult.<ref name=Barnes99 /> As the new polyp grows, it forms [[:Image:Coral polyp.jpg|its body parts]]. The distance between the new and adult polyps grows, and with it, the coenosarc (the common body of the colony). Budding can be intratentacular, from its oral discs, producing same-sized polyps within the ring of tentacles, or extratentacular, from its base, producing a smaller polyp.

Division forms two polyps that each become as large as the original. Longitudinal division begins when a polyp broadens and then divides its coelenteron (body), effectively splitting along its length. The mouth divides and new tentacles form. The two polyps thus created then generate their missing body parts and exoskeleton. Transversal division occurs when polyps and the exoskeleton divide transversally into two parts. This means one has the basal disc (bottom) and the other has the oral disc (top); the new polyps must separately generate the missing pieces.

Asexual reproduction offers the benefits of high reproductive rate, delaying senescence, and replacement of dead modules, as well as geographical distribution.{{clarify|how does asexual reproduction have the benefit of geographical distribution?|date=August 2021}}<ref>{{cite book |title=Hawaiian Coral Reef Ecology |year=1998 |last=Gulko | first=David |publisher=Mutual Publishing |location=Honolulu, Hawaii |isbn=978-1-56647-221-0 |page=10}}</ref>

===Colony division===
Whole colonies can reproduce asexually, forming two colonies with the same genotype. The possible mechanisms include fission, bailout and fragmentation. Fission occurs in some corals, especially among the family [[Fungiidae]], where the colony splits into two or more colonies during early developmental stages. Bailout occurs when a single polyp abandons the colony and settles on a different substrate to create a new colony. Fragmentation involves individuals broken from the colony during storms or other disruptions. The separated individuals can start new colonies.<ref name="SheppardDavy2009">{{cite book |last1=Sheppard |first1=Charles R.C.|last2=Davy |first2=Simon K. |last3=Pilling |first3=Graham M. |title=The Biology of Coral Reefs |url=https://books.google.com/books?id=toIeBQAAQBAJ&pg=PT78 |date=25 June 2009|publisher=OUP Oxford|isbn=978-0-19-105734-2 |pages=78–81}}</ref>

==Coral microbiomes==
[[File:Bacterial OTUs from clone libraries and next-generation sequencing.png|thumb|upright=1.7| Phylogenetic tree representing bacterial [[operational taxonomic unit]]s (OTUs) from [[clone libraries]] and [[next-generation sequencing]]. OTUs from next-generation sequencing are displayed if the OTU contained more than two sequences in the unrarefied OTU table (3626 OTUs).<ref>{{cite journal |doi = 10.1186/s40168-017-0329-8|title = Season, but not symbiont state, drives microbiome structure in the temperate coral Astrangia poculata|year = 2017|last1 = Sharp|first1 = Koty H.|last2 = Pratte|first2 = Zoe A.|last3 = Kerwin|first3 = Allison H.|last4 = Rotjan|first4 = Randi D.|last5 = Stewart|first5 = Frank J.|journal = Microbiome|volume = 5|issue = 1|page = 120|pmid = 28915923|pmc = 5603060 | doi-access=free }}</ref>]]

{{see also|Marine microbiome}}

Corals are one of the more common examples of an animal host whose symbiosis with microalgae can turn to [[dysbiosis]], and is visibly detected as bleaching. Coral [[microbiome]]s have been examined in a variety of studies, which demonstrate how oceanic environmental variations, most notably temperature, light, and inorganic nutrients, affect the abundance and performance of the microalgal symbionts, as well as [[calcification]] and physiology of the host.<ref>Dubinsky, Z. and Jokiel, P.L. (1994) "Ratio of energy and nutrient fluxes regulates symbiosis between zooxanthellae and corals". ''Pacific Science'', '''48'''(3): 313–324.</ref><ref>Anthony, K.R., Kline, D.I., Diaz-Pulido, G., Dove, S. and Hoegh-Guldberg, O.(2008) "Ocean acidification causes bleaching and productivity loss in coral reef builders". ''Proceedings of the National Academy of Sciences'', '''105'''(45): 17442–17446. {{doi|10.1073/pnas.0804478105}}.</ref><ref name=Apprill2017>Apprill, A. (2017) "Marine animal microbiomes: toward understanding host–microbiome interactions in a changing ocean". ''Frontiers in Marine Science'', '''4''': 222. {{doi|10.3389/fmars.2017.00222|doi-access=free}}. [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>

Studies have also suggested that resident bacteria, archaea, and fungi additionally contribute to nutrient and organic matter cycling within the coral, with viruses also possibly playing a role in structuring the composition of these members, thus providing one of the first glimpses at a multi-domain marine animal symbiosis.<ref>Bourne, D.G., Morrow, K.M. and Webster, N.S. (2016) "Insights into the coral microbiome: underpinning the health and resilience of reef ecosystems". ''Annual Review of Microbiology'', '''70''': 317–340. {{doi|10.1146/annurev-micro-102215-095440}}.</ref> The [[gammaproteobacterium]] ''[[Endozoicomonas]]'' is emerging as a central member of the coral's microbiome, with flexibility in its lifestyle.<ref name="Neave2016">Neave, M.J., Apprill, A., Ferrier-Pagès, C. and Voolstra, C.R. (2016) "Diversity and function of prevalent symbiotic marine bacteria in the genus ''Endozoicomonas''". ''Applied Microbiology and Biotechnology'', '''100'''(19): 8315–8324. {{doi|10.1007/s00253-016-7777-0}}.</ref><ref>Neave, M.J., Michell, C.T., Apprill, A. and Voolstra, C.R. (2017) "Endozoicomonas genomes reveal functional adaptation and plasticity in bacterial strains symbiotically associated with diverse marine hosts". ''Scientific Reports'', '''7''': 40579. {{doi|10.1038/srep40579}}.</ref> Given the recent mass bleaching occurring on reefs,<ref>Hughes, T.P., Kerry, J.T., Álvarez-Noriega, M., Álvarez-Romero, J.G., Anderson, K.D., Baird, A.H., Babcock, R.C., Beger, M., Bellwood, D.R., Berkelmans, R. and Bridge, T.C. (2017) "Global warming and recurrent mass bleaching of corals". ''Nature'', '''543'''(7645): 373–377. {{doi|10.1038/nature21707}}.</ref> corals will likely continue to be a useful and popular system for symbiosis and dysbiosis research.<ref name=Apprill2017 />

''[[Astrangia poculata]]'', the northern star coral, is a temperate [[stony coral]], widely documented along the eastern coast of the United States. The coral can live with and without [[zooxanthellae]] (algal symbionts), making it an ideal [[model organism]] to study microbial community interactions associated with symbiotic state. However, the ability to develop [[Primer (molecular biology)|primers]] and [[Molecular probe|probes]] to more specifically target key microbial groups has been hindered by the lack of full-length [[16S rRNA]] sequences, since sequences produced by the Illumina platform are of insufficient length (approximately 250 base pairs) for the design of primers and probes.<ref>[https://www.usgs.gov/center-news/usgs-scientists-publish-long-read-microbiome-sequences-temperate-coral-providing?qt-news_science_products=3#qt-news_science_products USGS scientists publish long-read microbiome sequences from temperate coral, providing community resource for probe and primer design], United States Geological Survey, 6 March 2019. [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref> In 2019, Goldsmith et al. demonstrated [[Sanger sequencing]] was capable of reproducing the biologically relevant diversity detected by deeper [[next-generation sequencing]], while also producing longer sequences useful to the research community for probe and primer design (see diagram on right).<ref name=Goldsmith2019>{{cite journal |doi = 10.3934/microbiol.2019.1.62|title = Stability of temperate coral ''Astrangia poculata'' microbiome is reflected across different sequencing methodologies|year = 2019|last1 = b. Goldsmith|first1 = Dawn|last2 = a. Pratte|first2 = Zoe|last3 = a. Kellogg|first3 = Christina|last4 = e. Snader|first4 = Sara|last5 = h. Sharp|first5 = Koty|journal = AIMS Microbiology|volume = 5|issue = 1|pages = 62–76|pmid = 31384703|pmc = 6646935 | bibcode=2019AIMSM...5...62G }} [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>

===Holobionts===
{{clear}}
Reef-building corals are well-studied [[holobiont]]s that include the coral itself together with its symbiont [[zooxanthellae]] (photosynthetic dinoflagellates), as well as its associated bacteria and viruses.<ref name="Knowlton2003">Knowlton, N. and Rohwer, F. (2003) "Multispecies microbial mutualisms on coral reefs: the host as a habitat". ''The American Naturalist'', '''162'''(S4): S51-S62. {{doi|10.1086/378684}}.</ref> Co-evolutionary patterns exist for coral microbial communities and coral phylogeny.<ref>{{Cite journal|last1=Pollock|first1=F. Joseph|last2=McMinds|first2=Ryan|last3=Smith|first3=Styles|last4=Bourne|first4=David G.|last5=Willis|first5=Bette L.|last6=Medina|first6=Mónica|last7=Thurber|first7=Rebecca Vega|last8=Zaneveld|first8=Jesse R.|date=2018-11-22|title=Coral-associated bacteria demonstrate phylosymbiosis and cophylogeny|journal=Nature Communications|language=en|volume=9|issue=1|page=4921|bibcode=2018NatCo...9.4921P|doi=10.1038/s41467-018-07275-x|issn=2041-1723|pmc=6250698|pmid=30467310|doi-access=free}}</ref>[[File:Relationships between corals and their microbial symbionts.jpg|thumb|upright=2.4| {{center|Relationships between corals and their microbial [[symbiont]]s<ref>Peixoto, R.S., Rosado, P.M., Leite, D.C.D.A., Rosado, A.S. and Bourne, D.G. (2017) "Beneficial microorganisms for corals (BMC): proposed mechanisms for coral health and resilience". ''Frontiers in Microbiology'', '''8''': 341. {{doi|10.3389/fmicb.2017.00341|doi-access=free}}.</ref>}}]]

It is known that the coral's [[microbiome]] and symbiont influence host health, however, the historic influence of each member on others is not well understood. Scleractinian corals have been diversifying for longer than many other symbiotic systems, and their microbiomes are known to be partially species-specific.<ref>{{Cite journal|last1=Apprill|first1=Amy|last2=Weber|first2=Laura G.|last3=Santoro|first3=Alyson E.|title=Distinguishing between Microbial Habitats Unravels Ecological Complexity in Coral Microbiomes|journal=mSystems|year=2016|volume=1|issue=5|pages=e00143–16|doi=10.1128/mSystems.00143-16|pmc=5080407|pmid=27822559}}</ref> It has been suggested that ''[[Endozoicomonas]]'', a commonly highly abundant bacterium in corals, has exhibited [[codiversification]] with its host.<ref>{{Cite journal|last1=La Rivière|first1=Marie|last2=Garrabou|first2=Joaquim|last3=Bally|first3=Marc|date=2015-12-01|title=Evidence for host specificity among dominant bacterial symbionts in temperate gorgonian corals|journal=Coral Reefs|language=en|volume=34|issue=4|pages=1087–1098|doi=10.1007/s00338-015-1334-7|bibcode=2015CorRe..34.1087L|s2cid=14309443|issn=1432-0975}}</ref><ref>{{Cite journal|last1=van de Water|first1=Jeroen A. J. M.|last2=Melkonian|first2=Rémy|last3=Voolstra|first3=Christian R.|last4=Junca|first4=Howard|last5=Beraud|first5=Eric|last6=Allemand|first6=Denis|last7=Ferrier-Pagès|first7=Christine|date=2017-02-01|title=Comparative Assessment of Mediterranean Gorgonian-Associated Microbial Communities Reveals Conserved Core and Locally Variant Bacteria|journal=Microbial Ecology|language=en|volume=73|issue=2|pages=466–478|doi=10.1007/s00248-016-0858-x|pmid=27726033|bibcode=2017MicEc..73..466V |s2cid=22336906|issn=1432-184X}}</ref> This hints at an intricate set of relationships between the members of the coral holobiont that have been developing as [[evolution]] of these members occurs.

A study published in 2018<ref>{{Cite journal|last1=Pollock|first1=F. Joseph|last2=McMinds|first2=Ryan|last3=Smith|first3=Styles|last4=Bourne|first4=David G.|last5=Willis|first5=Bette L.|last6=Medina|first6=Mónica|last7=Thurber|first7=Rebecca Vega|last8=Zaneveld|first8=Jesse R.|date=2018-11-22|title=Coral-associated bacteria demonstrate phylosymbiosis and cophylogeny|journal=Nature Communications|language=en|volume=9|issue=1|page=4921|doi=10.1038/s41467-018-07275-x|pmid=30467310|pmc=6250698|bibcode=2018NatCo...9.4921P|issn=2041-1723}}</ref> revealed evidence of [[phylosymbiosis]] between corals and their tissue and skeleton microbiomes. The coral skeleton, which represents the most diverse of the three coral microbiomes, showed the strongest evidence of phylosymbiosis. Coral microbiome composition and [[Species richness|richness]] were found to reflect coral [[phylogeny]]. For example, interactions between bacterial and eukaryotic coral phylogeny influence the abundance of ''Endozoicomonas'', a highly abundant bacterium in the coral holobiont. However, host-microbial [[cophylogeny]] appears to influence only a subset of coral-associated bacteria.

[[File:Control of the microbiota structure in the coral holobiont.jpg|thumb|upright=2.3|left| {{center|'''Top-down and bottom-up control of microbiota structure in the coral holobiont'''}} Stable microbes may be introduced to the holobiont through horizontal or vertical transmission and persist in ecological niches within the coral polyp where growth (or immigration) rates balance removal pressures from biophysical processes and immune or ecological interactions. Transient microbes enter the holobiont from environmental sources (e.g., seawater, prey items, or suspension feeding) and removal rates exceed growth/immigration rates such that a dynamic and high-diversity microbiota results. Transient and stable populations compete for resources including nutrients, light and space and the outcome of resource-based competition (bottom-up control) ultimately determines population growth rate and thus ability to persist when subject to removal. Whether a population is categorized as stable or transient may depend on the timeframe considered.<ref>Thompson, J.R., Rivera, H.E., Closek, C.J. and Medina, M. (2015) "Microbes in the coral holobiont: partners through evolution, development, and ecological interactions". ''Frontiers in cellular and infection microbiology'', '''4''': 176. {{doi|10.3389/fcimb.2014.00176|doi-access=free}}. [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref><br/>{{space|25}}<small>AMP = antimicrobial peptides, ROS = reactive oxygen species</small>]]

[[File:Trophic connections of the coral holobiont in the planktonic food web.jpg|thumb|upright=1.3|right| {{center|Trophic connections of the coral [[holobiont]] in the planktonic food web<ref>Thompson, J.R., Rivera, H.E., Closek, C.J. and Medina, M. (2015) "Microbes in the coral holobiont: partners through evolution, development, and ecological interactions". ''Frontiers in cellular and infection microbiology'', '''4''': 176. {{doi|10.3389/fcimb.2014.00176|doi-access=free}}.</ref>}}]]

{{clear}}

==Reefs==
{{main|Coral reef}}
{{See also|Coral reef fish|List of reefs}}
[[File:Coral reef locations.jpg|thumb|upright=1.5|Locations of coral reefs around the world]]
Many corals in the order [[Scleractinia]] are [[hermatypic]], meaning that they are involved in building reefs. Most such corals obtain some of their energy from [[zooxanthellae]] in the genus ''Symbiodinium''. These are [[symbiotic]] photosynthetic [[dinoflagellate]]s which require sunlight; reef-forming corals are therefore found mainly in shallow water. They secrete calcium carbonate to form hard skeletons that become the framework of the reef. However, not all reef-building corals in shallow water contain zooxanthellae, and some deep water species, living at depths to which light cannot penetrate, form reefs but do not harbour the symbionts.<ref name=Schuhmacher>{{cite journal |author1=Schuhmacher, Helmut |author2=Zibrowius, Helmut |year=1985 |title=What is hermatypic? |journal=Coral Reefs |volume=4 |issue=1 |pages=1–9 |doi=10.1007/BF00302198 |bibcode=1985CorRe...4....1S|s2cid=34909110 }}</ref>

[[File:Hertshoon.jpg|thumb|left|[[Staghorn coral]] (''Acropora cervicornis'') is an important hermatypic coral from the Caribbean.]]
There are various types of shallow-water coral reef, including fringing reefs, barrier reefs and atolls; most occur in tropical and subtropical seas. They are very slow-growing, adding perhaps one centimetre (0.4&nbsp;in) in height each year. The [[Great Barrier Reef]] is thought to have been laid down about two million years ago. Over time, corals fragment and die, sand and rubble accumulates between the corals, and the shells of clams and other molluscs decay to form a gradually evolving calcium carbonate structure.<ref>{{cite encyclopedia|publisher=MSN Encarta |year=2006 |title=Great Barrier Reef |url=http://encarta.msn.com/encyclopedia_761575831/Great_Barrier_Reef.html |access-date=April 25, 2015 |archive-url=https://web.archive.org/web/20091028020755/http://encarta.msn.com/encyclopedia_761575831/Great_Barrier_Reef.html |archive-date=October 28, 2009 }}</ref> Coral reefs are extremely diverse marine [[ecosystem]]s hosting over 4,000 species of fish, massive numbers of cnidarians, [[molluscs]], [[crustacean]]s, and many other animals.<ref name=Spalding>{{cite book |author1=Spalding, Mark |author2=Ravilious, Corinna |author3=Green, Edmund | year = 2001 | title = World Atlas of Coral Reefs |url=https://archive.org/details/worldatlasofcora01spal | pages = [https://archive.org/details/worldatlasofcora01spal/page/205 205–45] | publisher = University of California Press and UNEP/WCMC | location = Berkeley, CA | isbn = 978-0-520-23255-6}}</ref>

{{clear}}

==Evolution==
[[File:Life in the Ediacaran sea.jpg|thumb| {{center|Artist's depiction of life on the ocean floor as it may have appeared prior to the evolution of corals}}{{right|– [[Smithsonian Institution]]<ref>[https://www.flickr.com/photos/ideonexus/albums/72157603838941938 Smithsonian National Museum] ''flickr''.</ref>}}]]

At certain times in the geological past, reef-building organisms similar to corals were very abundant. Like modern corals, these ancient organisms built reefs, some of which ended as great structures in [[sedimentary rocks]]. Fossils of fellow reef-dwellers algae, sponges, and the remains of many [[Echinoderm|echinoids]], [[brachiopod]]s, [[bivalve]]s, [[gastropod]]s, and [[trilobite]]s appear along with coral fossils. This makes some corals useful [[index fossil]]s.<ref>{{cite web |last1=Alden |first1=Andrew |title=Index Fossils |url=http://geology.about.com/od/glossaryofgeology/g/Index-Fossils.htm |publisher=About education |access-date=25 April 2015 |archive-date=1 December 2016 |archive-url=https://web.archive.org/web/20161201184508/http://geology.about.com/od/glossaryofgeology/g/Index-Fossils.htm }}</ref> Coral fossils are not restricted to reef remnants, and many solitary fossils are found elsewhere, such as ''Cyclocyathus'', which occurs in England's [[Gault clay]] formation.

===Early corals===
Reef-building organisms similar to modern corals first appeared in the [[Cambrian]] about {{Ma|535}}.<ref name=Pratt>{{cite book | last1=Pratt | first1=B.R. | last2=Spincer | first2=B.R. | first3=R.A. | last3=Wood | first4=A.Yu. | last4=Zhuravlev | title=Ecology of the Cambrian Radiation | year=2001 | chapter-url=ftp://gis.dipbsf.uninsubria.it/zoo/The%20Ecology%20of%20the%20Cambrian%20Radiation%20-%20Andrey%20Zhuravlev.pdf | access-date=2007-04-06 | publisher=Columbia University Press | isbn=978-0-231-10613-9 | page=259 | chapter=12: Ecology and Evolution of Cambrian Reefs }}{{dead link|date=January 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> [[Fossil]]s are extremely rare until the [[Ordovician]] period, 100 million years later, when Heliolitida, [[Rugosa|rugose]], and [[tabulate]] corals became widespread. [[Paleozoic]] corals often contained numerous endobiotic symbionts.<ref name="VinnMotus2008">{{cite journal | doi=10.1666/07-056.1 | title=The earliest endosymbiotic mineralized tubeworms from the Silurian of Podolia, Ukraine | year=2008 | author=Vinn, O. | author2=Mõtus, M.-A. | journal=Journal of Paleontology | volume=82 | issue=2 | pages=409–14 | bibcode=2008JPal...82..409V | s2cid=131651974 | url=https://www.researchgate.net/publication/222089801 | access-date=2014-06-11}}</ref><ref name="VinnMotus2012">{{cite journal | title=Diverse early endobiotic coral symbiont assemblage from the Katian (Late Ordovician) of Baltica | year=2012 | author=Vinn, O. | author2=Mõtus, M.-A. | journal=Palaeogeography, Palaeoclimatology, Palaeoecology | volume=321–322 | pages=137–41 |doi=10.1016/j.palaeo.2012.01.028 | bibcode=2012PPP...321..137V }}</ref>

Tabulate corals occur in [[limestone]]s and calcareous [[shale]]s of the Ordovician period, with a gap in the fossil record due to [[Ordovician–Silurian extinction events|extinction events]] at the end of the Ordovician. Corals reappeared some millions of years later during the [[Silurian]] period, and tabulate corals often form low cushions or branching masses of [[calcite]] alongside rugose corals. Tabulate coral numbers began to decline during the middle of the Silurian period.<ref name=Tabulata>{{cite web |title=Introduction to the Tabulata |url=http://www.ucmp.berkeley.edu/cnidaria/tabulata.html |publisher=UCMP Berkeley |access-date=25 April 2015 |archive-url=https://web.archive.org/web/20150419112928/http://www.ucmp.berkeley.edu/cnidaria/tabulata.html |archive-date=19 April 2015 }}</ref>

Rugose or horn corals became dominant by the middle of the Silurian period, and during the Devonian, corals flourished with more than 200 genera. The rugose corals existed in solitary and colonial forms, and were also composed of calcite.<ref>{{cite web|title=Introduction to the Rugosa|url=http://www.ucmp.berkeley.edu/cnidaria/rugosa.html|publisher=UCMP Berkeley|access-date=25 April 2015|archive-url=https://web.archive.org/web/20150419112923/http://www.ucmp.berkeley.edu/cnidaria/rugosa.html|archive-date=19 April 2015}}</ref> Both rugose and tabulate corals became extinct in the [[Permian–Triassic extinction event]]<ref name=Tabulata /><ref>Xiang-Dong Wang and Xiao-Juan Wang (2007). [https://www.sciencedirect.com/science/article/abs/pii/S1871174X07000170 "Extinction patterns of Late Permian (Lopingian) corals in China"], ''Palaeoworld, 16,'' No. 1–3, 31–38</ref> {{Ma|250}} (along with 85% of marine species), and there is a gap of tens of millions of years until new forms of coral evolved in the [[Triassic]].

<gallery mode="packed" heights="150px" style="float:left;">
File:Syringoporid.jpg| Tabulate coral (a syringoporid); Boone limestone (Lower [[Carboniferous]]) near Hiwasse, Arkansas, scale bar is 2.0&nbsp;cm
File:AuloporaDevonianSilicaShale.jpg| Tabulate coral ''[[Aulopora]]'' from the [[Devonian]] period
File:RugosaOrdovician.jpg| Solitary rugose coral (''[[Grewingkia]]'') in three views; Ordovician, southeastern Indiana
</gallery>

{{clear}}

===Modern corals===
The currently ubiquitous stony corals, [[Scleractinia]], appeared in the [[Middle Triassic]] to fill the niche vacated by the extinct rugose and tabulate orders and is not closely related to the earlier forms. Unlike the corals prevalent before the Permian extinction, which formed skeletons of a form of calcium carbonate known as [[calcite]], modern stony corals form skeletons composed of the [[aragonite]].<ref name=Ries>{{cite journal | vauthors=Ries JB, Stanley SM, Hardie LA | date=July 2006 | title=Scleractinian corals produce calcite, and grow more slowly, in artificial Cretaceous seawater | journal=Geology | volume=34 | issue =7 | pages=525–28 | doi=10.1130/G22600.1 | bibcode=2006Geo....34..525R }}</ref> Their fossils are found in small numbers in rocks from the Triassic period, and become common in the [[Jurassic]] and later periods.<ref>{{cite web|title=Evolutionary history |url=http://coral.aims.gov.au/info/evolution.jsp |archive-url= https://web.archive.org/web/20181013124550/http://coral.aims.gov.au/info/evolution.jsp |archive-date= 13 October 2018 |website=AIMS |access-date=25 April 2015}}</ref> Although they are geologically younger than the tabulate and rugose corals, the aragonite of their skeletons is less readily preserved, and their fossil record is accordingly less complete.

{{Coral fossil record timeline}}

==Status==
{{Main|Environmental issues with coral reefs}}

===Threats===
[[File:Reef0484.jpg|thumb|upright|A healthy coral reef has a striking level of biodiversity in many forms of marine life.]]
Coral reefs are under stress around the world.<ref name="Coral reefs around the world">{{cite news |url=https://www.theguardian.com/environment/interactive/2009/sep/02/coral-world-interactive |title=Coral reefs around the world |work=The Guardian |date=2 September 2009}}</ref> In particular, coral mining, [[agricultural runoff|agricultural]] and [[urban runoff]], [[pollution]] (organic and inorganic), [[overfishing]], [[blast fishing]], disease, and the digging of [[canal]]s and access into islands and bays are localized threats to coral ecosystems. Broader threats are sea temperature rise, [[sea level rise]] and [[pH]] changes from [[ocean acidification]], all associated with [[greenhouse gas emissions]].<ref name=cra10>{{cite web|title=Threats to Coral Reefs|url=http://www.coral.org/resources/about_coral_reefs/threats_to_coral_reefs|publisher=[[Coral Reef Alliance]]|year=2010|access-date=5 December 2011|archive-url=https://web.archive.org/web/20111201035325/http://www.coral.org/resources/about_coral_reefs/threats_to_coral_reefs|archive-date=1 December 2011}}</ref> In 1998, 16% of the world's reefs died as a result of increased water temperature.<ref>[http://blogs.ei.columbia.edu/2011/06/13/losing-our-coral-reefs/ Losing Our Coral Reefs – Eco Matters – State of the Planet]. Blogs.ei.columbia.edu. Retrieved on 2011-11-01.</ref>

Approximately 10% of the world's coral reefs are dead.<ref name="Kleypas, J.A. 2006">{{Cite web |last1=Kleypas |first1=J.A. |first2=R.A. |last2=Feely |first3=V.J. |last3=Fabry |first4=C. |last4=Langdon |first5=C.L. |last5=Sabine |first6=L.L. |last6=Robbins |year=2006 |title=Impacts of Ocean Acidification on Coral Reefs and Other Marine Calcifiers: A guide for Future Research |publisher=[[National Science Foundation]], [[NOAA]], and [[United States Geological Survey]] |url=http://www.ucar.edu/communications/Final_acidification.pdf |access-date=April 7, 2011 |archive-url=https://web.archive.org/web/20110720101953/http://www.ucar.edu/communications/Final_acidification.pdf |archive-date=July 20, 2011 }}</ref><ref>Save Our Seas, 1997 Summer Newsletter, Cindy Hunter and Alan Friedlander</ref><ref>{{cite book |chapter=Status of Coral Reefs, Coral Reef Monitoring and Management in Southeast Asia, 2004 |last1=Tun |first1=K. |first2=L.M. |last2=Chou |first3=A. |last3=Cabanban |first4=V.S. |last4=Tuan |first5=S. |last5=Philreefs |first6=T. |last6=Yeemin |last7=Suharsono |first8=K. |last8=Sour |first9=D. |last9=Lane |year=2004 |pages=235–76 |editor-first=C. |editor-last=Wilkinson |title=Status of Coral Reefs of the world: 2004 |publisher=Australian Institute of Marine Science |location=Townsville, Queensland, Australia |access-date=2019-04-23 |chapter-url=https://www.researchgate.net/publication/292724900 }}</ref> About 60% of the world's reefs are at risk due to human-related activities.<ref>{{cite book |last=Burke |first=Lauretta |title=Reefs at risk revisited |url=https://archive.org/details/reefsatriskrevis00burk |url-access=limited |year=2011 |publisher=World Resources Institute |location=Washington, DC |isbn=978-1-56973-762-0 |page=[https://archive.org/details/reefsatriskrevis00burk/page/n51 38] |author2=Reytar, K. |author3=Spalding, M. |author4=Perry, A. }}</ref> The threat to reef health is particularly strong in [[Southeast Asia]], where 80% of reefs are [[endangered]].<ref>{{cite web |last1=Bryant|first1=Dirk |last2=Burke|first2=Lauretta |last3=McManus|first3=John |last4=Spalding|first4=Mark |title=Reefs at Risk: A Map-Based Indicator of Threats to the World's Coral Reef |url=http://coralreef.noaa.gov/aboutcrcp/strategy/reprioritization/wgroups/resources/climate/resources/reefsatrisk.pdf |publisher=NOAA |access-date=25 April 2015 |archive-url=https://web.archive.org/web/20130218220752/http://coralreef.noaa.gov/aboutcrcp/strategy/reprioritization/wgroups/resources/climate/resources/reefsatrisk.pdf |archive-date=2013-02-18 }}</ref> Over 50% of the world's [[coral reef]]s may be destroyed by 2030; as a result, most nations protect them through environmental laws.<ref name=Norlander>{{cite journal | 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.thefreelibrary.com/Coral+crisis!+Humans+are+killing+off+these+bustling+underwater...-a0112022348}}</ref>

In the Caribbean and tropical Pacific, direct contact between ~40–70% of common seaweeds and coral causes bleaching and death to the coral via transfer of [[lipid]]-soluble [[metabolite]]s.<ref>{{cite journal |vauthors=Rasher DB, Hay ME |title=Chemically rich seaweeds poison corals when not controlled by herbivores |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=107 |issue=21 |pages=9683–88 |date=May 2010 |pmid=20457927 |pmc=2906836 |doi=10.1073/pnas.0912095107 |bibcode=2010PNAS..107.9683R |doi-access=free }}</ref> Seaweed and algae proliferate given adequate [[nutrients]] and limited grazing by [[herbivore]]s such as [[parrotfish]].

Water temperature changes of more than {{cvt|1–2|C-change|abbr=on|1}} or [[salinity]] changes can kill some species of coral. Under such environmental stresses, corals expel their [[Symbiodinium]]; without them, coral tissues reveal the white of their skeletons, an event known as [[coral bleaching]].<ref name=Hoegh>{{cite journal | author=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=839–66 | volume=50 | issue=8 | doi=10.1071/MF99078 | author-link=Ove Hoegh-Guldberg (biologist) | doi-access= }}</ref>

Submarine springs found along the coast of Mexico's [[Yucatán Peninsula]] produce water with a naturally low pH (relatively high acidity) providing conditions similar to those expected to become widespread as the oceans absorb carbon dioxide.<ref name=Stephens>{{cite web |last1=Stephens |first1=Tim |title=Submarine springs offer preview of ocean acidification effects on coral reefs |url=http://news.ucsc.edu/2011/11/coral-reefs.html |publisher=University of California Santa Cruz |date=28 November 2011 |access-date=25 April 2015 |archive-date=10 May 2020 |archive-url=https://web.archive.org/web/20200510165726/https://news.ucsc.edu/2011/11/coral-reefs.html }}</ref> Surveys discovered multiple species of live coral that appeared to tolerate the acidity. The colonies were small and patchily distributed and had not formed structurally complex reefs such as those that compose the nearby [[Mesoamerican Barrier Reef System]].<ref name=Stephens/>

===Coral health===
To assess the threat level of coral, scientists developed a coral imbalance ratio, Log (Average abundance of disease-associated taxa / Average abundance of healthy associated taxa). The lower the ratio the healthier the microbial community is. This ratio was developed after the microbial mucus of coral was collected and studied.<ref>{{Cite web|title=Health and Disease Signatures of the Coral Microbiome • iBiology|url=https://www.ibiology.org/microbiology/coral-microbiome/|website=iBiology|language=en-US|access-date=2020-05-14|archive-date=2020-07-28|archive-url=https://web.archive.org/web/20200728205431/https://www.ibiology.org/microbiology/coral-microbiome/}}</ref>

===Climate change impacts===
Increasing [[sea surface temperature]]s in tropical regions (~{{convert|1|C-change|F-change|abbr=on|1}}) the last century have caused major [[coral bleaching]], death, and therefore shrinking coral populations. Although coral are able to adapt and acclimate, it is uncertain if this evolutionary process will happen quickly enough to prevent major reduction of their numbers.<ref name="Hoegh-Guldberg">{{cite journal |author=Hoegh-Guldberg O. |year=1999 |title=Climate change, coral bleaching and the future of the world's coral reefs |journal=Marine and Freshwater Research |volume=50 |issue=8 |pages=839–99 |doi=10.1071/mf99078 |doi-access=}}</ref> Climate change causes more frequent and more severe storms that can destroy [[coral reef]]s.<ref>{{Cite web|url=https://oceanservice.noaa.gov/facts/coralreef-climate.html|title=How does climate change affect coral reefs?|first=National Oceanic and Atmospheric Administration|last=US Department of Commerce|website=oceanservice.noaa.gov}}</ref>

Annual growth bands in some corals, such as the [[deep sea]] [[bamboo coral]]s (''Isididae''), may be among the first signs of the effects of ocean acidification on marine life.<ref>{{cite web |title=National Oceanic and Atmospheric Administration – New Deep-Sea Coral Discovered on NOAA-Supported Mission |url=http://www.noaanews.noaa.gov/stories2009/20090305_coral.html |access-date=2009-05-11 |publisher=noaanews.noaa.gov}}</ref> The growth rings allow [[geologist]]s to construct year-by-year chronologies, a form of [[incremental dating]], which underlie high-resolution records of past [[paleoclimatology|climatic]] and [[paleoecology|environmental]] changes using [[geochemical]] techniques.<ref name="Schrag">{{cite journal |author1=Schrag, D.P. |author2=Linsley, B.K. |year=2002 |title=Corals, chemistry, and climate |journal=Science |volume=296 |issue=8 |pages=277–78 |doi=10.1126/science.1071561 |pmid=11951026 |s2cid=82449130}}</ref>

Certain species form communities called [[microatoll]]s, which are colonies whose top is dead and mostly above the water line, but whose perimeter is mostly submerged and alive. Average [[tide]] level limits their height. By analyzing the various growth morphologies, microatolls offer a low-resolution record of sea level change. Fossilized microatolls can also be dated using [[radiocarbon dating]]. Such methods can help to reconstruct [[Holocene]] [[sea level]]s.<ref name="Smithers">{{cite journal |last1=Smithers |first1=Scott G. |last2=Woodroffe |first2=Colin D. |year=2000 |title=Microatolls as sea-level indicators on a mid-ocean atoll |journal=Marine Geology |volume=168 |issue=1–4 |pages=61–78 |bibcode=2000MGeol.168...61S |doi=10.1016/S0025-3227(00)00043-8}}</ref>

Though coral have large sexually-reproducing populations, their evolution can be slowed by abundant [[asexual reproduction]].<ref name="Hughes et al.">{{cite journal |author1=Hughes, T. |author2=Baird, A. |author3=Bellwood, D. |author4=Card, M. |author5=Connolly, S. |author6=Folke, C. |author7=Grosberg, R. |author8=Hoegh-Guldberg, O. |author9=Jackson, J. |author10=Klepas, J. |author11=Lough, J. |author12=Marshall, P. |author13=Nystrom, M. |author14=Palumbi, S. |author-link14=Stephen Palumbi |author15=Pandolfi, J. |year=2003 |title=Climate change, human impacts, and the resilience of coral reefs |journal=Science |volume=301 |issue=5635 |pages=929–33 |bibcode=2003Sci...301..929H |doi=10.1126/science.1085046 |pmid=12920289 |author16=Rosen, B. |author17=and Roughgarden, J. |s2cid=1521635}}</ref> [[Gene flow]] is variable among coral species.<ref name="Hughes et al." /> According to the [[biogeography]] of coral species, gene flow cannot be counted on as a dependable source of adaptation as they are very stationary organisms. Also, coral longevity might factor into their adaptivity.<ref name="Hughes et al." />

However, [[adaptation to climate change]] has been demonstrated in many cases, which is usually due to a shift in coral and zooxanthellae [[genotype]]s. These shifts in [[allele frequency]] have progressed toward more tolerant types of zooxanthellae.<ref name="Parmesan">{{cite journal |author=Parmesan, C. |year=2006 |title=Ecological and evolutionary responses to recent climate change |journal=Annual Review of Ecology, Evolution, and Systematics |volume=37 |pages=637–69 |doi=10.1146/annurev.ecolsys.37.091305.110100}}</ref> Scientists found that a certain [[scleractinia]]n zooxanthella is becoming more common where sea temperature is high.<ref name="Baker">{{cite journal |author=Baker, A. |year=2004 |title=Corals' adaptive response to climate change |journal=Nature |volume=430 |issue=7001 |page=741 |bibcode=2004Natur.430..741B |doi=10.1038/430741a |pmid=15306799 |doi-access=free |s2cid=32092741}}</ref><ref name="Donner et al.">{{cite journal |author=Donner, S. |author2=Skirving, W. |author3=Little, C. |author4=Oppenheimer, M. |author5=Hoegh-Guldberg, O. |year=2005 |title=Global assessment of coral bleaching and required rates of adaptation under climate change |url=http://www.princeton.edu/step/people/faculty/michael-oppenheimer/recent-publications/Global-assessment-of-coral-bleaching-and-required-rates.pdf |journal=Global Change Biology |volume=11 |issue=12 |pages=2251–65 |bibcode=2005GCBio..11.2251D |citeseerx=10.1.1.323.8134 |doi=10.1111/j.1365-2486.2005.01073.x |pmid=34991281 |archive-url=https://web.archive.org/web/20170814144112/http://www.princeton.edu/step/people/faculty/michael-oppenheimer/recent-publications/Global-assessment-of-coral-bleaching-and-required-rates.pdf |archive-date=2017-08-14 |access-date=2017-10-25 |s2cid=84890014}}</ref> Symbionts able to tolerate warmer water seem to photosynthesise more slowly, implying an evolutionary trade-off.<ref name="Donner et al." />

In the Gulf of Mexico, where sea temperatures are rising, cold-sensitive [[Staghorn coral|staghorn]] and [[elkhorn coral]] have shifted in location.<ref name="Parmesan" />
Not only have the symbionts and specific species been shown to shift, but there seems to be a certain growth rate favorable to selection. Slower-growing but more heat-tolerant corals have become more common.<ref name="Baskett et al.">{{cite journal |author1=Baskett, M. |author2=Gaines, S. |author3=Nisbet, R. |name-list-style=amp |year=2009 |title=Symbiont diversity may help coral reefs survive moderate climate change |url=https://escholarship.org/content/qt3d58b2w2/qt3d58b2w2.pdf?t=o17qaf |journal=Ecological Applications |volume=19 |issue=1 |pages=3–17 |doi=10.1890/08-0139.1 |pmid=19323170 |bibcode=2009EcoAp..19....3B |s2cid=1189125}}</ref> The changes in temperature and acclimation are complex. Some reefs in current shadows represent a [[Refugium (population biology)|refugium]] location that will help them adjust to the disparity in the environment even if eventually the temperatures may rise more quickly there than in other locations.<ref name="McClanahan et al.">{{cite journal |author1=McClanahan, T. |author2=Ateweberhan, M. |author3=Muhando, C. |author4=Maina, J. |author5=Mohammed, M. |name-list-style=amp |year=2007 |title=Effects of Climate and Seawater Temperature Variation on Coral Bleaching and Morality |journal=Ecological Monographs |volume=77 |issue=4 |pages=503–25 |citeseerx=10.1.1.538.970 |doi=10.1890/06-1182.1|bibcode=2007EcoM...77..503M }}</ref> This [[vicariance|separation of populations]] by climatic barriers causes a [[realized niche]] to shrink greatly in comparison to the old [[fundamental niche]].

====Geochemistry====
Corals are shallow, colonial organisms that integrate oxygen and trace elements into their skeletal [[aragonite]] ([[Polymorphism (materials science)|polymorph]] of [[calcite]]) crystalline structures as they grow. Geochemical anomalies within the crystalline structures of corals represent functions of temperature, salinity and oxygen isotopic composition. Such geochemical analysis can help with climate modeling.<ref>{{cite journal |last1=Kilbourne |first1=K. Halimeda |last2=Quinn |first2=Terrence M. |last3=Taylor |first3=Frederick W. |last4=Delcroix |first4=Thierry |last5=Gouriou |first5=Yves |year=2004 |title=El Niño-Southern Oscillation-related salinity variations recorded in the skeletal geochemistry of a ''Porites'' coral from Espiritu Santo, Vanuatu |journal=Paleoceanography |volume=19 |issue=4 |pages=PA4002 |bibcode=2004PalOc..19.4002K |doi=10.1029/2004PA001033|doi-access=free }}</ref> The [[δ18O|ratio of oxygen-18 to oxygen-16]] (δ<sup>18</sup>O), for example, is a proxy for temperature.

=====Strontium/calcium ratio anomaly=====
Time can be attributed to coral geochemistry anomalies by correlating [[strontium]]/[[calcium]] minimums with [[sea surface temperature|sea surface temperature (SST)]] maximums to data collected from [http://www.esrl.noaa.gov/psd/gcos_wgsp/Timeseries/Nino34/ NINO 3.4 SSTA].<ref name="Ren, L. 2002">{{cite journal |last1=Ren |first1=Lei |last2=Linsley |first2=Braddock K. |last3=Wellington |first3=Gerard M. |last4=Schrag |first4=Daniel P. |last5=Hoegh-guldberg |first5=Ove |year=2003 |title=Deconvolving the δ<sup>18</sup>O seawater component from subseasonal coral δ<sup>18</sup>O and Sr/Ca at Rarotonga in the southwestern subtropical Pacific for the period 1726 to 1997 |journal=Geochimica et Cosmochimica Acta |volume=67 |issue=9 |pages=1609–21 |bibcode=2003GeCoA..67.1609R |doi=10.1016/S0016-7037(02)00917-1}}</ref>
<!-- [[WP:NFCC]] violation: [[File:Linsley2006 figure1.png|thumb|The position of the [[South Pacific convergence zone]] as a function of precipitation and salinity]] -->

=====Oxygen isotope anomaly=====
The comparison of coral strontium/calcium minimums with sea surface temperature maximums, data recorded from [http://www.esrl.noaa.gov/psd/gcos_wgsp/Timeseries/Nino34/ NINO 3.4 SSTA], time can be correlated to coral strontium/calcium and [[Δ18O|δ<sup>18</sup>O]] variations. To confirm the accuracy of the annual relationship between Sr/Ca and [[Δ18O|δ<sup>18</sup>O]] variations, a perceptible association to annual coral growth rings confirms the age conversion. [[Geochronology]] is established by the blending of Sr/Ca data, growth rings, and [[Stable isotope ratio|stable isotope]] data. [[El Nino-Southern Oscillation|El Nino-Southern Oscillation (ENSO)]] is directly related to climate fluctuations that influence coral [[Δ18O|δ<sup>18</sup>O]] ratio from local salinity variations associated with the position of the [[South Pacific convergence zone|South Pacific convergence zone (SPCZ)]] and can be used for [[ENSO]] modeling.<ref name="Ren, L. 2002" />

=====Sea surface temperature and sea surface salinity=====
[[File:Global Sea Surface Temperature - GPN-2003-00032.jpg|thumb|Global sea surface temperature (SST)]]
The global moisture budget is primarily being influenced by tropical sea surface temperatures from the position of the [[Intertropical Convergence Zone]] (ITCZ).<ref>{{cite journal |last1=Wu |first1=Henry C. |last2=Linsley |first2=Braddock K. |last3=Dassié |first3=Emilie P. |last4=Schiraldi |first4=Benedetto |last5=deMenocal |first5=Peter B. |year=2013 |title=Oceanographic variability in the South Pacific Convergence Zone region over the last 210 years from multi-site coral Sr/Ca records |journal=Geochemistry, Geophysics, Geosystems |volume=14 |issue=5 |pages=1435–53 |bibcode=2013GGG....14.1435W |doi=10.1029/2012GC004293 |doi-access=free}}</ref> The [[Southern Hemisphere]] has a unique meteorological feature positioned in the southwestern Pacific Basin called the [[South Pacific convergence zone|South Pacific Convergence Zone (SPCZ)]], which contains a perennial position within the Southern Hemisphere. During [[ENSO]] warm periods, the [[SPCZ]] reverses orientation extending from the equator down south through [[Solomon Islands]], [[Vanuatu]], [[Fiji]] and towards the French [[Polynesian islands|Polynesian Islands]]; and due east towards [[South America]] affecting geochemistry of corals in tropical regions.<ref>{{cite journal |last1=Kiladis |first1=George N. |last2=von Storch |first2=Hans |last3=van Loon |first3=Harry |year=1989 |title=Origin of the South Pacific Convergence Zone |journal=Journal of Climate |volume=2 |issue=10 |pages=1185–95 |bibcode=1989JCli....2.1185K |doi=10.1175/1520-0442(1989)002<1185:OOTSPC>2.0.CO;2 |doi-access=free}}</ref>

Geochemical analysis of skeletal coral can be linked to sea surface salinity (SSS) and [[sea surface temperature]] (SST), from [http://www.esrl.noaa.gov/psd/gcos_wgsp/Timeseries/Nino34/ El Nino 3.4 SSTA] data, of tropical oceans to seawater [[Δ18O|δ<sup>18</sup>O]] ratio anomalies from corals. [[ENSO]] phenomenon can be related to variations in sea surface salinity (SSS) and [[sea surface temperature|sea surface temperature (SST)]] that can help model tropical climate activities.<ref name="Lukas, R. 1991">{{cite journal |last1=Lukas |first1=Roger |last2=Lindstrom |first2=Eric |year=1991 |title=The mixed layer of the western equatorial Pacific Ocean |journal=Journal of Geophysical Research |volume=96 |issue=S1 |pages=3343–58 |bibcode=1991JGR....96.3343L |doi=10.1029/90JC01951}}</ref>

=====Limited climate research on current species=====
[[File:Porites lutea.jpg|thumb|''Porites lutea'']]
Climate research on live coral species is limited to a few studied species. Studying ''[[Porites]]'' coral provides a stable foundation for geochemical interpretations that is much simpler to physically extract data in comparison to ''[[Platygyra]]'' species where the complexity of ''[[Platygyra]]'' species skeletal structure creates difficulty when physically sampled, which happens to be one of the only multidecadal living coral records used for coral [[paleoclimate]] modeling.<ref name="Lukas, R. 1991" />

== Protection ==
{{Main|Coral reef protection}}

Marine Protected Areas, [[Biosphere reserves]], [[marine park]]s, [[national monument]]s [[world heritage]] status, [[fishery management]] and [[habitat (ecology)|habitat protection]] can protect reefs from anthropogenic damage.<ref>{{cite magazine |date=January 2011 |title=Phoenix Rising |url=http://ngm.nationalgeographic.com/2011/01/phoenix-islands/stone-text |archive-url=https://web.archive.org/web/20101218213927/http://ngm.nationalgeographic.com/2011/01/phoenix-islands/stone-text |archive-date=December 18, 2010 |magazine=National Geographic Magazine |access-date=April 30, 2011}}</ref>

Many governments now prohibit removal of coral from reefs, and inform coastal residents about reef protection and ecology. While local action such as habitat restoration and herbivore protection can reduce local damage, the longer-term threats of acidification, temperature change and sea-level rise remain a challenge.<ref name="cra10" />

Protecting networks of diverse and healthy reefs, not only climate [[Refugium (population biology)|refugia]], helps ensure the greatest chance of [[genetic diversity]], which is critical for coral to adapt to new climates.<ref name=":0">{{Cite journal |last1=Walsworth | first1 = T.E. | last2 = Schindler | first2 = D.E. | last3 = Colton | first3 = M.A.| last4 = Webster | first4 = M.S. | last5 = Palumbi | first5 = S.R. | last6 = Mumby | first6 = P.J. | last7 = Essington | first7 = T.E. | last8 = Pinsky | first8 = M.L. |date=July 1, 2019 |title=Management for network diversity speeds evolutionary adaptation to climate change |url=https://www.nature.com/articles/s41558-019-0518-5.epdf?author_access_token=P8wTmOVZpLkKcslba2guA9RgN0jAjWel9jnR3ZoTv0MfsDpsr-XFeaym1-pv8WErp3wvwWdkVHp-xxQar4ROnGw5GpCATCYx5cv3yLdU3H0Yd0zwLASpSOCiN5WSFidDI_GNaKqZ7ZiaG4o1CQ6xdw%3D%3D |journal=Nature Research |volume=9 |issue=8 |pages=632–636 |doi=10.1038/s41558-019-0518-5 |bibcode=2019NatCC...9..632W |url-access=subscription }}</ref> A variety of conservation methods applied across marine and terrestrial threatened ecosystems makes coral adaption more likely and effective.<ref name=":0" />

To eliminate destruction of corals in their indigenous regions, projects have been started to grow corals in non-tropical countries.<ref>[http://www.ecodeco.nl/html/applications.htm EcoDeco EcologicalTechnology] {{webarchive|url=https://web.archive.org/web/20110307104000/http://www.ecodeco.nl/html/applications.htm|date=2011-03-07}}. Ecodeco.nl. Retrieved on 2011-11-29.</ref><ref>[http://www.koraalwetenschap.nl/content/view/277/1/ KoralenKAS project] {{webarchive|url=https://web.archive.org/web/20120426021608/http://www.koraalwetenschap.nl/content/view/277/1/|date=2012-04-26}}. Koraalwetenschap.nl. Retrieved on 2011-11-29.</ref>

==Relation to humans==
Local economies near major coral reefs benefit from an abundance of fish and other marine creatures as a food source. Reefs also provide recreational [[scuba diving]] and [[snorkeling]] tourism. These activities can damage coral but international projects such as [[Green Fins]] that encourage dive and snorkel centres to follow a Code of Conduct have been proven to mitigate these risks.<ref>{{cite journal |last1=Hunt |first1=Chloe V. |last2=Harvey |first2=James J. |last3=Miller |first3=Anne |last4=Johnson |first4=Vivienne |last5=Phongsuwan |first5=Niphon |year=2013 |title=The Green Fins approach for monitoring and promoting environmentally sustainable scuba diving operations in South East Asia |journal=Ocean & Coastal Management |volume=78 |pages=35–44 |doi=10.1016/j.ocecoaman.2013.03.004|bibcode=2013OCM....78...35H }}</ref>

===Jewelry===
{{main|Precious coral}}
[[File:6-Strand Necklace, Navajo (Native American), ca. 1920s, cropped.jpg|thumb|upright|6-strand necklace, [[Navajo people|Navajo]] (Native American), {{Circa|1920s}}, [[Brooklyn Museum]]]]
Corals' many colors give it appeal for necklaces and other [[jewelry]]. Intensely red coral is prized as a gemstone. Sometimes called fire coral, it is not the same as [[fire coral]]. Red coral is very rare because of [[overharvesting]].<ref>{{cite news |title= Coral makes a splash|first= Melissa|last= Magsaysay|url= https://www.latimes.com/archives/la-xpm-2009-jun-21-ig-coral21-story.html|newspaper= Los Angeles Times|date= June 21, 2009|access-date=January 12, 2013}}</ref> In general, it is inadvisable to give coral as gifts since they are in decline from stressors like climate change, pollution, and unsustainable fishing.

Always considered a precious mineral, "the Chinese have long associated red coral with auspiciousness and longevity because of its color and its resemblance to deer antlers (so by association, virtue, long life, and high rank".<ref>Welch, Patricia Bjaaland, ''Chinese Art: A Guide to Motifs and Visual Imagery''. Tokyo, Rutland and Singapore: Tuttle, 2008, p. 61</ref> It reached its height of popularity during the Manchu or Qing Dynasty (1644–1911) when it was almost exclusively reserved for the emperor's use either in the form of coral beads (often combined with pearls) for court jewelry or as decorative [[Penjing]] (decorative miniature mineral trees). Coral was known as ''shanhu'' in Chinese. The "early-modern 'coral network' [began in] the Mediterranean Sea [and found its way] to Qing China via the English [[East India Company]]".<ref>Lacey, Pippa, "The Coral Network: The trade of red coral to the Qing imperial court in the eighteenth century" in ''The Global Lives of Things'', ed. by Anne Gerritsen and Giorgio Aiello, London: Rutledge, 2016, p. 81</ref> There were strict rules regarding its use in a code established by the [[Qianlong Emperor]] in 1759.

By the middle of the 19th century, "coral fisheries" existed in the Mediterranean, the Red Sea, Persian Gulf and elsewhere. An instrument used to retrieve the coral consisted of two beams of heavy wood attached to each other at right angles. Heavy stones were added to make the apparatus sink and netting was attached beneath the device. It is then lowered by a strong rope over an outcrop of coral and the boat trawls over the coral causing it to break off and be caught in the netting. The device is then drawn to the surface by the boat crew.<ref>{{cite book |title=The National Cyclopaedia of Useful Knowledge, Vol IV |date=1848 |publisher=Charles Knight |location=London |page=935 |edition=First}}</ref>

===Medicine===
[[Image:ViennaDioscoridesCoral.jpg|thumb|left|upright|Depiction of coral in the [[Juliana Anicia Codex]], a 6th-century copy of [[Dioscorides]]' ''[[De Materia Medica]]''. The facing page states that coral can be used to treat ulcers.<ref>Folio 391, [[Juliana Anicia Codex]]</ref>]]
In medicine, chemical compounds from corals can potentially be used to treat cancer, neurological diseases, inflammation including arthritis, pain, bone loss, high blood pressure and for other therapeutic uses.<ref>{{Cite journal|last1=Copper|first1=Edwin|last2=Hirabayashi|first2=K.|last3=Strychar|first3=K. B.|last4=Sammarco|first4=P. W.|date=2014|title=Corals and their Potential Applications to Integrative Medicine|journal=Evidence-Based Complementary and Alternative Medicine|volume=2014|page=9|article-number=184959 |pmc=3976867|pmid=24757491|doi=10.1155/2014/184959|doi-access=free}}</ref><ref>{{Cite journal|last1=Senthilkumar|first1=Kalimuthu|last2=Se-Kwon|first2=Kim|date=2013|title=Marine Invertebrate Natural Products for Anti-Inflammatory and Chronic Diseases.|journal=Evidence-Based Complementary and Alternative Medicine|volume=2013|article-number=572859|pmc=3893779|pmid=24489586|doi=10.1155/2013/572859|doi-access=free}}</ref> Coral skeletons, e.g. ''[[Isididae]]'' are being researched for their potential near-future use for [[bone graft]]ing in humans.<ref name="wiley">{{cite journal |title=Biomaterial structure in deep-sea bamboo coral (Anthozoa: Gorgonacea: Isididae): perspectives for the development of bone implants and templates for tissue engineering |journal=Materialwissenschaft und Werkstofftechnik |volume=37 |issue=6 |pages=552–57 |doi= 10.1002/mawe.200600036 |year=2006 |last1=Ehrlich|first1=H. |last2=Etnoyer|first2=P. |last3=Litvinov|first3=S. D. |last4=Olennikova|first4=M.M. |last5=Domaschke|first5=H. |last6=Hanke|first6=T. |last7=Born|first7=R. |last8=Meissner|first8=H. |last9=Worch|first9=H.|s2cid=97972721 }}</ref>
Coral Calx, known as Praval Bhasma in [[Sanskrit]], is widely used in traditional system of [[Ayurveda|Indian medicine]] as a supplement in the treatment of a variety of bone metabolic disorders associated with calcium deficiency.<ref>{{cite journal |vauthors=Reddy PN, Lakshmana M, Udupa UV |title=Effect of Praval bhasma (Coral calx), a natural source of rich calcium on bone mineralization in rats |journal=Pharmacological Research |volume=48 |issue=6 |pages=593–99 |date=December 2003 |pmid=14527824 |doi=10.1016/S1043-6618(03)00224-X }}</ref> In classical times ingestion of pulverized coral, which consists mainly of the weak base [[calcium carbonate]], was recommended for calming stomach ulcers by [[Galen]] and [[Dioscorides]].<ref>Pedanius Dioscorides – Der Wiener Dioskurides, Codex medicus Graecus 1 der Österreichischen Nationalbibliothek Graz: [[Akademische Druck- und Verlagsanstalt]] 1998 fol. 391 verso (Band 2), Kommentar S. 47 und 52. {{ISBN|3-201-01725-6}}</ref>

===Construction===
Coral reefs in places such as the East African coast are used as a source of [[building material]].<ref name="Pouwels2002">{{cite book |last=Pouwels |first=Randall L.|title=Horn and Crescent: Cultural Change and Traditional Islam on the East African Coast, 800–1900 |url=https://books.google.com/books?id=iyw-_NMk0bgC&pg=PA26 |date=6 June 2002 |publisher=Cambridge University Press |isbn=978-0-521-52309-7 |page=26}}</ref> Ancient (fossil) coral limestone, notably including the [[Coral Rag Formation]] of the hills around [[Oxford]] (England), was once used as a building stone, and can be seen in some of the oldest buildings in that city including the Saxon tower of [[St Michael at the Northgate]], St. George's Tower of [[Oxford Castle]], and the medieval walls of the city.<ref>{{cite web |title=Strategic Stone Study: A Building Stone Atlas of Oxfordshire |url=https://www.bgs.ac.uk/downloads/start.cfm?id=1617 |publisher=English Heritage |access-date=23 April 2015 |date=March 2011}}</ref>

===Shoreline protection===
Healthy coral reefs absorb 97 percent of a wave's energy, which buffers shorelines from currents, waves, and storms, helping to prevent loss of life and property damage. Coastlines protected by coral reefs are also more stable in terms of erosion than those without.<ref name=Ferrario>{{cite journal |author1=Ferrario, F. |author2=Beck, M.W. |author3=Storlazzi, C.D. |author4=Micheli, F. |author5=Shepard, C.C. |author6=Airoldi, L. | title=The effectiveness of coral reefs for coastal hazard risk reduction and adaptation | journal=Nature Communications | year=2014 | volume=5 | issue=3794 |page=3794 | doi= 10.1038/ncomms4794|pmid=24825660 |pmc=4354160 | bibcode=2014NatCo...5.3794F }}</ref>

===Local economies===
Coastal communities near coral reefs rely heavily on them. Worldwide, more than 500 million people depend on coral reefs for food, income, coastal protection, and more.<ref>{{cite web|url=http://www.icriforum.org/sites/default/files/scr2004v1-all.pdf|title=Status of Coral Reefs of the World: 2004 Volume 1|publisher=Global Coral Reef Monitoring Network|access-date=2019-01-14|archive-url=https://web.archive.org/web/20190617115005/http://www.icriforum.org/sites/default/files/scr2004v1-all.pdf|archive-date=2019-06-17}}</ref> The total economic value of coral reef services in the United States – including fisheries, tourism, and coastal protection – is more than $3.4 billion a year.

===Aquaria===
{{main|Reef aquarium}}
[[File:Zoanthus-dragon-eye.jpg|right|thumb|This dragon-eye zoanthid is a popular source of color in reef tanks.]]
The saltwater fishkeeping hobby has expanded, over recent years, to include [[reef tanks]], fish tanks that include large amounts of [[live rock]] on which coral is allowed to grow and spread.<ref>[http://www.advancedaquarist.com/2011/1/corals Aquarium Corals: Collection and Aquarium Husbandry of Northeast Pacific Non-Photosynthetic Cnidaria] {{Webarchive|url=https://web.archive.org/web/20170606164618/http://www.advancedaquarist.com/2011/1/corals |date=2017-06-06 }}. Advancedaquarist.com (2011-01-14). Retrieved on 2016-06-13.</ref> These tanks are either kept in a natural-like state, with algae (sometimes in the form of an [[algae scrubber]]) and a deep sand bed providing filtration,<ref>[http://reefkeeping.com/issues/2008-10/newbie/index.php Reefkeeping 101 – Various Nutrient Control Methods]. Reefkeeping.com. Retrieved on 2016-06-13.</ref> or as "show tanks", with the rock kept largely bare of the algae and [[microfauna]] that would normally populate it,<ref>[http://saltaquarium.about.com/od/aquariummaintenancecare/a/sandlrcleaning.htm Aquarium Substrate & Live Rock Clean Up Tips] {{Webarchive|url=https://web.archive.org/web/20160806201853/http://saltaquarium.about.com/od/aquariummaintenancecare/a/sandlrcleaning.htm |date=2016-08-06 }}. Saltaquarium.about.com. Retrieved on 2016-06-13.</ref> in order to appear neat and clean.

The most popular kind of coral kept is soft coral, especially [[zoanthid]]s and mushroom corals, which are especially easy to grow and propagate in a wide variety of conditions, because they originate in enclosed parts of reefs where water conditions vary and lighting may be less reliable and direct.<ref>[http://marinebio.org/oceans/coral-reefs.asp Coral Reefs] {{webarchive|url=https://archive.today/20130625040003/http://marinebio.org/oceans/coral-reefs.asp |date=2013-06-25 }}. Marinebio.org. Retrieved on 2016-06-13.</ref> More serious fishkeepers may keep small polyp [[stony coral]], which is from open, brightly lit reef conditions and therefore much more demanding, while large polyp stony coral is a sort of compromise between the two.

===Aquaculture===
{{main|Aquaculture of coral}}
[[Coral aquaculture]], also known as ''coral farming'' or ''coral gardening'', is the cultivation of corals for commercial purposes or coral reef restoration. Aquaculture is showing promise as a potentially effective tool for restoring [[coral reef]]s, which have been declining around the world.<ref name="Horoszowski-Fridman">{{cite journal |vauthors=Horoszowski-Fridman YB, Izhaki I, Rinkevich B| year=2011 |title=Engineering of coral reef larval supply through transplantation of nursery-farmed gravid colonies |journal=Journal of Experimental Marine Biology and Ecology |volume=399 |issue=2| pages=162–66 |doi=10.1016/j.jembe.2011.01.005| bibcode=2011JEMBE.399..162H }}</ref><ref name="Pomeroy">{{cite journal |last1=Pomeroy |first1=Robert S. |last2=Parks |first2=John E. |last3=Balboa |first3=Cristina M. |year=2006 |title=Farming the reef: Is aquaculture a solution for reducing fishing pressure on coral reefs? |journal=Marine Policy |volume=30 |issue=2 |pages=111–30 |doi=10.1016/j.marpol.2004.09.001|bibcode=2006MarPo..30..111P }}</ref><ref name="Rinkevich">{{cite journal|author=Rinkevich B |year=2008 |title=Management of coral reefs: We have gone wrong when neglecting active reef restoration |url=http://www.ocean.org.il/Eng/_documents/Management-of-coral-reefs.pdf |archive-url=https://web.archive.org/web/20130523175241/http://www.ocean.org.il/Eng/_documents/Management-of-coral-reefs.pdf |archive-date=2013-05-23 |journal=Marine Pollution Bulletin |volume=56 |issue=11 |pages=1821–24 |doi=10.1016/j.marpolbul.2008.08.014 |pmid=18829052 |bibcode=2008MarPB..56.1821R }}</ref> The process bypasses the early growth stages of corals when they are most at risk of dying. Coral fragments known as "seeds" are grown in nurseries then replanted on the reef.<ref name="Ferse">{{cite journal |last1=Ferse |first1=Sebastian C.A. |year=2010|title=Poor Performance of Corals Transplanted onto Substrates of Short Durability |journal=Restoration Ecology |volume=18 |issue=4 |pages=399–407 |doi=10.1111/j.1526-100X.2010.00682.x|bibcode=2010ResEc..18..399F |s2cid=83723761 }}</ref> Coral is farmed by coral farmers who live locally to the reefs and farm for reef [[Conservation movement|conservation]] or for income. It is also farmed by scientists for research, by businesses for the supply of the live and ornamental coral trade and by private [[aquarium]] hobbyists.

==Gallery==
''Further images: [[commons:Category:Coral reefs]] and [[commons:Category:Corals]]''
<gallery mode="packed">
File:Mushroom Coral (Fungia) Top Macro 91.JPG|''[[Fungia]]'' sp. skeleton
File:Eusmilia fastigiata large.jpg|Polyps of ''[[Eusmilia fastigiata]]''
File:Dendrogyra cylindrus (pillar coral) (San Salvador Island, Bahamas) 1 (15513345363).jpg|[[Pillar coral]], ''Dendrogyra cylindricus''
File:Brain coral.jpg|[[Brain coral]], ''[[Diploria labyrinthiformis]]''
File:Stony coral spawning 3.jpg|Brain coral releasing eggs
File:EilatFringingReef.jpg|Fringing [[coral reef]] off the coast of [[Eilat]], [[Israel]].
File:000324-Corals-IMG 0418-2.jpg|Corals, Tis Beach, [[Chabahar]], [[Iran]]
File:000407-Corals-IMG 0693-2.jpg|Corals, Tis Beach, [[Chabahar]], [[Iran]]
</gallery>

==See also==
*[[Keystone species]]
*[[Ringstead Coral Bed]]

==References==
{{Reflist}}

==Sources==
* {{cite book|author1=Allen, G.R. |author2=R. Steene | title=Indo-Pacific Coral Reef Field Guide | date=1994 |publisher=Tropical Reef Research | isbn=978-981-00-5687-2}}
* {{cite book| author=Calfo, Anthony | title=Book of Coral Propagation | isbn=978-0-9802365-0-7| year=2007 | publisher=Reading Trees Publications }}
* {{cite book|author1=Colin, P.L. |author2=C. Arneson | title=Tropical Pacific Invertebrates
| date=1995 |publisher=Coral Reef Press | isbn=978-0-9645625-0-9}}
* {{cite book| author=Fagerstrom, J.A. | title=The Evolution of Reef Communities | date=1987 | publisher=Wiley | isbn=978-0-471-81528-0}}
* {{cite book| author=Gosliner, T. |author2=D. Behrens |author3=G. Williams | title=Coral Reef Animals of the Indo-Pacific, Animals Life from Africa to Hawaiʻi (invertebrates)| date=1996 |publisher=Sea Challengers | isbn=978-0-930118-21-1}}
* {{cite book| author=Nybakken, J.W. | title=Marine Biology, An Ecological Approach | date=2004 | publisher=Pearson/Benjamin Cummings | isbn=978-0-8053-4582-7}}
* {{cite book| author=Redhill, Surrey | title=Corals of the World: Biology and Field Guide }}
* {{cite book |author1=Segaloff, Nat |author2=Paul Erickson |title=A Reef Comes to Life. Creating an Undersea Exhibit |date=1991 |publisher=F. Watts |isbn=978-0-531-10994-6 |url-access=registration |url=https://archive.org/details/reefcomestolifec00sega }}
* {{cite book |last1=Sheppard |first1=Charles R.C.|last2=Davy |first2=Simon K. |last3=Pilling |first3=Graham M. |title=The Biology of Coral Reefs |url=https://books.google.com/books?id=toIeBQAAQBAJ&pg=PT78 |date=25 June 2009 |publisher=OUP Oxford|isbn=978-0-19-105734-2}}
* {{cite book| author=Veron, J.E.N. | title=Corals of Australia and the Indo-Pacific | date=1993 | publisher=University of Hawaii Press | isbn=978-0-8248-1504-2}}
* {{cite book| author=Wells, Susan | title=Coral Reefs of the World | year=1988 | publisher=IUCN, UNEP | isbn=978-2-88032-944-0 | url=https://archive.org/details/coralreefsofworl88well }}

==External links==
{{Wikispecies|Anthozoa|Coral}}
{{Commons category multi|Corals|Anthozoa}}
* [http://ocean.si.edu/ocean-life-ecosystems/coral-reefs Coral Reefs] The Ocean Portal by the [[Smithsonian Institution]]
* [[NOAA]] – [https://coralreef.noaa.gov/ Coral Reef Conservation Program]
* [[NOAA]] CoRIS – [http://www.coris.noaa.gov/ Coral Reef Biology]
* [[NOAA]] Office for Coastal Management – [https://coast.noaa.gov/states/fast-facts/coral-reefs.html Fast Facts – Coral Reefs]
* [[NOAA]] Ocean Service Education – [https://oceanservice.noaa.gov/education/tutorial_corals/welcome.html Corals]
* {{cite web |title= What is a coral? |url= http://www.stanford.edu/group/microdocs/whatisacoral.html |access-date= 2017-02-04 |publisher= Stanford microdocs project |archive-date= 2014-01-06 |archive-url= https://web.archive.org/web/20140106080751/http://www.stanford.edu/group/microdocs/whatisacoral.html }}

{{Good article}}
{{Corals}}
{{Authority control}}
{{Taxonbar|from=Q27685078}}

[[Category:Anthozoa]]
[[Category:Coral reefs|*]]