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 animals in class Anthozoa of phylum Cnidaria typically living 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 corals can catch small fish and plankton, using stinging cells on their tentacles, most corals obtain the majority of their energy and nutrients from photosynthetic unicellular algae called zooxanthellae that live within the coral's tissue. 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 located 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.



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] Those with eight tentacles are called octocorallia or Alcyonaria and comprise soft corals, sea fans and sea pens. Those with more than eight in a multiple of six are called hexacorallia or Zoantharia. This group includes reef-building corals (scleractinians), sea anemones and zoanthids.


Anatomy of a coral polyp

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.[5]

While a coral head appears to be a single organism, it is actually a group of many individual, yet genetically identical, polyps. The polyps are multicellular organisms. 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 exoskeleton called the basal plate or calicle (L. small cup). The calicle is formed by a thickened calcareous ring (annular thickening) with six supporting radial ridges (as shown below). These structures grow vertically and project into the base of the polyp. When a polyp is physically stressed, its tentacles contract into the calyx so that virtually no part is exposed above the skeletal platform. This protects the organism from predators and the elements.[6][7]

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.

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 be as much as 10 g/m² of polyp/day (0.3 ounce/sq yd/day). This is light dependent, with night-time production 90% lower than that during the middle of the day.[8]

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.

Nematocysts are stinging cells at the tips of the calices that carry poison, which they rapidly release in response to contact with another organism. The tentacles also bear a contractile band of epithelium called the pharynx. Jellyfish and sea anemones also carry nematocysts.

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 (0.0020–0.020 in) in diameter, and allow transport of both metabolites and cellular components.[9]

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

Many corals, as well as other cnidarian groups such as sea anemones (e.g. Aiptasia), form a symbiotic relationship with a class of algae, zooxanthellae, of the genus Symbiodinium. Aiptasia, while considered a pest among coral reef aquarium hobbyists, serves as a valuable model organism in the study of cnidarian-algal symbiosis. Typically, a polyp harbors one species of algae. Via photosynthesis, these provide energy for the coral, and aid in calcification.[10] The algae benefit from a safe environment, and consume the carbon dioxide and nitrogenous waste produced by the polyp. Due to the strain the algae can put on the polyp, stress on the coral often 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 chances of surviving short-term stress—they can regain algae at a later time. If the stressful conditions persist, the polyp eventually dies.[11]


Polyps feed on a variety of small organisms, from microscopic plankton to small fish. The polyp's tentacles immobilize or kill prey using their nematocysts (also known as 'cnidocysts'). The tentacles then contract to bring the prey into the stomach. Once digested, the stomach reopens, allowing the elimination of waste products and the beginning of the next hunting cycle.

These poisons are usually too weak to harm humans. An exception is fire coral.


Corals can be both gonochoristic (unisexual) and hermaphroditic, each of which can reproduce sexually and asexually. Reproduction also allows coral to settle new areas.


Life cycles of broadcasters and brooders

Corals predominantly reproduce sexually. About 25% of hermatypic corals (stony corals) form single sex (gonochoristic) colonies, while the rest are hermaphroditic.[12] 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.[13]

Planulae exhibits positive phototaxis, swimming towards light to reach surface waters, where they drift and grow before descending to seek a hard surface to which it can attach and establish a new colony. They also exhibit positive sonotaxis, moving towards sounds that emanate from the reef and away from open water.[14] High failure rates afflict many stages of this process, and even though millions of gametes are released by each colony, very few new colonies form. The time from spawning to settling is usually two to three days, but can be up to two months.[15] The larva grows into a polyp and eventually becomes a coral head by asexual budding and growth.

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

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 must rely on environmental cues, varying from species to species, to determine the proper time to release gametes into the water. The cues involve lunar changes, sunset time, and possibly chemical signalling.[12] Synchronous spawning may form hybrids and is perhaps involved in coral speciation.[16] In some places, the spawn can be visually dramatic, clouding the usually clear water with gametes, typically at night.

Corals use two methods for sexual reproduction, which differ in whether the female gametes are released:

  • Broadcasters, the majority of which mass spawn, rely heavily on environmental cues, because they release both sperm and eggs into the water. The corals use long-term cues, such as day length, water temperature, and/or rate of temperature change. The short-term cue is most often the lunar cycle, with sunset cuing the release.[12] About 75% of coral species are broadcasters, the majority of which are hermatypic, or reef-building corals.[12] The positively buoyant gametes float towards the surface, where fertilization produces planula larvae. The larvae swim towards the surface light to enter into currents, where they remain usually for two days, but can be up to three weeks, and in one known case two months,[15] after which they settle and metamorphose into polyps and form colonies.
  • Brooders are most often ahermatypic (not reef-building) in areas of high current or wave action. Brooders release only sperm, which is negatively buoyant, and can harbor unfertilized eggs for weeks, lowering the need for mass synchronous spawning events, which do sometimes occur.[12] After fertilization, the corals release planula larvae which are ready to settle.[10]


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

Within a coral head, the genetically identical polyps reproduce asexually, either via gemmation (budding) or division, both shown in the photo of Orbicella annularis. Budding involves a new polyp growing from an adult, whereas division forms two polyps each as large as the original.[13]

  • Budding expands colony size. It occurs when a new corallite grows out from an adult polyp. As the new polyp grows, it produces 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:
    • Intratentacular—from its oral discs, producing same-sized polyps within the ring of tentacles
    • Extratentacular—from its base, producing a smaller polyp
  • Longitudinal division begins when a polyp broadens and then divides its coelenteron. 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). The two new polyps must generate the missing pieces.

Asexual reproduction has several benefits for these sessile colonial organisms:[17]

  • Cloning allows fast 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.

Colony division

  • Fission occurs in some corals, especially among the family Fungiidae, where the colony splits into two or more colonies during early developmental stages.

Whole colonies can reproduce asexually through fragmentation or bailout, forming another individual colony with the same genotype.

  • 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 situations. The separated individuals can start new colonies.


Locations of coral reefs

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 cnidarians, mollusks, crustaceans, and many other animals.[18]


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.[19]

Hermatypic corals

Hermatypic or stony corals build reefs. With the help of zooxanthellae, they convert surplus food to calcium carbonate, forming a hard skeleton. Hermatypic species include Scleractinia, Millepora, Tubipora and Heliopora.[20]

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

  • Brain coral grow to 1.8 meters (5.9 ft) in width.
  • Acropora and staghorn coral grow fast and large, and are important reef-builders. Staghorn coral displays large, antler-like branches, and grows in areas with strong surf.
  • Galaxea fascicularis or star coral is another important reef-builder.
  • Pillar coral forms pillars which can grow to 3 meters (9.8 ft) in height.
  • Leptopsommia, or rock coral, appears almost everywhere in the Caribbean.[21]

Ahermatypic corals

Ahermatypic corals have no zooxanthellae and do not build reefs. They include Alcyonacea, as well as some Anthipatharia species (black coral, Cirripathes, Antipathes).[20] Ahermatypic corals, such as sea whips, sea feathers, and sea pens,[21] are also known as soft corals. Unlike stony corals, they are flexible, undulating back and forth 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.

Evolutionary history

Solitary rugose coral (Grewingkia) in three views; Ordovician, southeastern Indiana

Although corals first appeared in the Cambrian period,[22] some 542 million years ago, fossils are extremely rare until the Ordovician period, 100 million years later, when rugose and tabulate corals became widespread.

Tabulate corals occur in the 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 finally became extinct at the end of the Permian period, 250 million years ago. The skeletons of tabulate corals are composed of a form of calcium carbonate known as calcite.

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

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 become common in the Jurassic and later periods. Scleractinian skeletons are composed of a form of calcium carbonate known as aragonite.[23] 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.

Timeline of the major coral fossil record and developments from 650 m.y.a. to present.[24][25]

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

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, enabling geologists to date the age the rocks in which they are found.

Coral fossils are not restricted to reef remnants, and many solitary corals may be found elsewhere, such as Cyclocyathus, which occurs in England's Gault clay formation.

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.


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

Corals are highly sensitive to environmental changes. Scientists have predicted over 50% of the world's coral reefs may be destroyed by 2030;[26] as a result most nations protect them through environmental laws.

Seaweed/algae can destroy a coral reef. In the Caribbean and tropical Pacific, direct contact between ~40 to 70% of common seaweeds and coral causes bleaching and death to the coral via transfer of lipid–soluble metabolites.[27] Seaweed and algae proliferate given adequate nutrients and limited grazing by herbivores. Corals die if surrounding water temperature changes by more than a degree or two beyond their normal range or if water salinity drops. In an early symptom of environmental stress, corals expel their zooxanthellae; without their symbiotic algae, coral tissues become colorless as they reveal the white of their calcium carbonate skeletons, an event known as coral bleaching.[28]

Many governments now prohibit removal of coral from reefs, and use education to inform their populations about reef protection and ecology. However, many other human activities damage reefs, including runoff, mooring, fishing, diving, mining and construction.

Coral's narrow niche and the stony corals' reliance on calcium carbonate deposition makes them susceptible to changes in water pH. The increase in atmospheric carbon dioxide has caused enough dissolution of carbon dioxide to lower the ocean's pH, in a process known as ocean acidification. Lowered pH reduces corals' ability to produce calcium carbonate, and at the extreme, can dissolve their skeletons. Without deep and immediate cuts in anthropogenic CO2, many scientists fear acidification will severely degrade or destroy coral ecosystems.[29]

A section through a coral, dyed to determine growth rate

Importance 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. Unfortunately, these activities can have deleterious effects, such as the destruction of coral.

Coral reefs provide medical benefits for humans. Chemical compounds taken from corals are used in medicine for cancer, AIDS, pain, and other uses[citation needed]. Coral skeletons are also commonly used for bone grafting in humans.

Live coral is highly sought after for aquaria. Given the proper ecosystem, live coral makes a stunning addition to any saltwater aquarium. Soft corals are easier to maintain in captivity than hard corals.[30] Aquariums with corals are used by hobbyists in both tropical countries, as well as in countries in colder climates, where corals do not occur in the wild (sea). To eliminate destruction of corals in their indigenous regions, projects have been started to grow corals in colder countries, too. One such project is the KoralenKAS, which has been executed in practice by EcoDeco BV.[31][32]

Isididae may be usable as living bone implants and in aquatic cultivation, because of their potential to mimic valuable biological properties.[33]

In jewelry

Coral's many colors give it appeal for necklaces and other jewelry. Intensely red coral is prized as a gemstone. It is sometimes called fire coral, but is not the same as fire coral. Red coral is very rare because of overharvesting due to the great demand for perfect specimens.

In construction

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

Ancient 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.

In climate research

The annual growth bands in bamboo corals and others allow geologists to construct year-by-year chronologies, a form of incremental dating, which underlie high-resolution records of past climatic and environmental changes using geochemical techniques.[34]

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.[35]

Deep sea bamboo corals (Isididae) may be among the first organisms to display the effects of ocean acidification. They produce growth rings similar to those of tree, and can provide a view of changes in the condition in the deep sea over time.[36]


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.[37][38][39] 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.[40] 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.

See also


Further images: commons:Category:Coral reefs and commons:Category:Coral


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  • coral — [kôr′əl, kär′əl] n. [OFr < L coralium < Gr korallion < ? Heb gōrál, pebble or Ar garal, small stone] 1. the hard, stony skeleton secreted by certain marine polyps (class Anthozoa) and often deposited in extensive masses forming reefs and …   English World dictionary

  • coral — adjetivo 1. Área: música De la música de coro: Los cantos corales encierran cierta solemnidad. sustantivo masculino 1. Corallium rubrum. Animal celentéreo que tiene un esqueleto externo de color rojo o rosado y vive en colonias en mares… …   Diccionario Salamanca de la Lengua Española

  • coral — Es masculino cuando significa ‘celentéreo que vive en colonias, cuyo esqueleto calcáreo pulimentado se usa en joyería’: «Bajo el mar se pueden encontrar algas y corales gigantescos» (Lux Chile [Chile 1997]); y ‘composición vocal propia del culto… …   Diccionario panhispánico de dudas

  • coral — c.1300, from O.Fr. coral (12c., Mod.Fr.corail), from L. corallium, from Gk. korallion; perhaps of Sem. origin (Cf. Heb. goral small pebble, Arabic garal small stone ), originally just the red variety found in the Mediterranean, hence use of the… …   Etymology dictionary

  • Coral — (Франгокастелло,Греция) Категория отеля: Адрес: Frangokastello, Франгокастелло, 73011, Греция …   Каталог отелей

  • Coral — Le nom se rencontre en Lorraine, où il semble désigner un personnage amical, sympathique (sens de l adjectif coral en ancien français). On le trouve également en Catalogne et dans le Sud Ouest, où il peut avoir trois sens : soit un nom de… …   Noms de famille

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