Coral

For other uses, see Coral (disambiguation).
Orders
Scleractinia

Corals are gastrovascular marine cnidarians (phylum Cnidaria; class Anthozoa) existing as small sea anemone-like polyps, typically forming colonies of many individuals. The group includes the important reef builders known as hermatypic corals, found in tropical oceans, and belonging to the subclass Zoantharia of order Scleractinia (formerly Madreporaria). The latter are also known as stony corals in as much as the living tissue thinly covers a skeleton composed of calcium carbonate. A coral "head" is formed of many individual polyps, each polyp only a few millimetres in diameter. The colony of polyps functions essentially as a single organism by sharing nutrients via a well developed gastrovascular network, and the polyps are clones, each having the same genetic structure. Each polyp generation grows on the skeletal remains of previous generations, forming a structure that has a shape characteristic of the species, but subject to environmental influences.

The hermatypic corals obtain much of their nutrient requirement from symbiotic unicellular algae called zooxanthellae, and so are dependent upon growing in sunlight. As a result, these corals are usually found not far beneath the surface, although in clear waters corals can grow at depths of 60 m (200 ft). Other corals, notably the cold-water genus Lophelia, do not have associated algae, and can live in much deeper water, with recent finds as deep as 3000 m. Corals breed by spawning, with many corals of the same species in a region releasing gametes simultaneously over a period of one to several nights around a full moon.

Corals are major contributors to the physical structure of coral reefs that develop only in tropical and subtropical waters. Some corals exist in cold waters, such as off the coast of Norway (north to at least 69° 14.24' N) and the Darwin Mounds off western Scotland. The most extensive development of extant coral reef is the Great Barrier Reef off the coast of Queensland, Australia. Indonesia is home to 581 of the world's 793 known coral reef-building coral species.

Coral types

There are several other types of corals, notably the octocorals (subclass Octocorallia) and corals classified in other orders of subclass Zoantharia: to wit, the black corals (order Antipatharia) and the soft corals (order Zoanthinaria). Extinct corals include rugose corals and tabulate coral. These two groups went extinct at the end of the Paleozoic. Most other anthozoans would be treated under the common name of "sea anemone".

Geological history

Fossil coral Heliophyllum halli from the Devonian of Canada.

Although corals first appeared in the Cambrian period, some 570 million years ago, they are extremely rare as fossils until the Ordovician period, 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. 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 may be either solitary or colonial, and like the Tabulate corals their skeletons are also composed of calcite. The finest details of their skeletal structures are often well preserved, and such fossils may be cut and polished.

Coral skeletons in a zoological display

Scleractinian corals diversified during the Mesozoic and Cenozoic eras and are at the height of their development today. Their fossils may be found in small numbers in rocks from the Triassic period, and they are relatively common fossils in rocks from the Jurassic and Cretaceous periods as well as the Caenozoic era. 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, the aragonite skeleton Scleractinian corals does not tend to preserve well, so it is often easier to find fossils of the more ancient Tabulate and Rugose corals.

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. And like modern corals their fossil ancestors built reefs beneath the ancient seas. Some of these reefs now lie as great structures in the midst of sedimentary rocks. Such reefs can be found in the rocks of many parts of the world including those of the Ordovician period of Vermont, the Silurian period of the Michigan Basin and in many parts of Europe, the Devonian period of Canada and the Ardennes in Belgium, and the Cretaceous period of South America and Denmark. Reefs from both the Silurian and Carboniferous periods have been recorded as far north as Siberia, and as far south as Australia.

Brain coral off the coast of Belize

However, these ancient reefs are not composed entirely of corals. Algae and sponges, as well as the fossilized remains of many echinoids, brachiopods, bivalves, gastropods, and trilobites that lived on the reefs help to build them. These fossil reefs are prime locations to look for fossils of many different types, besides the corals themselves.

Corals are not restricted to just reefs, many solitary corals may be found in rocks where reefs are not present (such as Cyclocyathus which occurs in the Cretaceous period Gault clay formation of England).

As well as being important rock builders, some corals are useful as zone (or index) fossils, enabling geologists to date the age the rocks in which they are found, particularly those found in the limestones of the Carboniferous period.

Environmental effects on coral

A coral reef can be an oasis of marine life.

Coral can be sensitive to environmental changes, and as a result 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 are colorless, revealing the white of their calcium carbonate skeletons, an event known as 'coral bleaching'.

A combination of temperature changes, pollution, and overuse has led to the destruction of many coral reefs around the world. This has increased the importance of coral biology as a subject of study. Climatic variations, such as El Niño, can cause the temperature changes that destroy corals.

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 kind of incremental dating, which combined with geochemical analysis of each band, can provide high-resolution records of paleoclimatic and paleoenvironamental change.

Uses

Living corals underwater are more colorful than dead coral

Coral reefs are a great source of tourism for scuba diving or snorkelling, however this has conservational implications due to damage from removal or destruction of coral.

Ancient coral reefs on land are often mined for limestone. An example of this is the quarrying of Portland limestone from the Isle of Portland.

Reddish coral is sometimes used as a gemstone especially in Tibet. Pure red coral is known as 'fire coral' and it is very rare because of the demand for perfect fire coral for jewellery-making purposes.


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Pure red coral is known as 'fire coral' and it is very rare because of the demand for perfect fire coral for jewellery-making purposes. The term "paperless office" seems to have been finally discredited and integrations of future desk forms with future computer systems are usually discussed under the more neutral term of office of the future. Reddish coral is sometimes used as a gemstone especially in Tibet. In a sense, the typical desk is becoming gradually virtual through a combination of the GUI and printouts, and is now expanding in size instead of shrinking, because of the exploitation of cubicle walls and of the theoretically infinite size of the GUI desktop. An example of this is the quarrying of Portland limestone from the Isle of Portland. An example of this is the recycle bin appearing on the microsoft desktop. Ancient coral reefs on land are often mined for limestone. The desk's working surface served as the inspiration for our present direct manipulation interface, which we usually know as the GUI, the graphical user interface or as the virtual desktop.

Coral reefs are a great source of tourism for scuba diving or snorkelling, however this has conservational implications due to damage from removal or destruction of coral. Even computer monitor frames themselves are used to attach reminder notes and business cards. In fossil and modern corals these bands allow geologists to construct year-by-year chronologies, a kind of incremental dating, which combined with geochemical analysis of each band, can provide high-resolution records of paleoclimatic and paleoenvironamental change. The cubicle walls have become new homes for papers and other items once left on the horizontal desktop surface. Some coral species exhibit banding in their skeletons resulting from annual variations in their growth rate. Since this last paper boom again produced a rise in the number of office workers and rises in office space rent, the infamous cubicle desk became widely accepted in the USA as an economical way of putting more desk workers in the same space without actually shrinking the size of their working surfaces. Climatic variations, such as El Niño, can cause the temperature changes that destroy corals. As information work was shifted to computers, users constantly printed out what was on their screens because the computer monitors had a resolution which was much inferior to that of paper, and because monitors were too costly to occupy entire desks, like sheets of papers laid out for comparison and/or for "reminding" purposes.

This has increased the importance of coral biology as a subject of study. The exact opposite happened. A combination of temperature changes, pollution, and overuse has led to the destruction of many coral reefs around the world. There would be no need for paper since all documents would be perfectly organized and accessible on the computers. In an early symptom of environmental stress, corals expel their zooxanthellae; without their symbiotic unicellular algae, coral tissues are colorless, revealing the white of their calcium carbonate skeletons, an event known as 'coral bleaching'. The beginning of this paper boom gave birth to the concept of the "paperless office", in which all information would appear on computer monitors. 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. The modular nature of the personal computer and its printer and other peripherals gave a boost to the existing but recently invented ergonomic desk, which was adapted to the peculiar needs of computer users.

A coral reef can easily be swamped in algae if there are too many nutrients in the water. The biggest paper boom occurred in the last decades of the 20th century with the introduction of mainframe computer printers and personal computer printers. Coral can be sensitive to environmental changes, and as a result are generally protected through environmental laws. Even executive or management desks became mass-produced, built of cheap plywood or fiberboard covered with wood veneer, as the number of persons managing the white collar workers became even greater. As well as being important rock builders, some corals are useful as zone (or index) fossils, enabling geologists to date the age the rocks in which they are found, particularly those found in the limestones of the Carboniferous period. Modular desks seating several co-workers close by became common. Corals are not restricted to just reefs, many solitary corals may be found in rocks where reefs are not present (such as Cyclocyathus which occurs in the Cretaceous period Gault clay formation of England). Paperwork drove even higher the number of desk workers, whose work surface diminished in size as office rents rose, and the paper itself was moved more and more directly to filing cabinets or sent to records management centers, or transformed into microfilm, or both.

These fossil reefs are prime locations to look for fossils of many different types, besides the corals themselves. Another big boom occurred after the Second World War with the spread of photocopying. Algae and sponges, as well as the fossilized remains of many echinoids, brachiopods, bivalves, gastropods, and trilobites that lived on the reefs help to build them. The L-shaped desk became popular, with the "leg" being used as an annex for the typewriter. However, these ancient reefs are not composed entirely of corals. Steel desks were introduced to take heavier loads of paper and withstand the pounding meted out on the typewriters. Reefs from both the Silurian and Carboniferous periods have been recorded as far north as Siberia, and as far south as Australia. A smaller boom in office work and desk production occurred at the end of the 19th century and the beginning of the 20th with the introduction of smaller and cheaper electrical presses and efficient carbon papers coupled with the general acceptance of the typewriter.

Such reefs can be found in the rocks of many parts of the world including those of the Ordovician period of Vermont, the Silurian period of the Michigan Basin and in many parts of Europe, the Devonian period of Canada and the Ardennes in Belgium, and the Cretaceous period of South America and Denmark. The famous Wooton desk and others were the last, monstrous manifestations of the dying "pigeonhole" era. Some of these reefs now lie as great structures in the midst of sedimentary rocks. Correspondence and other documents were now too numerous to get enough attention to be rolled up or folded again, then summarized and tagged before being pigeonholed in a small compartment over or under the work surface of the desk. And like modern corals their fossil ancestors built reefs beneath the ancient seas. Paper documents started leaving the desk as a "home", with the general introduction of filing cabinets. 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. It provided a relatively fast and cheap way to lock up the ever increasing flow of paper without having to file everything by the end of the day.

Although they are geologically younger than the Tabulate and Rugose corals, the aragonite skeleton Scleractinian corals does not tend to preserve well, so it is often easier to find fossils of the more ancient Tabulate and Rugose corals. More paper and more correspondence drove the need for more complex desks and more specialized desks, such as the rolltop desk which was a mass produced, slatted variant of the classical cylinder desk. The skeletons of Scleractinian corals are composed of a form of calcium carbonate known as aragonite. Thus, age alone does not guarantee that an antique desk is a masterpiece, since this shift took place more than a hundred years ago. Their fossils may be found in small numbers in rocks from the Triassic period, and they are relatively common fossils in rocks from the Jurassic and Cretaceous periods as well as the Caenozoic era. From then on, limited quantities of finely crafted desks have been constructed by master cabinetmakers for the homes and offices of the rich while the vast majority of desks were assembled rapidly by unskilled labor, from components turned out in batches by machine tools. Scleractinian corals diversified during the Mesozoic and Cenozoic eras and are at the height of their development today. This was the first sharp division in desk manufacturing.

The finest details of their skeletal structures are often well preserved, and such fossils may be cut and polished. As these office workers grew in number, desks were mass-produced for them in large quantities, using newer, steam-driven woodworking machinery. The Rugose corals may be either solitary or colonial, and like the Tabulate corals their skeletons are also composed of calcite. This produced a boom in the number of, or some might say the birth of, the white-collar worker. Rugose corals became dominant by the middle of the Silurian period, and became extinct early in the Triassic period. Refinements to those first desk forms were considerable through the 19th century, as steam-driven machinery made cheap wood-based paper possible in the last periods of the first phase of the industrial revolution. The skeletons of Tabulate corals are composed of a form of calcium carbonate known as calcite. The ergonomic desk of the last decades is the newest addition to a long list of desk forms, but in a way it is only a refinement of the mechanically complex drawing table or drafting table of the end of the 18th century.

Their numbers began to decline during the middle of the Silurian period and they finally became extinct at the end of the Permian period. The desk forms we are familiar with in this beginning of the millennium were born mostly in the 17th and 18th centuries. 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. It is often possible to find out if a table or other piece of furniture of those times was designed to be used as a desk by looking for a drawer with three small separations (one each for the ink pot, the blotter and the powder tray) and room for the pens. Although corals first appeared in the Cambrian period, some 570 million years ago, they are extremely rare as fossils until the Ordovician period, when Rugose and Tabulate corals became widespread. Desks of the Renaissance and later eras had relatively slimmer structures, and more and more drawers as woodworking became more precise and cabinet-making became a distinct trade. Most other anthozoans would be treated under the common name of "sea anemone". Desks of the period usually had massive structures.

These two groups went extinct at the end of the Paleozoic. The absence of regular movable type printing also influenced desk size and shape because of the bigger volumes required for manuscript documents. Extinct corals include rugose corals and tabulate coral. The desks were designed, consequentially, with slots and hooks for bookmarks as well as writing implements. There are several other types of corals, notably the octocorals (subclass Octocorallia) and corals classified in other orders of subclass Zoantharia: to wit, the black corals (order Antipatharia) and the soft corals (order Zoanthinaria). Before the invention of the movable type printing press in the 15th century, any reader was potentially a writer or publisher or both, since any book or other document had to be copied by hand. . Medieval illustrations show the first pieces of furniture which seem to have been designed and constructed for the specific goals of reading and writing.

Indonesia is home to 581 of the world's 793 known coral reef-building coral species. Desk forms might have existed in classical antiquity or in other ancient centers of civilization in the Middle East or Far East, but we have no specific proof. The most extensive development of extant coral reef is the Great Barrier Reef off the coast of Queensland, Australia. . Some corals exist in cold waters, such as off the coast of Norway (north to at least 69° 14.24' N) and the Darwin Mounds off western Scotland. At the other end one finds the portable desk, which, in its smallest forms, is light enough to be placed on a lap or on small supports on a bed. Corals are major contributors to the physical structure of coral reefs that develop only in tropical and subtropical waters. At one extreme in size one finds the Armoire desk, encased in a very large cabinet looking like a traditional wardrobe from the exterior, when the doors are closed.

Corals breed by spawning, with many corals of the same species in a region releasing gametes simultaneously over a period of one to several nights around a full moon. To many the ideal or generic concept of a desk is the pedestal desk, which is often called an executive desk. Other corals, notably the cold-water genus Lophelia, do not have associated algae, and can live in much deeper water, with recent finds as deep as 3000 m. For instance, an Armoire desk is a desk built within a large wardrobe-like cabinet usually having the height of a man or a woman. As a result, these corals are usually found not far beneath the surface, although in clear waters corals can grow at depths of 60 m (200 ft). Not all desks have the form of a table. The hermatypic corals obtain much of their nutrient requirement from symbiotic unicellular algae called zooxanthellae, and so are dependent upon growing in sunlight. Unlike a regular table, only one side of a desk is suitable to sit on, except for some unusual desks like a partners desk.

Each polyp generation grows on the skeletal remains of previous generations, forming a structure that has a shape characteristic of the species, but subject to environmental influences. The list of desk forms and types gives the most common desk variations. The colony of polyps functions essentially as a single organism by sharing nutrients via a well developed gastrovascular network, and the polyps are clones, each having the same genetic structure. Desks often have one or more drawers. A coral "head" is formed of many individual polyps, each polyp only a few millimetres in diameter. It is often used in a work or office setting to read or write on, using simple implements like a pencil and paper or complicated ones like a computer. The latter are also known as stony corals in as much as the living tissue thinly covers a skeleton composed of calcium carbonate. A desk is a furniture form and a class of table.

The group includes the important reef builders known as hermatypic corals, found in tropical oceans, and belonging to the subclass Zoantharia of order Scleractinia (formerly Madreporaria). Corals are gastrovascular marine cnidarians (phylum Cnidaria; class Anthozoa) existing as small sea anemone-like polyps, typically forming colonies of many individuals.

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