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Pillar coral, Dendrogyra cylindricus
Scientific classification
Kingdom: Animalia
Phylum: Cnidaria
Class: Anthozoa
Ehrenberg, 1831
Extant Subclasses and Orders

[1][2]  See Anthozoa for details

Corals are marine invertebrates in the class (biology) Anthozoa of phylum Cnidaria. They typically live in compact colonies of many identical individual polyps. The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.

A coral "head" is a colony of myriad genetically identical polyps. Each polyp is a spineless animal typically only a few millimeters in diameter and a few centimeters in length. A set of tentacles surround a central mouth opening. An exoskeleton is excreted near the base. Over many generations, the colony thus creates a large skeleton that is characteristic of the species. Individual heads grow by asexual reproduction of polyps. Corals also breed sexually by spawning: polyps of the same species release gametes simultaneously over a period of one to several nights around a full moon.

Although some corals can catch small fish and plankton, using stinging cells on their tentacles, like those in sea anemone and jellyfish, most corals obtain the majority of their energy and nutrients from photosynthetic unicellular algae that live within the coral's tissue called zooxanthella (also known as Symbiodinium). Such corals require sunlight and grow in clear, shallow water, typically at depths shallower than 60 metres (200 ft). Corals can be major contributors to the physical structure of the coral reefs that develop in tropical and subtropical waters, such as the enormous Great Barrier Reef off the coast of Queensland, Australia. Other corals do not have associated algae and can live in much deeper water, with the cold-water genus Lophelia surviving as deep as 3,000 metres (9,800 ft).[3] Examples live on the Darwin Mounds, north-west of Cape Wrath, Scotland. Corals have also been found off the coast of the U.S. in Washington State and the Aleutian Islands in Alaska.


  • Taxonomy 1
    • Hermatypic corals 1.1
    • Ahermatypic corals 1.2
    • Perforate corals 1.3
  • Anatomy 2
    • Colonial form 2.1
    • Polyp 2.2
    • Exoskeleton 2.3
    • Tentacles 2.4
    • Nervous system 2.5
  • Ecology 3
    • Feeding 3.1
    • Intracellular symbionts 3.2
  • Reproduction 4
    • Sexual 4.1
      • Broadcasters 4.1.1
      • Brooders 4.1.2
      • Planulae 4.1.3
    • Asexual 4.2
    • Colony division 4.3
  • Reefs 5
  • Evolutionary history 6
  • Status 7
    • Threats 7.1
    • Protection 7.2
  • Relation to humans 8
    • Jewelry 8.1
    • Medicine 8.2
    • Construction 8.3
    • Climate research 8.4
    • Aquaria 8.5
    • Aquaculture 8.6
    • Slowing down green house effect 8.7
  • See also 9
  • Gallery 10
  • References 11
  • Further reading 12
  • External links 13


A short tentacle plate coral in Papua New Guinea

Corals divide into two subclasses, depending on the number of tentacles or lines of symmetry, and a series of orders corresponding to their exoskeleton: nematocyst type and mitochondrial genetic analysis.[1][2][4] Common coral typing crosses suborder/class boundaries.

Hermatypic corals

Hermatypic corals in the order Scleractinia are stony corals that build reefs. They mostly obtain at least part of their energy requirements from zooxanthella (Symbiodinium), symbiotic photosynthetic microalgae. They secrete calcium carbonate to form a hard skeleton. Those having six or fewer lines of symmetry in their body structure are called hexacorallia or Zoantharia. This group includes reef-building corals (scleractinians), sea anemones and zoanthids. Hermatypic genera include Scleractinia, Millepora, Tubipora and Heliopora.[5]

In the Caribbean alone, at least 50 species of uniquely structured hard coral exist. Well-known types include:

  • Brain corals grow to 1.8 meters (6 ft) in width.
  • Acropora and staghorn corals grow fast and large, and are important reef-builders. Staghorn coral displays large, antler-like branches, and grows in areas with strong surf.
  • Pillar coral forms pillars which can grow to 3 meters (10 ft) in height.
  • Leptopsammia, or rock coral, appears almost everywhere in the Caribbean.[6]

Ahermatypic corals

Ahermatypic corals have no zooxanthella (Symbiodinium). They have eight tentacles and are also called octocorallia. They include corals in subclass Alcyonacesdasda, as well as some species in order Anthipatharia (black coral, Cirripathes, Antipathes).[5] Ahermatypic corals, such as sea whips, sea feathers, and sea pens,[6] are also known as soft corals. Unlike stony corals, they are flexible, undulating in the current, and often are perforated, with a lacy appearance. Their skeletons are proteinaceous, rather than calcareous. Soft corals are somewhat less plentiful (in the Caribbean, twenty species appear) than stony corals.

Perforate corals

Corals can be perforate or imperforate. Perforate corals have porous skeletons, which allows their polyps to connect with each other through the skeleton. Imperforate corals have hard solid skeletons.[7][8]


Anatomy of a coral polyp
Flight through a µCT image stack of an Acropora coral from three views – note that the "arms" are mostly hollow. This coral had been hot glued into a stone and late grew over it.
Flight around a 3D object created from the data above.

The Muslim polymath Al-Biruni (d. 1048) classified sponges and corals as animals arguing that they respond to touch.[9] Nevertheless, people believed coral to be a plant until the 18th century, when William Herschel used a microscope to establish that coral had the characteristic thin cell membranes of an animal.[10]

Colonial form

The polyps interconnect by a complex and well-developed system of gastrovascular canals, allowing significant sharing of nutrients and symbiotes. In soft corals, these range in size from 50–500 micrometres (0.002–0.020 in) in diameter, and allow transport of both metabolites and cellular components.[11]

Close-up of Montastraea cavernosa polyps, the tentacles are clearly visible.


While the coral head is the familiar visual form of a single organism, it is actually a group of many individual, yet polyps. Polyps are usually a few millimeters in diameter, and are formed by a layer of outer epithelium and inner jellylike tissue known as the mesoglea. They are radially symmetrical, with tentacles surrounding a central mouth, the only opening to the stomach or coelenteron, through which food is ingested and waste expelled.


The stomach closes at the base of the polyp, where the epithelium produces an

  • What is a coral?
  • NOAA CoRIS – Coral Reef Biology
  • NOAA Ocean Service Education – Corals
  • coral reef identification guide for the Red Sea and Indo-Pacific

External links

  • Coral Reefs The Ocean Portal by Smithsonian Institution.
  • Coral, The Reef & Marine Aquarium Magazine. ISSN 1556-5769 Coral Magazine
  • Book of Coral Propagation by Anthony Calfo. ISBN 0-9802365-0-9
  • Coral Reefs of the World by Susan Wells
  • Corals of the World: Biology and Field Guide by Surrey Redhill
  • Marine Biology, An Ecological Approach, 6th edition, Nybakken, J.W. 2004. ISBN 0-8053-4582-5
  • Indo-Pacific Coral Reef Field Guide by Allen, G.R & R. Steene. 1994. ISBN 981-00-5687-7
  • Coral Reef Animals of the Indo-Pacific, Animals Life from Africa to Hawai‘i (invertebrates) by Gosliner, T., D. Behrens & G. Williams. 1996. ISBN 0-930118-21-9
  • Tropical Pacific Invertebrates by Colin, P.L. & C. Arneson. 1995. ISBN 0-9645625-0-2
  • Corals of Australia and the Indo-Pacific by Veron, J.E.N. 1993. ISBN 0-8248-1504-1
  • The Evolution of Reef Communities by Fagerstrom, J.A. 1987. ISBN 0-471-81528-4
  • A Reef Comes to Life. Creating an Undersea Exhibit by Segaloff, Nat, and Paul Erickson. 1991. ISBN 0-531-10994-1
  • SeaWorld – Coral reef bibliography

Further reading

  1. ^ a b Daly, M., Fautin, D.G., and Cappola, V.A. (March 2003). "Systematics of the Hexacorallia (Cnidaria: Anthozoa)". Zoological Journal of the Linnean Society 139 (3): 419–437.  
  2. ^ a b McFadden, C.S., France, S.C., Sanchez, J.A., and Alderslade, P. (December 2006). "A molecular phylogenetic analysis of the Octocorallia (Cnidaria: Anthozoa) based on mitochondrial protein-coding sequences". Molecular Phylogenentics and Evolution 41 (3): 413–527.  
  3. ^ Squires, D.F. (1959). "Deep sea corals collected by the Lamont Geological Observatory. 1. Atlantic corals". American Museum Novitates 1965: 1–42. 
  4. ^ France, S. C., P. E. Rosel, J. E. Agenbroad, L. S. Mullineaux, and T. D. Kocher (March 1996). "DNA sequence variation of mitochondrial large-subunit rRNA provides support for a two subclass organization of the Anthozoa (Cnidaria)". Molecular Marine Biology and Biotechnology 5 (1): 15–28.  
  5. ^ a b The Greenpeace Book of Coral Reefs
  6. ^ a b National Geographic Traveller:The Caribbean
  7. ^ Triefeldt, Laurie (2007) Plants & Animals Page 65. Quill Driver Books. ISBN 978-1-884956-72-0
  8. ^ Chisolm, Hugh (1911). The Encyclopædia britannica: a dictionary of arts, sciences, literature and general information 11th edition. Encyclopedia Britannica. p. 104. 
  9. ^ Egerton, Frank N. (2012). Roots of Ecology: Antiquity to Haeckel.  
  10. ^ The Light of Reason 8 August 2006 02:00 BBC Four
  11. ^ D. Gateno, A. Israel, Y. Barki and B. Rinkevich (1998). "Gastrovascular Circulation in an Octocoral: Evidence of Significant Transport of Coral and Symbiont Cells". The Biological Bulletin (Marine Biological Laboratory) 194 (2): 178–186.  
  12. ^ Barnes, R.D.k (1987). Invertebrate Zoology (5th ed.). Orlando, FL, USA: Harcourt Brace Jovanovich, Inc. pp. 149–163. 
  13. ^ Sumich, J. L. (1996). An Introduction to the Biology of Marine Life (6th ed.). Dubuque, IA, USA: Wm. C. Brown. pp. 255–269. 
  14. ^ "Anatomy of Coral". Marine Reef. 
  15. ^ a b c Murphy, Richard C. (2002). Coral Reefs: Cities Under The Seas. The Darwin Press, Inc.  
  16. ^ a b Madl, P. and Yip, M. (2000). "Field Excursion to Milne Bay Province – Papua New Guinea". Retrieved 2006-03-31. 
  17. ^ W. W. Toller, R. Rowan and N. Knowlton (2001). following Experimental and Disease-Associated Bleaching"M. faveolata and Montastraea annularis"Repopulation of Zooxanthellae in the Caribbean Corals . The Biological Bulletin (Marine Biological Laboratory) 201 (3): 360–373.  
  18. ^ a b c d Veron, J.E.N. (2000). Corals of the World. Vol 3 (3rd ed.). Australia: Australian Institute of Marine Sciences and CRR Qld Pty Ltd.  
  19. ^ a b Barnes, R. and; Hughes, R. (1999). An Introduction to Marine Ecology (3rd ed.). Malden, MA: Blackwell Science, Inc. pp. 117–141.  
  20. ^ Hatta, M., Fukami, H., Wang, W., Omori, M., Shimoike, K., Hayashibara, T., Ina, Y., Sugiyama, T. (1999). "Reproductive and genetic evidence for a reticulate evolutionary theory of mass spawning corals" (PDF). Molecular Biology and Evolution 16 (11): 1607–1613.  
  21. ^ Vermeij, Mark J. A.; Marhaver, Kristen L.; Huijbers, Chantal M.; Nagelkerken, Ivan; Simpson, Stephen D. (2010). "Coral Larvae Move toward Reef Sounds". PLoS ONE 5 (5): e10660.  
  22. ^ Jones, O.A. and R. Endean. (1973). Biology and Geology of Coral Reefs. New York, USA: Harcourt Brace Jovanovich. pp. 205–245.  
  23. ^ Gulko, David (1998). Hawaiian Coral Reef Ecology. Honolulu, Hawaii: Mutual Publishing. p. 10.  
  24. ^ Spalding, Mark, Corinna Ravilious, and Edmund Green (2001). World Atlas of Coral Reefs. Berkeley, CA, USA: University of California Press and UNEP/WCMC. pp. 205–245.  
  25. ^ Pratt, B.R.; Spincer, B.R., R.A. Wood and A.Yu. Zhuravlev (2001). "12: Ecology and Evolution of Cambrian Reefs". Ecology of the Cambrian Radiation. Columbia University Press. p. 259.  
  26. ^ Ries, J.B., Stanley, S.M., Hardie, L.A.; Stanley; Hardie (July 2006). "Scleractinian corals produce calcite, and grow more slowly, in artificial Cretaceous seawater". Geology 34 (7): 525–528.  
  27. ^ Ben M. Waggoner (2000). David Smith and Allen Collins, ed. "Anthozoa: Fossil Record". Anthozoa.  
  28. ^ William A. Oliver, Jr. (2003). "Corals: Table 1". Fossil Groups.  
  29. ^ "Coral reefs around the world".  
  30. ^ a b "Threats to Coral Reefs".  
  31. ^ Losing Our Coral Reefs – Eco Matters – State of the Planet. Retrieved on 2011-11-01.
  32. ^ Kleypas, J.A.; Feely, R.A.; Fabry, V.J.; Langdon, C.; Sabine, C.L.; Robbins (2006). "Impacts of Ocean Acidification on Coral Reefs and Other Marine Calcifiers: A guide for Future Research".  
  33. ^ Save Our Seas, 1997 Summer Newsletter, Dr. Cindy Hunter and Dr. Alan Friedlander
  34. ^ Tun, K.; Chou, L.M.; Cabanban, A.; Tuan, V.S.; Philreefs; Yeemin, T.; Suharsono; Sour, K.; Lane, D. (2004). "Status of Coral Reefs, Coral Reef Monitoring and Management in Southeast Asia, 2004". In Wilkinson, C. Status of Coral Reefs of the world: 2004. Townsville, Queensland, Australia: Australian Institute of Marine Science. pp. 235–276. 
  35. ^ Burke, Lauretta; Reytar, K.; Spalding, M.; Perry, A. (2011). Reefs at risk revisited. Washington, DC: World Resources Institute. p. 38.  
  36. ^ Norlander (8 December 2003). "Coral crisis! Humans are killing off these bustling underwater cities. Can coral reefs be saved? (Life science: corals)". Science World. 
  37. ^ Rasher DB, Hay ME; Hay (May 2010). "Chemically rich seaweeds poison corals when not controlled by herbivores". Proceedings of the National Academy of Sciences of the United States of America 107 (21): 9683–8.  
  38. ^  
  39. ^ "Submarine Springs Offer Preview of Ocean Acidification Effects On Coral Reefs". ScienceDaily. Nov 28, 2011. Retrieved 5 December 2011. 
  40. ^ "Phoenix Rising". National Geographic Magazine. January 2011. Retrieved April 30, 2011. 
  41. ^ EcoDeco EcologicalTechnology. Retrieved on 2011-11-29.
  42. ^ KoralenKAS project. Retrieved on 2011-11-29.
  43. ^ Hunt, Chloe V.; Harvey, James J.; Miller, Anne; Johnson, Vivienne; Phongsuwan, Niphon (2013). "The Green Fins approach for monitoring and promoting environmentally sustainable scuba diving operations in South East Asia". Ocean & Coastal Management 78: 35–44.  
  44. ^ "Eight great soft corals for new reefkeepers". AquaDaily. 2008-12-05. Retrieved 2009-01-02. 
  45. ^ Magsaysay, Melissa (June 21, 2009). "Coral makes a splash". Los Angeles Times. Retrieved January 12, 2013. 
  46. ^ H. Ehrlich, P. Etnoyer, S. D. Litvinov, et al; Etnoyer, P.; Litvinov, S. D.; Olennikova, M.M.; Domaschke, H.; Hanke, T.; Born, R.; Meissner, H.; Worch, H. (2006). "Biomaterial structure in deep-sea bamboo coral (Anthozoa: Gorgonacea: Isididae)". Materialwissenschaft und Werkstofftechnik ( 37 (6): 552.  
  47. ^ Reddy PN, Lakshmana M, Udupa UV; Lakshmana; Udupa (December 2003). "Effect of Praval bhasma (Coral calx), a natural source of rich calcium on bone mineralization in rats". Pharmacological Research 48 (6): 593–9.  
  48. ^ "National Oceanic and Atmospheric Administration – New Deep-Sea Coral Discovered on NOAA-Supported Mission". Retrieved 2009-05-11. 
  49. ^ Schrag, D.P. and Linsley, B.K. (2002). "Corals, Chemistry, and Climate". Science 296 (8): 277–278.  
  50. ^ Smithers, Scott G.; Woodroffe, Colin D. (2000). "Microatolls as sea-level indicators on a mid-ocean atoll". Marine Geology 168 (1–4): 61–78.  
  51. ^ Hoegh-Guldberg O. (1999). "Climate change, coral bleaching and the future of the world’s coral reefs". Marine and Freshwater Research 50: 839–99.  
  52. ^ a b c Hughes, T., Baird, A., Bellwood, D., Card, M., Connolly, S., Folke, C., Grosberg, R., Hoegh-Guldberg, O., Jackson, J., Klepas, J., Lough, J., Marshall, P., Nystrom, M., Palumbi, S., Pandolfi, J., Rosen, B., and Roughgarden, J. (2003). "Climate change, Human Impacts, and the Reslience of Coral Reefs". Science 301: 929–33.  
  53. ^ a b Parmesan, C. (2006). "Ecological and Evolutionary Responses to Recent Climate Change". Annu Rev Ecol Evol and Syst 37: 637–69.  
  54. ^ Baker, A. (2004). "Corals’ adaptive response to climate change". Nature 430: 741.  
  55. ^ a b Donner, S., Skirving, W., Little, C., Oppenheimer, M., and Hoegh-Guldenberg (2005). "Global assessment of coral bleaching and required rates of adaptation under climate change". Glob Chang Biol 11: 2251–65.  
  56. ^ Baskett, M., Gaines, S., and Nisbet, R. (2009). "Symbiont diversity may help coral reefs survive moderate climate change.". Ecol Appl 19: 3–17.  
  57. ^ McClanahan, T., Ateweberhan, M., Muhando, C., Maina, J., and Mohammed, M. (2007). "Effects of Climate and Seawater Temperature Variation on Coral Bleaching and Morality". Ecol Monogr 77: 503–25.  
  58. ^ Aquarium Corals: Collection and Aquarium Husbandry of Northeast Pacific Non-Photosynthetic Cnidaria
  59. ^ Reefkeeping 101 - Various Nutrient Control Methods
  60. ^ Aquarium Substrate & Live Rock Clean Up Tips
  61. ^ Excellent Beginner Corals for a Reef Aquarium
  62. ^ Coral Reefs
  63. ^ Horoszowski-Fridman YB, Izhaki I, Rinkevich B; Izhaki; Rinkevich (2011). "Engineering of coral reef larval supply through transplantation of nursery-farmed gravid colonies". Journal of Experimental Marine Biology and Ecology 399 (2): 162–166.  
  64. ^ Pomeroy, Robert S.; Parks, John E.; Balboa, Cristina M. (2006). "Farming the reef: Is aquaculture a solution for reducing fishing pressure on coral reefs?". Marine Policy 30 (2): 111–30.  
  65. ^ Rinkevich B (2008). "Management of coral reefs: We have gone wrong when neglecting active reef restoration" (PDF). Marine pollution bulletin 56 (11): 1821–1824.  
  66. ^ Ferse, Sebastian C.A. (2010). "Poor Performance of Corals Transplanted onto Substrates of Short Durability". Restoration Ecology 18 (4): 399–407.  
  67. ^ Ho, Lih Der (2002). Taiwan de shan hu jiao. Taiwan: Walkers Cultural. p. 196. 


Further images: and


See also

When corals secrete calcium carbonate to form their skeletons, the saturation of carbonite in sea will be lowered. This will make carbon dioxide in air dissolve into the sea so that carbon dioxide in air will be reduced, in which slowing down green house effect.[67]

Slowing down green house effect

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 reefs, which have been declining around the world.[63][64][65] The process bypasses the early growth stages of corals when they are most at risk of dying. Coral seeds are grown in nurseries then replanted on the reef.[66] Coral is farmed by coral farmers who live locally to the reefs and farm for reef 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.


The most popular kind of coral kept is soft coral, especially zoanthids and mushroom corals, which are especially easy to grow and propagate in a wide variety of conditions,[61] because they originate in enclosed parts of reefs where water conditions vary and lighting may be less reliable and direct.[62] 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.

The saltwater fishkeeping hobby has increasingly 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.[58] 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,[59] or as "show tanks", with the rock kept largely bare of the algae and microfauna that would normally populate it,[60] in order to appear neat and clean.

This dragon-eye zoanthid is a popular source of color in reef tanks


Another coral evolutionary response are refugia population shifts as a reaction to temperature pressures. In the Gulf of Mexico, where sea temperatures are indeed rising, there has been a migration of cold-sensitive staghorn and elkhorn coral.[53] Not only have the symbionts and specific species been shown to shift, there seems to be a certain growth rate favorable to selection. It has been found that slower growing coral have become more common as they are more heat tolerant.[56] However, the changes in temperature and acclimation are complex. Some reefs in current shadows represent a refugium location that will help them adjust to the disparity in the environment even if eventually the temperatures may be rising more quickly there than other locations.[57] This vicariance due to climatic barriers causes a realized niche to shrink greatly in comparison to the old fundamental niche. Scientists have found direct evidence of selection acting upon this coral life system by exploring different angles of adaptation, but conclusions are difficult to draw.

This trade off indicates an evolutionary shift of what traits are most selected for in the current environment in terms of survival. [55] However, evidence of coral

Though coral have big populations and phenomenal sexual capabilities as well as zooxanthellae being diverse, evolution is sometimes slowed by abundant selection, adaptation, and successful evolution of a species.

Increasing sea temperatures in tropical regions (~1 degree C) the last century have caused major coral bleaching, death, and therefore shrinking coral populations since although they are able to adapt and acclimate, it is uncertain if this evolutionary process will happen quickly enough to prevent major reduction of their numbers.[51]

Certain species form communities called microatolls, 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 radioactive carbon dating. Such methods can help to reconstruct Holocene sea levels.[50]

[49] techniques.geochemical changes using environmental and climatic, which underlie high-resolution records of past incremental dating to construct year-by-year chronologies, a form of geologists They allow [48] The annual growth bands in

Climate research

Coral reefs on land provide lime for use as building blocks ("coral rag"). Coral rag is an important local building material in places such as the East African coast.

Tabulate coral (a syringoporid); Boone limestone (Lower Carboniferous) near Hiwasse, Arkansas, scale bar is 2.0 cm


In medicine, chemical compounds from corals are used for cancer, AIDS, pain, and other uses. Coral skeletons, e.g. Isididae are also used for bone grafting in humans.[46] Coral Calx, known as Praval Bhasma in Sanskrit, is widely used in traditional system of Indian medicine as a supplement in the treatment of a variety of bone metabolic disorders associated with calcium deficiency.[47]


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.[45]

6-Strand Necklace, Navajo (Native American), ca. 1920s, Brooklyn Museum


Live coral is highly sought after for aquaria. Soft corals are easier to maintain in captivity than hard corals.[44]

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 has been proven to mitigate these risks.[43]

Relation to humans

To eliminate destruction of corals in their indigenous regions, projects have been started to grow corals in non-tropical countries.[41][42]

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.[30]

A section through a coral, dyed to determine growth rate

Marine Protected Areas (MPAs), Biosphere reserves, marine parks, national monuments world heritage status, fishery management and habitat protection can protect reefs from anthropogenic damage.[40]

A diversity of corals


Submarine springs found along the coast of Mexico's Yucatán Peninsula produce water with a naturally low pH (a measure of acidity) providing conditions similar to those expected to become widespread as the oceans absorb carbon dioxide. 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.[39]

Water temperature changes of more than 1–2 °C (1.8–3.6 °F) 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.[38]

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 metabolites.[37] Seaweed and algae proliferate given adequate nutrients and limited grazing by herbivores such as parrotfish.

General estimates show approximately 10% of the world's coral reefs are dead.[32][33][34] About 60% of the world's reefs are at risk due to human-related activities.[35] The threat to reef health is particularly strong in Southeast Asia, where 80% of reefs are endangered. Over 50% of the world's coral reefs may be destroyed by 2030; as a result, most nations protect them through environmental laws.[36]

Coral reefs are under stress around the world.[29] In particular, coral mining, agricultural and urban runoff, overfishing, blast fishing, disease, and the digging of canals 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.[30] In 1998, 16% of the world's reefs died as a result of increased water temperature.[31]

A healthy coral reef has a striking level of biodiversity in many forms of marine life.



A Petoskey stone is a rock and a fossil, often pebble-shaped, that is composed of a fossilized coral, Hexagonaria percarinata. They are found predominantly in Michigan's Upper Peninsula, and the northwestern portion of Michigan's Lower Peninsula.

Fossils of fellow reef-dwellers algae, sponges, and the remains of many echinoids, brachiopods, bivalves, gastropods, and trilobites appear along with coral fossils. This makes some corals useful index fossils that enabled geologists to date the rocks in which they are found. Coral fossils are not restricted to reef remnants, and many solitary fossils may be found elsewhere, such as Cyclocyathus, which occurs in England's Gault clay formation.

At certain times in the geological past, corals were very abundant. Like modern corals, these ancestors built reefs, some of which ended as great structures in sedimentary rocks.

The scleractinian corals filled the niche vacated by the extinct rugose and tabulate species. Their fossils may be found in small numbers in rocks from the Triassic period, and became common in the Jurassic and later periods. Scleractinian skeletons are composed of a form of calcium carbonate known as aragonite.[26] Although they are geologically younger than the tabulate and rugose corals, their aragonitic skeleton is less readily preserved, and their fossil record is less complete.

Rugose corals became dominant by the middle of the Silurian period, and became extinct early in the Triassic period. The rugose corals existed in solitary and colonial forms, and were also composed of calcite.

Tabulate corals occur in limestones and calcareous shales of the Ordovician and Silurian periods, and often form low cushions or branching masses alongside rugose corals. Their numbers began to decline during the middle of the Silurian period, and they became extinct at the end of the Permian period, . The skeletons of tabulate corals are composed of a form of calcium carbonate known as calcite.

Although corals first appeared in the Cambrian period,[25] some , fossils are extremely rare until the Ordovician period, 100 million years later, when rugose and tabulate corals became widespread.

Right frame 
Horn coral fossil.
Solitary rugose coral (Grewingkia) in three views; Ordovician, southeastern Indiana

Evolutionary history

The hermatypic, stony corals are often found in coral reefs, large calcium carbonate structures generally found in shallow, tropical water. Reefs are built up from coral skeletons, and are held together by layers of calcium carbonate produced by coralline algae. Reefs are extremely diverse marine ecosystems hosting over 4,000 species of fish, massive numbers of cnidaria, mollusks, crustacea, and many other animals.[24]

Locations of coral reefs


  • 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.

Whole colonies can reproduce asexually, forming two colonies with the same genotype.

Colony division

  • Cloning allows high reproduction rates, supporting rapid habitat exploitation.
  • Modular growth allows biomass to increase without a corresponding decrease in surface-to-volume ratio.
  • Modular growth delays senescence, by allowing the clone-type to survive the loss of one or more modules.
  • New modules can replace dead modules, reducing clone-type mortality and preserving the colony's territory.
  • Spreading the clone type to distant locations reduces clone-type mortality from localized threats.

Asexual reproduction has several benefits for these sessile colonial organisms:[23]

Division forms two polyps each as large as the original. Longitudinal division begins when a polyp broadens and then divides its coelenteron, analogous to splitting a log along its length. The mouth also divides and new tentacles form. The two "new" polyps 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), similar to cutting the end off a log. The new polyps must separately generate the missing pieces.

  • Intratentacular—from its oral discs, producing same-sized polyps within the ring of tentacles
  • Extratentacular—from its base, producing a smaller polyp

Budding involves splitting a smaller polyp from an adult.[19] As the new polyp grows, it forms its body parts. The distance between the new and adult polyps grows, and with it, the coenosarc (the common body of the colony; see coral anatomy). Budding can be:

Within a coral head, the genetically identical polyps reproduce asexually, either via gemmation (budding) or by longitudinal or transversal division, both shown in the photo of Orbicella annularis.

The tabulate coral Aulopora (Devonian) showing initial budding from protocorallite
Calices (basal plates) of Orbicella annularis showing multiplication by gemmation (small central calice) and division (large double calice)


Planulae exhibit positive phototaxis, swimming towards light to reach surface waters, where they drift and grow before descending to seek a hard surface to which they can attach and begin a new colony. They also exhibit positive sonotaxis, moving towards sounds that emanate from the reef and away from open water.[21] High failure rates afflict many stages of this process, and even though millions of gametes are released by each colony, few new colonies form. The time from spawning to settling is usually two to three days, but can be up to two months.[22] The larva grows into a polyp and eventually becomes a coral head by asexual budding and growth.


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 on to the waiting egg carriers who harbor unfertilized eggs for weeks. Synchronous spawning events sometimes occurs even with these species.[18] After fertilization, the corals release planula that are ready to settle.[16]


Synchronous spawning is very typical on the coral reef, and often, even when multiple species are present, all corals spawn on the same night. This synchrony is essential so male and female gametes can meet. Corals rely on environmental cues, varying from species to species, to determine the proper time to release gametes into the water. The cues involve temperature change, lunar cycle, day length, and possibly chemical signalling.[18] Synchronous spawning may form hybrids and is perhaps involved in coral speciation.[20] The immediate cue is most often sunset, which cues the release.[18] The spawning event can be visually dramatic, clouding the usually clear water with gametes.

A male star coral, Montastraea cavernosa, releases sperm into the water.

About 75% of all hermatypic corals "broadcast spawn" by releasing gameteseggs and sperm—into the water to spread offspring. The gametes fuse during fertilization to form a microscopic larva called a planula, typically pink and elliptical in shape. A typical coral colony forms several thousand larvae per year to overcome the odds against formation of a new colony.[19]


Corals predominantly reproduce sexually. About 25% of hermatypic corals (stony corals) form single sex (gonochoristic) colonies, while the rest are hermaphroditic.[18]

Life cycles of broadcasters and brooders


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.


The algae benefit from a safe place to live and consume the polyp's carbon dioxide and nitrogenous waste. Due to the strain the algae can put on the polyp, stress on the coral often drives them to eject the algae. Mass ejections are known as coral bleaching, because the algae contribute to coral's brown coloration; other colors, however, are due to host coral pigments, such as green fluorescent proteins (GFPs). Ejection increases the polyp's chance of surviving short-term stress—they can regain algae, possibly of a different species at a later time. If the stressful conditions persist, the polyp eventually dies.[17]

Light and confocal images of Symbiodinium cells in hospite (living in a host cell) within scyphistomae of the jellyfish Cassiopea xamachana.

Many corals, as well as other calcification.[16] As much as 30% of the tissue of a polyp may be plant material.[15]:23

Intracellular symbionts

:24[15] Polyps feed on a variety of small organisms, from microscopic



Modified epidermal cells -- neurons -- contain long, thin strand-like processes, called neurites, synapsing into each other and creating a basic neural network. Corals and other cnidarians have two separate networks: one fast conducting and one slow conducting. It is believed that corals were one of the first animals to evolve such networks -- sponges have ion channels and inactive DNA for neuron production, but corals realise them fully.

Nervous system

epithelium called the pharynx. Jellyfish and sea anemones also carry nematocysts.


Nematocyst discharge: A dormant nematocyst discharges response to nearby prey touching the cnidocil, the operculum flap opens, and its stinging apparatus fires the barb into the prey, leaving a hollow filament through which poisons are injected to immobilise the prey, then the tentacles manoeuvre the prey to the mouth.

Formation of the calcareous exoskeleton involves deposition of the mineral aragonite by the polyps from calcium and carbonate ions they acquire from seawater. The rate of deposition, while varying greatly across species and environmental conditions, can reach 10 g/m² per day (0.3 ounce/sq yd/day). This is light dependent, with night-time production 90% lower than that during the middle of the day.[14]

The polyp grows by extension of vertical calices which occasionally septate to form a new, higher, basal plate. Over many generations, this extension forms the large calcareous structures of corals and ultimately coral reefs.


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