Definitions

coral-red

Coral

[kawr-uhl, kor-]

Corals are 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 formed from 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. Examples of these can be found living on 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

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 reef-building corals (Scleractinians), sea anemones and zoanthids.

Anatomy

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. When polyps are physically stressed, they contract into the calyx so that virtually no part is exposed above the skeletal platform. This protects the organism from predators and the elements (Barnes, R.D., 1987; Sumich, 1996).

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.

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.

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. 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 protein). 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.

Reproduction

Corals maintain a variety of ways to propagate and settle new areas, the two main methods being by sexual and asexual means. Corals can be both gonochoristic and hermaphroditic, each of which can utilise sexual and asexual means of reproduction.

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 larvae 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 usually 2 or 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.

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.

Corals must rely on environmental cues, varying from species to species, to determine the proper time to release gametes into the water. There are two methods corals use 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 in contrast to brooders they release both sperm and eggs into the water. The corals use long-term cues such as day length, water temperature, and/or rate of temperature change; and the short-term cue is most often the lunar cycle, with the sunset cuing the time of 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 occurs to produce planula larvae. The planula 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 (non-reef building), or some hermatypic corals which are in areas of high current or wave action. Brooders release only sperm, which is negatively buoyant, and can store unfertilized eggs for weeks, lowering the need for mass synchronous spawning events, but can still occur. After fertilization the corals release planula larvae which are ready to settle.

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.

  • Budding expands the size of a coral colony. It occurs when a new corallite grows out from the adult polyp. As the new polyp grows it produces a coelenteron (stomach), tentacles and a mouth. 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 occur by means of:
  • Longitudinal division begins with broadening of a polyp, which then divides the coelenteron. The mouth divides and new tentacles form. The difference with this is that each polyp must complete its missing parts of the body and exoskeleton.
  • Intra-tentacular budding forms from the oral discs of a polyp, meaning that both polyps are the same size and are within the same ring of tentacles.
  • Extra-tentacular budding forms from the base of a polyp, and the new polyp is smaller.
  • Transversal division occurs when polyps and the exoskeleton divide transversally into two parts. This means that one has the basal disc (bottom) and the other has the oral disc (top). The two new polyps must again complete the missing parts.
  • Fission occurs in some corals, especially among the family Fungiidae, where the colony is able to split into two or more colonies during the early stages of their development.

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

  • Polyp bailout occurs when a single polyp abandons the colony and re-establishes on a new substrate to create a new adult colony.
  • Fragmentation, which can actually be included as a type of fission, involves individuals broken from the colony during storms, or other situations where breaking can occur. The separated individuals can start new coral colonies.

Reefs

The hermatypic, stony corals are often found in coral reefs, large calcium carbonate structures generally found in shallow, tropical water. Reefs are built up from coral skeletons and 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.

Types

Stony corals

In the Caribbean area alone, 50 species of hard coral exist, each of which has its own unique structure. The most known types are:

  • Brain coral that can become 1,8 meters wide and which looks like human brains (hence the name)
  • Acropora and Staghorn coral grow fast and large and are important reefbuilders. Staghorn coral makes large branches of coral and grows in the area which normally receives mosts surfs.
  • Galaxea fascicularis or starcoral is another important reefbuilder
  • Pillar coral, which forms pillars which can grow up to 3meters in height
  • Leptopsommia or rock coral, appears almost everywhere in the Caribbean

Soft corals

Soft corals are somewhat less plentiful (in the Caribbean, twenty species appear). They consist of sponges (which are an important host for minuscule, spineless animals and create a shelter for fish), as well as sea whips, sea feathers, sea pens, and Black corals.

Geological history

Although corals first appeared in the Cambrian period, some , 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.

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

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. 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.

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 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.

Live coral is also highly sought after in the aquarium trade. Although difficult to maintain in some or most cases, they add a striking beauty. Provided the proper ecosystem, live coral makes a stunning addition to any salt water aquarium.

Coral as a gemstone

See: Precious coral.

Red shades of coral are sometimes used as a gemstone, especially in Tibet. In vedic astrology, red coral represents Mars.

Intensely red coral is sometimes known as fire coral (but this is not at all the same thing as fire coral). This extremely red coral is very rare now because of overharvesting due to the great demand for perfect red coral in jewelry-making.

Ancient corals

Ancient coral reefs on land are often mined for lime or use as building blocks ("coral rag"). 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

Further images: and

References

External links

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