Coral

Coral
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 skeletal remains of marine organisms 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 but 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 shallower than 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. Squires, D.F. (1959). "Deep sea corals collected by the Lamont Geological Observatory. 1. Atlantic corals". American Museum Novitates 1965: 1-42.  An example of these are the Darwin Mounds located north-west of Cape Wrath, Scotland. Corals have also been found off the coast of Washington State and the Aleutian Islands in Alaska.

Phylogeny

Main article: Anthozoa

Mushroom coral in Papua New Guinea

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. Daly, M., Fautin, D.G., and Cappola, V.A. (March 2003). "Systematics of the Hexacorallia (Cnidaria: Anthozoa)". Zoological Journal of the Linnean Society 139: 419-437.  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. PubMed.  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. PubMed.  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

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, and although allowing it to retreat into the exoskeleton, little or no protection is given, as the polyps grow as a rug over the surface of the exoskeleton. It is a popular misconception that polyps can \'hide in their little holes\' 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 acquire from seawater. The rate of deposition, while varying greatly between species and environmental conditions, can be as much as 10 g / m² of polyp / day (0.3 ounce / sq yd / day). This is light dependent, with night-time production 90% lower than that during the middle of the day. Anatomy of Coral. Marine Reef. Retrieved on 2006-03-31.

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

The polyp\'s tentacles trap prey using stinging cells called nematocysts. These are cells modified to capture and immobilize prey, such as plankton, by injecting poisons, firing very rapidly in response to contact. These poisons are usually weak but in fire corals they 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. 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 194 (2): 178-186. 

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

Aside from feeding on plankton, many corals as well as other cnidarian groups such as sea anemones (e.g. Aiptasia), form a symbiotic relationship with a class of algae, zooxanthellae, of the genus Symbiodinium. The sea anemone Aiptasia, while considered a pest among coral reef aquarium hobbyists, has served as a valuable model organism in the scientific study of cnidarian-algal symbiosis. Typically a polyp will harbor one particular species of algae. Via photosynthesis, these provide energy for the coral, and aid in calcification. Madl, P. and Yip, M. (2000). Field Excursion to Milne Bay Province - Papua New Guinea. Retrieved on 2006-03-31. The algae benefit from a safe environment, and use the carbon dioxide and nitrogenous waste produced by the polyp. Due to the strain the algae can put on the polyp, stress on the coral often triggers ejection of the algae, known on a large scale as coral bleaching, as it is the algae that contribute to the brown coloration of corals; other colors, however, are due to host coral pigments, such as GFPs (green fluorescent proteins). Ejecting the algae increases the polyps\' chances of surviving stressful periods - they can regain the algae at a later time. If the stressful conditions persist, the polyps, and corals, will eventually die. W. W. Toller, R. Rowan and N. Knowlton (2001). "Repopulation of Zooxanthellae in the Caribbean Corals Montastraea annularis and M. faveolata following Experimental and Disease-Associated Bleaching". The Biological Bulletin 201: 360-373. 

Reproduction

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.Spalding, Mark, Corinna Ravilious, and Edmund Green (2001). World Atlas of Coral Reefs. Berkeley, CA, USA: University of California Press and UNEP/WCMC, 205-245. 

Geological history

Although corals first appeared in the Cambrian period,

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, 259. ISBN 0231106130.  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.

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

These ancient reefs are not composed entirely of corals. Algae, sponges, and the remains of many 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

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; Norlander (8 December 2003). "Coral crisis! Humans are killing off these bustling underwater cities. Can coral reefs be saved? (Life science: corals)". Science World.  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. Hoegh-Guldberg, O. (1999). "Climate change, coral bleaching and the future of the world\'s coral reefs". Marine and Freshwater Research 50 (8): 839-866. 

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

The narrow niche that coral occupies, and the stony corals\' reliance on calcium carbonate deposition, means they are very susceptible to changes in water pH. Ocean acidification, caused by dissolution of carbon dioxide in the water that lowers pH, is currently occurring in the surface waters of the world\'s oceans due to increasing atmospheric carbon dioxide. Lowered pH reduces the ability of corals to produce calcium carbonate skeletons, and at the extreme, results in the dissolution of those skeletons entirely. Without deep and early cuts in anthropogenic CO₂, scientists fear that ocean acidification may inevitably result in the severe degradation or destruction of coral species and ecosystems.Gattuso, J.P., Frankignoulle, M., Bourge, I., Romaine, S. and Buddemeier, R.W. (1998). "Effect of calcium carbonate saturation of seawater on coral calcification". Global Planet Change 18: 37-46. 

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. Glynn, P.W. (2001). "History of significant coral bleaching events and insights regarding amelioration". Coral Bleaching and Marine Protected Areas: Proceedings of the Workshop on Mitigating Coral Bleaching Impact Through MPA Design. Bishop Museum, Honolulu, Hawaii, 29-31 May 2001: 36-39. 

Uses

Live corals

Corals at Papua New Guinea

Local economies near major coral reefs benefit from an abundance of fish and octopus as a food source. Reefs also provide recreational scuba diving and snorkeling tourism. Unfortunately all these activities can also have deleterious effects, such as removal or accidental destruction of coral. Besides the recreational use, coral is also useful as a protection against hurricanes and other extreme weather.

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 jewelry-making.

Live corals in Papua New Guinea

Ancient corals

Ancient coral reefs on land are often mined for lime or use as building blocks ("coral rag"), 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. Schrag, D.P. and Linsley, B.K. (2002). "Corals, Chemistry, and Climate". Science 296 (8): 277-278. PubMed. 

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. Smithers, S.G. and Woodroffe, C.D. (August 2000). "Microatolls as sea-level indicators on a mid-ocean atoll.". Marine Geology 168 (1-4): 61-78. 

Gallery

References

External links

Wikispecies has information related to:

Coral

• Coral Identification by Classification and Morphology
• Corals can alter their skeleton to match the changing chemistry of seawater - LiveScience.com
• Biomineralisation in modern and fossil corals

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