Coral

Corals
Pillar coral, Dendrogyra cylindricus
Scientific classification
Kingdom:	Animalia
Phylum:	Cnidaria
Class:	Anthozoa Ehrenberg, 1831
Extant Subclasses and Orders
Alcyonaria    Alcyonacea    Helioporacea Zoantharia    Antipatharia    Corallimorpharia    Scleractinia    Zoanthidea   See Anthozoa for details

For other uses, see Coral (disambiguation).

Corals are marine animals from the class Anthozoa and exist as small sea anemone-like polyps, typically in colonies of many identical individuals. The group includes the important reef builders that are found in tropical oceans, which secrete calcium carbonate to form a hard skeleton.

A coral "head", commonly perceived to be a single organism, is actually formed of thousands of individual, genetically identical polyps, each polyp only a few millimeters in diameter. Over thousands of generations, the polyps lay down a skeleton that is characteristic of their species. A head of coral grows by asexual reproduction of the individual polyps. Corals also breed sexually by spawning, with corals of the same species releasing gametes simultaneously over a period of one to several nights around a full moon.

Although corals can catch plankton using stinging cells on their tentacles, these animals obtain most of their nutrients from symbiotic unicellular algae called zooxanthellae. Consequently, most corals depend on sunlight and grow in clear and shallow water, typically at depths under 60 m (200 ft). These 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, such as in the Atlantic, with the cold-water genus Lophelia surviving as deep as 3000 m.

Phylogeny

Main article: Anthozoa

Corals belong to the class Anthozoa and are divided into two subclasses, depending on the number of tentacles or lines of symmetry, and a series of orders corresponding to their exoskeleton, nematocyst type, and mitochondrial genetic analysis. 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 the reef-building corals, sea anemones and zoanthids.

Anatomy

Anatomy of a coral polyp

While a coral head appears to be a single organism, it is actually a head of many individual, yet genetically identical, polyps. The polyps are multicellular organisms that feed on a variety of small organisms, from microscopic plankton to small fish.

Polyps are usually a few millimeters in diameter, and are formed by a layer of outer epithelium and inner jellylike tissue known as the mesoglea. They are radially symmetrical with tentacles surrounding a central mouth, the only opening to the stomach or coelenteron, through which both food is ingested and waste expelled.

The stomach closes at the base of the polyp, where the epithelium produces an exoskeleton called the basal plate or calicle (L. small cup). This is formed by a thickened calciferous ring (annular thickening) with six supporting radial ridges (as shown below). These structures grow vertically and project into the base of the polyp allowing it to retreat into the exoskeleton for protection.

The polyp grows by extension of vertical calices which are occasionally septated to form a new, higher, basal plate. Over many generations this extension forms the large calciferous (Calcium containing) structures of corals and ultimately coral reefs.

Formation of the calciferous exoskeleton involves deposition of the mineral aragonite by the polyps from calcium ions they aquire from seawater. The rate of deposition, while varying greatly between species and environmental conditions, can be as much as 10 g / m² of polyp / day (0.3 ounce / sq yd / day). This is light dependent, with night-time production 90% lower than that at midday.

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

The polyp's tentacles trap prey using stinging cells called nematocysts. These are cells modified to capture and immobilize prey, such as plankton, by injecting poisons, firing very rapidly in response to contact. In fire corals these poisons are potent enough to harm humans. Nematocysts can also be found in jellyfish and sea anemones. The toxins injected by nematocysts immobilize or kill prey, which can then be drawn into the polyp's stomach by the tentacles through a contractile band of epithelium called the pharynx.

The polyps are interconnected by a complex and well developed system of gastrovascular canals allowing significant sharing of nutrients and symbiotes. In soft corals these range in size from 50-500 μm in diameter and to allow transport of both metabolites and cellular components.

Close-up of Montastrea cavernosa polyps. Tentacles are clearly visible.

Aside from feeding on plankton, many corals enjoy a symbiotic relationship with a class of algae, zooxanthellae, of the genus Symbiodinium. Typically a polyp will harbour one particular species of algae. Via photosynthesis, these provide energy for the coral, and aid in calcification. The algae benefit from a safe environment, and use the carbon dioxide and nitrogenous waste produced by the polyp. Due to the strain the algae can put on the polyp, stress on the coral often triggers ejection of the algae, known on a large scale as coral bleaching as it is the algae that give coral colour. This increases the polyps' chances of surviving stressful periods - they can regain the algae at a later time. If the conditions persist, the polyps - and corals - will eventually die.

Reproduction

Life cycles of broadcasters and brooders

Sexual

Corals predominantly reproduce sexually, with 25% of hermatypic corals (stony corals) forming single sex (gonochoristic) colonies, whilst the rest are hermaphroditic. About 75% of all hermatypic corals "broadcast spawn" by releasing gametes - eggs and sperm - into the water to spread colonies over large distances. The gametes fuse during fertilisation to form a microscopic larvum called a planula, typically pink and elliptical in shape; a moderately sized coral colony can form several thousands of these larva per year to overcome the huge odds against formation of a new colony.

The planula swims towards light, exhibiting positive phototaxis, to surface waters where they drift and grow for a time before swimming back down to locate a surface on which it can attach and establish a new colony. At many stages of this process there are high failure rates, and even though millions of gametes are released by each colony very few new colonies are formed. The time from spawning to settling is often 2-3 days, but can be up to 2 months. The larva grows into a coral polyp and eventually becomes a coral head by asexual budding and growth, creating new polyps.

Corals that do not broadcast spawn are called brooders, with most non-stony corals displaying this characteristic. These corals release sperm but harbour the eggs, allowing larger, negatively buoyant, planulae to form which are later released ready to settle. The larva grows into a coral polyp and eventually becomes a coral head by asexual budding and growth to create new polyps.

Calices (basal plates) of Orbicella annularis showing two methods of multiplication: gemmation (small central calicle) and division (large double calicle).

Synchronous spawning is very typical on a coral reef and often, even when there are multiple species present, all the corals on the reef release gametes during the same night. This synchrony is essential so that male and female gametes can meet and form planula. The cues that guide the release are complex, but over the short term involve lunar changes, sunset time, and possibly chemical signalling. Synchronous spawning may have the result of forming coral hybrids, perhaps involved in coral speciation. In some places the coral spawn can be dramatic, usually occurring at night, where the usually clear water becomes cloudy with gametes.

Asexual

Within a head of coral the genetically identical polyps reproduce asexually to allow growth of the colony. This is achieved either through gemmation or budding or through division, both shown in the diagrams of Orbicella annularis. Budding involves a new polyp growing from an adult, whereas division forms two polyps each as large as the original.

Whole colonies can reproduce asexually through fragmentation, where a piece broken off a coral head and moved by wave action can continue to grow in a new location.

Coral reefs

Locations of coral reefs

Main article: Coral reef

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 held together by layers of calcium carbonate produced by coralline algae. Reefs are extremely diverse marine ecosystems being host to over 4,000 species of fish, massive numbers of cnidarians, molluscs, crustaceans, and many other animals.

Geological history

The fossil coral Heliophyllum halli from the Devonian of Canada

Although corals first appeared in the Cambrian period, 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 like the Tabulate corals their skeletons are also composed of calcite.

The Scleractinian corals filled the niche vacated by the extinct Rugose and Tabulate corals. Their fossils may be found in small numbers in rocks from the Triassic period, and become relatively common in rocks from the Jurassic and later periods. The skeletons of Scleractinian corals are composed of a form of calcium carbonate known as aragonite. 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.

At certain times in the geological past corals were very abundant, just as modern corals are in the warm clear tropical waters of certain parts of the world today. Like modern corals their ancestors built reefs, some of which now lie as great structures in sedimentary rocks.

Fossil tabulate coral, Favosites sp.

These ancient reefs are not composed entirely of corals. Algae, sponges, and the remains of many echinoids, brachiopods, bivalves, gastropods, and trilobites that lived on the reefs are preserved within them. This makes some corals useful index fossils, enabling geologists to date the age the rocks in which they are found.

Corals are not restricted to reefs, and many solitary corals may be found in rocks where reefs are not present, such as Cyclocyathus which occurs in England's Gault clay formation.

Environmental effects on coral

A coral reef can be an oasis for marine life.

Corals are highly sensitive to environmental changes. Scientists have predicted that over 50% of the coral reefs in the world may be destroyed by the year 2030; as a result they are generally protected through environmental laws. A coral reef can easily be swamped in algae if there are too many nutrients in the water. Coral will also die if the water temperature changes by more than a degree or two beyond its normal range or if the salinity of the water drops. In an early symptom of environmental stress, corals expel their zooxanthellae; without their symbiotic unicellular algae, coral tissues become colorless as they reveal the white of their calcium carbonate skeletons, an event known as coral bleaching.

Many governments now prohibit removal of coral from reefs to reduce damage by divers. Damage is still caused by anchors dropped by dive boats or fishermen. In places where local fishing causes reef damage, education schemes have been run to inform the population about reef protection and ecology.

The narrow niche that coral occupies, and the stony corals' reliance on calcium carbonate deposition, means they are very susceptible to changes in pH. Ocean acidification, caused by dissolution of carbon dioxide in the water, is currently occurring due to an increase in atmospheric carbon dioxide. This changes the balance of production and dissolution of calcium carbonate, leading to destruction of corals.

A section through a coral, dyed to determine growth rate

A combination of temperature changes, pollution, and overuse by divers and jewelry producers has led to the destruction of many coral reefs around the world. This has increased the importance of coral biology as a discipline
Climatic variations can cause temperature changes that destroy corals. For example, during the 1997-98 warming event all the hydrozoan Millepora boschmai colonies near Panamá were bleached and died within six years - this species is now thought to be extinct.

Uses

Live corals

Local economies near major coral reefs benefit from an abundance of fish and octopus. Reefs also provide recreational scuba diving and snorkeling tourism. Alas, all these activities can also have deleterious effects, such as removal or accidental destruction of coral.

Red shades of coral are sometimes used as a gemstone, especially in Tibet. In vedic astrology, red coral represents Mars. Pure red coral is known as 'fire coral' and is very rare because of the demand for perfect fire coral in jewellery-making.

Corals are more colourful when alive than dead. Photo credit:William Rafti

Ancient corals

Ancient coral reefs on land are often mined for lime or use as building blocks ("coral rag"), for example the Portland limestone of the Isle of Portland. Coral rag is an important local building material in places such as the east African coast.

Some coral species exhibit banding in their skeletons resulting from annual variations in their growth rate. In fossil and modern corals these bands allow geologists to construct year-by-year chronologies, a form of incremental dating, which can provide high-resolution records of past climatic and environmental changes when combined with geochemical analysis of each band.

Certain species of corals form communities called microatolls. The vertical growth of microatolls is limited by average tidal height. By analyzing the various growth morphologies, microatolls can be used as a low resolution record of patterns of sea level change. Fossilized microatolls can also be dated using radioactive carbon dating to obtain a chronology of patterns of sea level change. Such methods have been used to used to reconstruct Holocene sea levels.

Gallery

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  Brain coral, Diploria labyrinthiformis
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  Polyps of Eusmilia fastigiata
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  Staghorn coral, Acropora
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  Orange cup coral, Balanophyllia elegans
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  Brain coral spawning
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  Magnified Zooxanthellae
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  Brain coral releasing eggs
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  Release of sperm

External links

• Coral Reefs and Hard Grounds information from from the Florida Fish and Wildlife Conservation Commission Fish and Wildlife Research Institute
• Coral Reefs of the Tropics: facts, photos and movies from The Nature Conservancy
• Australian Coral Records Research Group
• Coral Identification by Classification and Morphology
• International Council for the Exploration of the Sea: deep-water corals in the northeast Atlantic
• Institute of Marine Research: Coral reefs in Norway
• Reef Corals of the Indo-Malayan Seas: provides an identification key to the black corals, stony corals, soft corals, fire corals and lace corals in the waters of Indonesia and adjacent territories
• Corals can alter their skeleton to match the changing chemistry of seawater - LiveScience.com
• Biomineralisation in modern and fossil corals