Coral

Coral

Overview
Corals are marine animals in class
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 Anthozoa
Anthozoa
Anthozoa is a class within the phylum Cnidaria that contains the sea anemones and corals. Unlike other cnidarians, anthozoans do not have a medusa stage in their development. Instead, they release sperm and eggs that form a planula, which attaches to some substrate on which the cnidarian grows...

 of phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....

 Cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...

 typically living in compact colonies
Colony (biology)
In biology, a colony reference to several individual organisms of the same species living closely together, usually for mutual benefit, such as stronger defense or the ability to attack bigger prey. Some insects live only in colonies...

 of many identical individual "polyp
Polyp
A polyp in zoology is one of two forms found in the phylum Cnidaria, the other being the medusa. Polyps are approximately cylindrical in shape and elongated at the axis of the body...

s". The group includes the important reef
Coral reef
Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...

 builders that inhabit tropical ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

s and secrete calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

 to form a hard skeleton.

A coral "head" is a colony of myriad genetically identical
Cloning
Cloning in biology is the process of producing similar populations of genetically identical individuals that occurs in nature when organisms such as bacteria, insects or plants reproduce asexually. Cloning in biotechnology refers to processes used to create copies of DNA fragments , cells , or...

 polyps. Each polyp is a spineless animal typically only a few millimeters in diameter and a few centimeters in length.
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Encyclopedia
Corals are marine animals in class
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 Anthozoa
Anthozoa
Anthozoa is a class within the phylum Cnidaria that contains the sea anemones and corals. Unlike other cnidarians, anthozoans do not have a medusa stage in their development. Instead, they release sperm and eggs that form a planula, which attaches to some substrate on which the cnidarian grows...

 of phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....

 Cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...

 typically living in compact colonies
Colony (biology)
In biology, a colony reference to several individual organisms of the same species living closely together, usually for mutual benefit, such as stronger defense or the ability to attack bigger prey. Some insects live only in colonies...

 of many identical individual "polyp
Polyp
A polyp in zoology is one of two forms found in the phylum Cnidaria, the other being the medusa. Polyps are approximately cylindrical in shape and elongated at the axis of the body...

s". The group includes the important reef
Coral reef
Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...

 builders that inhabit tropical ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

s and secrete calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

 to form a hard skeleton.

A coral "head" is a colony of myriad genetically identical
Cloning
Cloning in biology is the process of producing similar populations of genetically identical individuals that occurs in nature when organisms such as bacteria, insects or plants reproduce asexually. Cloning in biotechnology refers to processes used to create copies of DNA fragments , cells , or...

 polyps. Each polyp is a spineless animal typically only a few millimeters in diameter and a few centimeters in length. A set of tentacle
Tentacle
A tentacle or bothrium is one of usually two or more elongated flexible organs present in animals, especially invertebrates. The term may also refer to the hairs of the leaves of some insectivorous plants. Usually, tentacles are used for feeding, feeling and grasping. Anatomically, they work like...

s surround a central mouth opening. An exoskeleton
Exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...

 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
Asexual reproduction
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...

 of polyps. Corals also breed sexually by spawning: polyps of the same species release gamete
Gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...

s simultaneously over a period of one to several nights around a full moon
Full moon
Full moon lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun. More precisely, a full moon occurs when the geocentric apparent longitudes of the Sun and Moon differ by 180 degrees; the Moon is then in opposition with the Sun.Lunar eclipses can only occur at...

.

Although corals can catch small fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

 and plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

, using stinging cells
Cnidocyte
A cnidocyte, cnidoblast, or nematocyte is a type of venomous cell unique to the phylum Cnidaria . The cnidocyte cell provides a means for them to catch prey and defend themselves from predators. Despite being morphologically simple, lacking a skeleton and usually being sessile, cnidarians prey on...

 on their tentacles, most corals obtain the majority of their energy and nutrients from photosynthetic
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

 unicellular algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

 called zooxanthella
Zooxanthella
Zooxanthellae are flagellate protozoa that are golden-brown intracellular endosymbionts of various marine animals and protozoa, especially anthozoans such as the scleractinian corals and the tropical sea anemone, Aiptasia....

e that live within the coral's tissue. Such corals require sunlight and grow in clear, shallow water, typically at depths shallower than 60 metres (196.9 ft). Corals can be major contributors to the physical structure of the coral reef
Coral reef
Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...

s that develop in tropical and subtropical waters, such as the enormous Great Barrier Reef
Great Barrier Reef
The Great Barrier Reef is the world'slargest reef system composed of over 2,900 individual reefs and 900 islands stretching for over 2,600 kilometres over an area of approximately...

 off the coast of Queensland
Queensland
Queensland is a state of Australia, occupying the north-eastern section of the mainland continent. It is bordered by the Northern Territory, South Australia and New South Wales to the west, south-west and south respectively. To the east, Queensland is bordered by the Coral Sea and Pacific Ocean...

, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

. Other corals do not have associated algae and can live in much deeper water, with the cold-water genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 Lophelia surviving as deep as 3000 metres (9,842.5 ft). Examples live on the Darwin Mounds
Darwin Mounds
Darwin Mounds describes a vast field of undersea sand mounds situated off the north west coast of Scotland, first discovered in May 1998, they provide a unique habitat for ancient deep water coral reefs...

 located north-west of Cape Wrath
Cape Wrath
Cape Wrath is a cape in Sutherland, Highland, in northern Scotland. It is the most northwesterly point on the island of Great Britain. The land between the Kyle of Durness and the lighthouse that is situated right at the tip, is known as the Parph, two hundred and seven square kilometers of...

, Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

. Corals have also been found off the coast of the U.S. in Washington State and the Aleutian Islands in Alaska
Alaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...

.

Taxonomy




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
Cnidocyte
A cnidocyte, cnidoblast, or nematocyte is a type of venomous cell unique to the phylum Cnidaria . The cnidocyte cell provides a means for them to catch prey and defend themselves from predators. Despite being morphologically simple, lacking a skeleton and usually being sessile, cnidarians prey on...

 type and mitochondrial genetic analysis. Those with eight tentacles are called octocorallia or Alcyonaria
Alcyonaria
Octocorallia is a subclass of Anthozoa comprising ~3,000 species of water-based organisms formed of colonial polyps with 8-fold symmetry. It includes the blue coral, soft corals, sea pens, and gorgonians within three orders: Alcyonacea, Helioporacea, and Pennatulacea...

 and comprise soft corals
Alcyonacea
The Alcyonacea, or the soft corals are an order of corals which do not produce calcium carbonate skeletons. Soft corals contain minute, spiny skeletal elements called sclerites. Aside from their scientific utility in species identification, sclerites give these corals some degree of support and...

, sea fan
Sea fan
A gorgonian, also known as sea whip or sea fan, is an order of sessile colonial cnidarian found throughout the oceans of the world, especially in the tropics and subtropics. Gorgonians are similar to the sea pen, another soft coral. Individual tiny polyps form colonies that are normally erect,...

s and sea pen
Sea pen
Sea pens are colonial marine cnidarians belonging to the order Pennatulacea. There are 14 families within the order; they are thought to have a cosmopolitan distribution in tropical and temperate waters worldwide...

s. Those with more than eight in a multiple of six are called hexacorallia or Zoantharia
Zoantharia
Zoanthids are an order of cnidarians commonly found in coral reefs, the deep sea and many other marine environments around the world. These animals come in a variety of different colonizing formations and in numerous colors...

. This group includes reef-building corals (scleractinia
Scleractinia
Scleractinia, also called stony corals, are exclusively marine animals; they are very similar to sea anemones but generate a hard skeleton. They first appeared in the Middle Triassic and replaced tabulate and rugose corals that went extinct at the end of the Permian...

ns), sea anemones and zoanthids.

Anatomy



People believed coral to be a plant until the 18th century, when William Herschel
William Herschel
Sir Frederick William Herschel, KH, FRS, German: Friedrich Wilhelm Herschel was a German-born British astronomer, technical expert, and composer. Born in Hanover, Wilhelm first followed his father into the Military Band of Hanover, but emigrated to Britain at age 19...

 used a microscope to establish that coral had the characteristic thin cell membranes of an animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

.

While a coral head appears to be a single organism, it is actually a group of many individual, yet genetically identical
Cloning
Cloning in biology is the process of producing similar populations of genetically identical individuals that occurs in nature when organisms such as bacteria, insects or plants reproduce asexually. Cloning in biotechnology refers to processes used to create copies of DNA fragments , cells , or...

, polyp
Polyp
A polyp in zoology is one of two forms found in the phylum Cnidaria, the other being the medusa. Polyps are approximately cylindrical in shape and elongated at the axis of the body...

s. The polyps are multicellular organism
Multicellular organism
Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms. Most life that can be seen with the the naked eye is multicellular, as are all animals and land plants.-Evolutionary history:Multicellularity has evolved independently dozens of times...

s. Polyps are usually a few millimeters in diameter, and are formed by a layer of outer epithelium
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...

 and inner jellylike tissue known as the mesoglea
Mesoglea
Mesoglea is the translucent, inert, jelly-like substance that makes up most of the bodies of jellyfish, comb jellies and certain primitive sea creatures in the phyla Cnidaria and Ctenophora. It acts as the creatures' structural support in water, as they lack bones or cartilage, endo- or...

. 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
Exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...

 called the basal plate or calicle (L.
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 small cup). The calicle is formed by a thickened calcareous
Calcareous
Calcareous is an adjective meaning mostly or partly composed of calcium carbonate, in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.-In zoology:...

 ring (annular
Annulus (zoology)
In zoology, an annulus is an external circular ring. Annuli are commonly found in segmented animals such as earthworms and leeches. The bodies of these annelids are externally marked by annuli that are arranged in series with each other....

 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
Sepal
A sepal is a part of the flower of angiosperms . Collectively the sepals form the calyx, which is the outermost whorl of parts that form a flower. Usually green, sepals have the typical function of protecting the petals when the flower is in bud...

 so that virtually no part is exposed above the skeletal platform. This protects the organism from predators and the elements.

The polyp grows by extension of vertical calices which occasionally septate
Septate
Septate is a morphological term used in biology in two different instances.*In human biology, it is used to describe a divided uterus.*In mycology, it is used to describe the condition of spores or hyphae in terms of having or lacking a septa to divide...

 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
Aragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...

 by the polyps from calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

 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.
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
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...

 called the pharynx
Pharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...

. Jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...

 and sea anemone
Sea anemone
Sea anemones are a group of water-dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Zoantharia. Anthozoa often have large polyps that allow for digestion of larger...

s 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 – in diameter, and allow transport of both metabolite
Metabolite
Metabolites are the intermediates and products of metabolism. The term metabolite is usually restricted to small molecules. A primary metabolite is directly involved in normal growth, development, and reproduction. Alcohol is an example of a primary metabolite produced in large-scale by industrial...

s and cellular components.
Many corals, as well as other cnidarian groups such as sea anemones (e.g. Aiptasia
Aiptasia
Aiptasia is a genus of a symbiotic cnidarian belonging to the class Anthozoa . Other well known cnidarian groups include the jellyfish , the hydroids , and the box jellyfish...

), form a symbiotic relationship with a class of algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

, zooxanthellae, of the genus Symbiodinium. Aiptasia
Aiptasia
Aiptasia is a genus of a symbiotic cnidarian belonging to the class Anthozoa . Other well known cnidarian groups include the jellyfish , the hydroids , and the box jellyfish...

, while considered a pest among coral reef aquarium hobbyists, serves as a valuable model organism
Model organism
A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms. Model organisms are in vivo models and are widely used to...

 in the study of cnidarian-algal symbiosis
Symbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...

. Typically, a polyp harbors one species of algae. Via photosynthesis, these provide energy for the coral, and aid in calcification.
The algae benefit from a safe environment, and consume the carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 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
Coral bleaching
Coral bleaching is the loss of intracellular endosymbionts through either expulsion or loss of algal pigmentation.The corals that form the structure of the great reef ecosystems of tropical seas depend upon a symbiotic relationship with unicellular flagellate protozoa, called zooxanthellae, that...

, because the algae contribute to coral's brown coloration; other colors, however, are due to host coral pigments, such as green fluorescent protein
Green fluorescent protein
The green fluorescent protein is a protein composed of 238 amino acid residues that exhibits bright green fluorescence when exposed to blue light. Although many other marine organisms have similar green fluorescent proteins, GFP traditionally refers to the protein first isolated from the...

s (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.

Feeding


Polyps feed on a variety of small organisms, from microscopic plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

 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
Fire coral
Fire corals are colonial marine organisms that look rather like real coral. However they are technically not corals; they are actually more closely related to jellyfish and other stinging anemones...

.

Reproduction


Corals can be both gonochoristic
Gonochorism
In biology, gonochorism or unisexualism describes sexually reproducing species in which individuals have just one of at least two distinct sexes. The term is most often used with animals . The sex of an individual may change during its lifetime, this can for example be found in parrotfish...

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

Sexual



Corals predominantly reproduce sexually
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...

. About 25% of hermatypic coral
Hermatypic coral
Hermatypic corals or "stony corals" are reef-building corals, while corals that do not deposit aragonite structures and contribute to coral reef development are referred to as ahermatypic species....

s (stony corals) form single sex (gonochoristic) colonies, while the rest are hermaphroditic. About 75% of all hermatypic corals "broadcast spawn" by releasing gamete
Gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...

s—eggs
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...

 and sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...

—into the water to spread offspring. The gametes fuse during fertilization to form a microscopic larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...

 called a planula
Planula
A planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species. The planula forms from the fertilized egg of a medusa, as the case in scyphozoans and some hydrozoans, or from a polyp, as in the case of anthozoans...

, 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.

Planulae exhibits positive phototaxis
Phototaxis
Phototaxis is a kind of taxis, or locomotory movement, that occurs when a whole organism moves in response to the stimulus of light. This is advantageous for phototrophic organisms as they can orient themselves most efficiently to receive light for photosynthesis...

, 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. 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. The larva grows into a polyp and eventually becomes a coral head by asexual budding and growth.

Synchronous spawning
Reproductive synchrony
Reproductive synchrony is a term used in evolutionary biology and behavioural ecology. Reproductive synchrony — sometimes termed 'ovulatory synchrony' — may manifest itself as 'breeding seasonality'...

 is very typical on the coral reef, and often, even when multiple species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 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. Synchronous spawning may form hybrids and is perhaps involved in coral speciation
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...

. 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
    Day length
    Day length, or length of day, or length of daytime, refers to the time each day from the moment the upper limb of the sun's disk appears above the horizon during sunrise to the moment when the upper limb disappears below the horizon during sunset...

    , water temperature, and/or rate of temperature change. The short-term cue is most often the lunar cycle
    Lunar phase
    A lunar phase or phase of the moon is the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases change cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon, and Sun...

    , with sunset cuing the release. About 75% of coral species are broadcasters, the majority of which are hermatypic, or reef-building corals. The positively buoyant gametes float towards the surface, where fertilization produces planula
    Planula
    A planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species. The planula forms from the fertilized egg of a medusa, as the case in scyphozoans and some hydrozoans, or from a polyp, as in the case of anthozoans...

     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, 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. After fertilization, the corals release planula
    Planula
    A planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species. The planula forms from the fertilized egg of a medusa, as the case in scyphozoans and some hydrozoans, or from a polyp, as in the case of anthozoans...

     larvae which are ready to settle.

Asexual




Within a coral head, the genetically identical polyps reproduce asexually
Asexual reproduction
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...

, either via gemmation (budding
Budding
Budding is a form of asexual reproduction in which a new organism grows on another one. The new organism remains attached as it grows, separating from the parent organism only when it is mature. Since the reproduction is asexual, the newly created organism is a clone and is genetically identical...

) 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.
  • 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:
  • 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
    Fungiidae
    Fungiidae is a family of Cnidaria. It contains thirteen extant genera.-Characteristics:Species are generally solitary marine animals that are capable of benthic locomotion. These corals often appear to be bleached or dead. In most genera, a single polyp emerges from the center of the skeleton to...

    , 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.

Reefs




The hermatypic, stony corals are often found in coral reef
Coral reef
Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...

s, large calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

 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
Coralline algae
Coralline algae are red algae in the order Corallinales. They are characterized by a thallus that is hard because of calcareous deposits contained within the cell walls...

. Reefs are extremely diverse marine ecosystems hosting over 4,000 species of fish, massive numbers of cnidarians, mollusks, crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s, and many other animals.

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.

Hermatypic corals



Hermatypic
Hermatypic coral
Hermatypic corals or "stony corals" are reef-building corals, while corals that do not deposit aragonite structures and contribute to coral reef development are referred to as ahermatypic species....

 or stony corals build reefs. With the help of zooxanthellae, they convert surplus food to calcium carbonate, forming a hard skeleton. Hermatypic species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 include Scleractinia
Scleractinia
Scleractinia, also called stony corals, are exclusively marine animals; they are very similar to sea anemones but generate a hard skeleton. They first appeared in the Middle Triassic and replaced tabulate and rugose corals that went extinct at the end of the Permian...

, Millepora, Tubipora and Heliopora.

In the Caribbean alone, 50 species of uniquely structured hard coral exist. Well-known types include:
  • Brain coral
    Brain coral
    Brain coral is a common name given to corals in the family Faviidae so called due to their generally spheroid shape and grooved surface which resembles a brain...

     grow to 1.8 metres (5.9 ft) in width.
  • Acropora
    Acropora
    Acropora is a genus of scleractinian coral in the Phylum Cnidaria. Some of its species are known as table coral, elkhorn coral and staghorn coral. There are currently 149 described species...

    and staghorn coral
    Staghorn coral
    The Staghorn coral is a branching coral with cylindrical branches ranging from a few centimetres to over two metres in length and height. It occurs in back reef and fore reef environments from 0 to 30 m depth. The upper limit is defined by wave forces, and the lower limit is controlled by...

     grow fast and large, and are important reef-builders. Staghorn coral displays large, antler-like branches, and grows in areas with strong surf
    Breaking wave
    In fluid dynamics, a breaking wave is a wave whose amplitude reaches a critical level at which some process can suddenly start to occur that causes large amounts of wave energy to be transformed into turbulent kinetic energy...

    .
  • Galaxea fascicularis or star coral is another important reef-builder.
  • Pillar coral
    Pillar coral
    Pillar corals are a type of hard coral which live in the western Atlantic Ocean. They are one of the digitate corals which resemble fingers, or a cluster of cigars, growing up from the sea floor, but without any secondary branching.Pillar corals can grow to be up to 2.5 m tall...

     forms pillars which can grow to 3 metres (9.8 ft) in height.
  • Leptopsommia, or rock coral, appears almost everywhere in the Caribbean.

Ahermatypic corals



Ahermatypic corals have no zooxanthellae and do not build reefs. They include Alcyonacea
Alcyonacea
The Alcyonacea, or the soft corals are an order of corals which do not produce calcium carbonate skeletons. Soft corals contain minute, spiny skeletal elements called sclerites. Aside from their scientific utility in species identification, sclerites give these corals some degree of support and...

, as well as some Anthipatharia species (black coral
Black coral
Black corals are a group of deep water, tree-like corals related to sea anemones. They are also found in rare dark shallow water areas such as New Zealand's Milford Sound where they can be viewed from an underwater observatory. They normally occur in the tropics...

, Cirripathes, Antipathes). Ahermatypic corals, such as sea whips, sea feathers, and sea pen
Sea pen
Sea pens are colonial marine cnidarians belonging to the order Pennatulacea. There are 14 families within the order; they are thought to have a cosmopolitan distribution in tropical and temperate waters worldwide...

s, 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



Although corals first appeared in the Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...

 period, some , fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

s are extremely rare until the Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 period, 100 million years later, when rugose
Rugosa
Disambiguation:The Rugosa Rose is also sometimes just called "Rugosa". For the moon in Star Wars: The Clone Wars, see .The Rugosa, also called the Tetracoralla, are an extinct order of coral that were abundant in Middle Ordovician to Late Permian seas.Solitary rugosans are often referred to...

 and tabulate coral
Tabulate coral
The tabulate corals, forming the order Tabulata, are an extinct form of coral. They are almost always colonial, forming colonies of individual hexagonal cells known as corallites defined by a skeleton of calcite, similar in appearance to a honeycomb. Adjacent cells are joined by small pores...

s became widespread.

Tabulate corals occur in the limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

s and calcareous shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...

s of the Ordovician and Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

 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
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...

 period, 250 million years ago. The skeletons of tabulate corals are composed of a form of calcium carbonate known as calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...

.

Rugose corals became dominant by the middle of the Silurian period, and became extinct early in the Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

 period. The rugose corals existed in solitary and colonial forms, and are also composed of calcite.

The scleractinia
Scleractinia
Scleractinia, also called stony corals, are exclusively marine animals; they are very similar to sea anemones but generate a hard skeleton. They first appeared in the Middle Triassic and replaced tabulate and rugose corals that went extinct at the end of the Permian...

n 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
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

 and later periods. Scleractinian skeletons are composed of a form of calcium carbonate known as aragonite
Aragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...

. 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.
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
Echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....

, brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...

s, bivalves, gastropods, and trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...

s appear along with coral fossils. This makes some corals useful index fossil
Index fossil
Index fossils are fossils used to define and identify geologic periods . They work on the premise that, although different sediments may look different depending on the conditions under which they were laid down, they may include the remains of the same species of fossil...

s, 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
Gault Clay
Gault is a clay formation of stiff blue clay deposited in a calm, fairly deep water marine environment during the Lower Cretaceous Period...

 formation.

A Petoskey stone
Petoskey stone
A Petoskey stone is a rock and a fossil, often pebble-shaped, that is composed of a fossilized coral, Hexagonaria percarinata. The stones were formed as a result of glaciation, in which sheets of ice plucked stones from the bedrock, grinding off their rough edges and depositing them in Michigan's...

 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.

Threats



Corals are highly sensitive to environmental
Natural environment
The natural environment encompasses all living and non-living things occurring naturally on Earth or some region thereof. It is an environment that encompasses the interaction of all living species....

 changes. Scientists have predicted over 50% of the world's coral reef
Coral reef
Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...

s may be destroyed by 2030; as a result, most nations protect them through environmental laws.

Seaweed
Seaweed
Seaweed is a loose, colloquial term encompassing macroscopic, multicellular, benthic marine algae. The term includes some members of the red, brown and green algae...

/algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

 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
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...

–soluble metabolite
Metabolite
Metabolites are the intermediates and products of metabolism. The term metabolite is usually restricted to small molecules. A primary metabolite is directly involved in normal growth, development, and reproduction. Alcohol is an example of a primary metabolite produced in large-scale by industrial...

s. 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
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...

 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
Coral bleaching
Coral bleaching is the loss of intracellular endosymbionts through either expulsion or loss of algal pigmentation.The corals that form the structure of the great reef ecosystems of tropical seas depend upon a symbiotic relationship with unicellular flagellate protozoa, called zooxanthellae, that...

.

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
Scuba diving
Scuba diving is a form of underwater diving in which a diver uses a scuba set to breathe underwater....

, mining and construction.

Coral's narrow niche
Ecological niche
In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...

 and the stony corals
Scleractinia
Scleractinia, also called stony corals, are exclusively marine animals; they are very similar to sea anemones but generate a hard skeleton. They first appeared in the Middle Triassic and replaced tabulate and rugose corals that went extinct at the end of the Permian...

' reliance on calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

 deposition makes them susceptible to changes in water pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

. 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
Ocean acidification
Ocean acidification is the name given to the ongoing decrease in the pH and increase in acidity of the Earth's oceans, caused by the uptake of anthropogenic carbon dioxide from the atmosphere....

. Lowered pH reduces corals' ability to produce calcium carbonate, and at the extreme, can dissolve their skeletons. Without deep and immediate cuts in anthropogenic , many scientists fear acidification will severely degrade or destroy coral ecosystems.

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
Scuba diving
Scuba diving is a form of underwater diving in which a diver uses a scuba set to breathe underwater....

 and snorkeling
Snorkeling
Snorkeling is the practice of swimming on or through a body of water while equipped with a diving mask, a shaped tube called a snorkel, and usually swimfins. In cooler waters, a wetsuit may also be worn...

 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. Coral skeletons are also commonly used for bone grafting in humans.

Live coral is highly sought after for aquaria
Aquarium
An aquarium is a vivarium consisting of at least one transparent side in which water-dwelling plants or animals are kept. Fishkeepers use aquaria to keep fish, invertebrates, amphibians, marine mammals, turtles, and aquatic plants...

. 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. 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.

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

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
Fire coral
Fire corals are colonial marine organisms that look rather like real coral. However they are technically not corals; they are actually more closely related to jellyfish and other stinging anemones...

. Red coral is very rare because of overharvesting due to the great demand for perfect specimens.

In construction



Ancient coral reefs on land provide lime for use as building blocks ("coral rag
Coral rag
Coral rag is a rubbly limestone composed of ancient coral reef material. The term also refers to the building blocks quarried from these strata which are an important local building material in areas such as the east African coast and the Caribbean basin .It is also the name of a member — the Coral...

"). Coral rag is an important local building material
Building material
Building material is any material which is used for a construction purpose. Many naturally occurring substances, such as clay, sand, wood and rocks, even twigs and leaves have been used to construct buildings. Apart from naturally occurring materials, many man-made products are in use, some more...

 in places such as the East African coast.

In climate research


The annual growth bands in bamboo corals and others allow geologists
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...

 to construct year-by-year chronologies, a form of incremental dating
Incremental dating
Incremental dating techniques allow the construction of year-by-year annual chronologies, which can be temporally fixed or floating....

, which underlie high-resolution records of past climatic
Paleoclimatology
Paleoclimatology is the study of changes in climate taken on the scale of the entire history of Earth. It uses a variety of proxy methods from the Earth and life sciences to obtain data previously preserved within rocks, sediments, ice sheets, tree rings, corals, shells and microfossils; it then...

 and environmental
Paleoecology
Paleoecology uses data from fossils and subfossils to reconstruct the ecosystems of the past. It involves the study of fossil organisms and their associated remains, including their life cycle, living interactions, natural environment, and manner of death and burial to reconstruct the...

 changes using geochemical
Geochemistry
The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and...

 techniques.

Certain species form communities called microatoll
Microatoll
A microatoll is a discoid colony of coral, dead on the top but living around the perimeter. Growth is mainly lateral, as upward growth is limited by exposure to air. Microatolls may be up to in diameter. They are named for their resemblance to island atolls formed during the subsidence of...

s, which are colonies whose top is dead and mostly above the water line, but whose perimeter is mostly submerged and alive. Average tide
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....

 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
Radiocarbon dating
Radiocarbon dating is a radiometric dating method that uses the naturally occurring radioisotope carbon-14 to estimate the age of carbon-bearing materials up to about 58,000 to 62,000 years. Raw, i.e. uncalibrated, radiocarbon ages are usually reported in radiocarbon years "Before Present" ,...

. Such methods can help to reconstruct Holocene
Holocene
The Holocene is a geological epoch which began at the end of the Pleistocene and continues to the present. The Holocene is part of the Quaternary period. Its name comes from the Greek words and , meaning "entirely recent"...

 sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

s.

Deep sea
Deep sea
The deep sea, or deep layer, is the lowest layer in the ocean, existing below the thermocline and above the seabed, at a depth of 1000 fathoms or more. Little or no light penetrates this part of the ocean and most of the organisms that live there rely for subsistence on falling organic matter...

 bamboo coral
Bamboo coral
Bamboo coral, family Isididae, is a family of mostly deep-sea coral of the phylum Cnidaria, class Anthozoa, and subclass Octocorallia. It is a commonly recognized inhabitant of the deep sea, due to the clearly articulated skeletons of the species. Deep water coral species such as this are...

s (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.

Aquaculture


Coral aquaculture, also known as coral farming or coral gardening, is the cultivation of corals for commercial purposes or coral reef restoration. Aquaculture is showing promise as a potentially effective tool for restoring coral reef
Coral reef
Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...

s, which have been declining around the world. 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. Coral is farmed by coral farmers who live locally to the reefs and farm for reef conservation
Conservation
Conservation may refer to:* Conservation movement, to protect animals, fungi, plants and their habitats** Conservation biology, the science of the protection and management of biodiversity...

 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
Aquarium
An aquarium is a vivarium consisting of at least one transparent side in which water-dwelling plants or animals are kept. Fishkeepers use aquaria to keep fish, invertebrates, amphibians, marine mammals, turtles, and aquatic plants...

 hobbyists.

See also

  • Bamboo coral
    Bamboo coral
    Bamboo coral, family Isididae, is a family of mostly deep-sea coral of the phylum Cnidaria, class Anthozoa, and subclass Octocorallia. It is a commonly recognized inhabitant of the deep sea, due to the clearly articulated skeletons of the species. Deep water coral species such as this are...

  • Coral dermatitis
    Coral dermatitis
    Coral dermatitis are a cutaneous condition caused by injury from the exoskeleton of certain corals.- References :...

  • Coral Triangle
    Coral Triangle
    The Coral Triangle is a geographical term so named as it refers to a roughly triangular area of the tropical marine waters of Indonesia, Malaysia, Papua New Guinea, Philippines, Solomon Islands and Timor-Leste that contain at least 500 species of reef-building corals in each ecoregion...

  • Organ pipe coral
    Organ pipe coral
    The Organ pipe coral is an alcyonarian coral native to the waters of the Indian Ocean and the central and western regions of the Pacific Ocean. It is the only known species of the genus Tubipora. This species is a soft coral but with a unique, hard skeleton of calcium carbonate that contains many...

  • Sea Turtle Association of Japan, Kuroshima Research Station
    Sea Turtle Association of Japan, Kuroshima Research Station
    Kuroshima Research Station started in 1973, under the name of the UnderwaterPark Foundation, Yaeyama Underwater Park Research Institute, for the purpose of managing and utilizing the underwater area in Sekisei lagoon between Ishigaki Island and Iriomote Island including Kuroshima Island...


Gallery


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

Further reading

  • Coral Reefs The Ocean Portal by Smithsonian Institution.
  • Coral, The Reef & Marine Aquarium Magazine. ISSN 1556-5769 Coral Magazine
    Coral Magazine
    Coral, The Reef & Marine Aquarium Magazine, is an international periodical serving an audience of amateur and professional aquarists, marine biologists, and coral reef ecologists....

  • Book of Coral Propagation by Anthony Calfo. ISBN 0980236509
  • 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 0805345825
  • Indo-Pacific Coral Reef Field Guide by Allen, G.R & R. Steene. 1994. ISBN 9810056877
  • Coral Reef Animals of the Indo-Pacific, Animals Life from Africa to Hawai‘i (invertebrates) by Gosliner, T., D. Behrens & G. Williams. 1996. ISBN 0930118219
  • Tropical Pacific Invertebrates by Colin, P.L. & C. Arneson. 1995. ISBN 0964562502
  • Corals of Australia and the Indo-Pacific by Veron, J.E.N. 1993. ISBN 0824815041
  • The Evolution of Reef Communities by Fagerstrom, J.A. 1987. ISBN 0471815284
  • A Reef Comes to Life. Creating an Undersea Exhibit by Segaloff, Nat, and Paul Erickson. 1991. ISBN 0531109941
  • SeaWorld – Coral reef bibliography

External links