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 following types of crochet are derived from the basic method:. Reddish coral is sometimes used as a gemstone especially in Tibet. Although crochet underwent a subsequent decline in popularity, it has recently benefited from a revival of interest in handcrafts among the younger generation, as well as great strides in improvement of the quality and varieties of yarn. An example of this is the quarrying of Portland limestone from the Isle of Portland. Often using granny squares, a motif worked in the round, and incorporating bright colors, these designs became indicative of the era. Ancient coral reefs on land are often mined for limestone. The craft remained primarily a homemaker's art until the late 1960s when the younger generation picked up on crochet.

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. Modern practice In the 1950s, crocheters began to use thicker yarns to create less delicate clothing and home items, though thread crocheting remained more popular until about 1960. 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. Mass-produced steel hooks were used to work the thread beginning in about 1900. Some coral species exhibit banding in their skeletons resulting from annual variations in their growth rate. Crochet in the round or filet crochet, worked in rows of 'open' or 'closed' mesh to create patterns, were most common. Climatic variations, such as El Niño, can cause the temperature changes that destroy corals. From 1800 to 1950, crochet was done almost exclusively in thread.

This has increased the importance of coral biology as a subject of study. Riego de la Branchardiere around 1845 who published patterns and instructions for reproducing bobbin lace and needle lace via crochet. A combination of temperature changes, pollution, and overuse has led to the destruction of many coral reefs around the world. Irish crochet lace was boosted by Mlle. 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'. This impression was partially mitigated by Queen Victoria, who conspicuously purchased Irish-made crochet lace and even learned to crochet herself. 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. Those who could afford lace made by older and more expensive methods disdained crochet as a cheap copy.

A coral reef can easily be swamped in algae if there are too many nutrients in the water. The introduction of crochet as an imitation of a status symbol, rather than a unique craft in its own right, had stigmatized the practice as common. Coral can be sensitive to environmental changes, and as a result are generally protected through environmental laws. The finished items were purchased mainly by the emerging middle class. 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. Around the world, crochet became a thriving cottage industry, supporting communities whose traditional livelihoods had been displaced by imperialism. 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). Those that survive to this day are often ornately carved or inlaid with mother-of-pearl.

These fossil reefs are prime locations to look for fossils of many different types, besides the corals themselves. At this time, thread spun from natural fibers was used without dyeing, and worked with handmade hooks of ivory, brass, or hardwood. 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. It required minimal equipment and supplies, all easily accessible to persons of all social classes. However, these ancient reefs are not composed entirely of corals. Beginning in the 1800s in Europe, crochet began to be used as a less costly substitute for other forms of lace. Reefs from both the Silurian and Carboniferous periods have been recorded as far north as Siberia, and as far south as Australia. These writers point to the tambour hooks used in tambour embroidery in France in the seventeenth century, and contend that the hooking of loops through fine fabric in tambour work evolved into "crochet in the air." Most samples of early work claimed to be crochet turn out to actually be samples of naalebinding.

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. Other writers point out that woven, knit and knotted textiles survive from very early periods, but that there are no surviving samples of crocheted fabric in any ethnological collection, or archeological source prior to 1800. Some of these reefs now lie as great structures in the midst of sedimentary rocks. These writers point to the "simplicity" of the technique and claim that it "must" have been early. And like modern corals their fossil ancestors built reefs beneath the ancient seas. Many find it likely that crochet was in fact used by early cultures but that a bent forefinger was used in place of a fashioned hook; therefore, there were no artifacts left behind to attest to the practice. 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. Some theorize that crochet evolved from traditional practices in Arabia, South America, or China, but there is no decisive evidence of the craft being performed before its popularity in Europe during the 1800s.

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. . The skeletons of Scleractinian corals are composed of a form of calcium carbonate known as aragonite. This method distinguishes crochet from other methods of fabric-making such as knitting, as it is composed entirely of loops made with a single hook and is only secured when the free end of the strand is pulled through the final loop. 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. Stitches are made by pulling one or more loops through each loop of the chain. Scleractinian corals diversified during the Mesozoic and Cenozoic eras and are at the height of their development today. Rounds can also be created by working many stitches into a single loop.

The finest details of their skeletal structures are often well preserved, and such fossils may be cut and polished. The chain is either turned and worked in rows, or joined end-to-end and worked in rounds. The Rugose corals may be either solitary or colonial, and like the Tabulate corals their skeletons are also composed of calcite.
Crocheted fabric in the modern sense is begun by placing a slip-knot loop on the hook, pulling another loop through the first loop, and so on to create a chain. Rugose corals became dominant by the middle of the Silurian period, and became extinct early in the Triassic period. The word is not to be confused with "crotchet", otherwise known as a quarter note. The skeletons of Tabulate corals are composed of a form of calcium carbonate known as calcite. The origin of the crochet technique is a subject of considerable controversy.

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 word crochet is derived from the Middle French word croc or croche, meaning hook. It describes the process of creating fabric from a length of cord, yarn, or thread with a Crochet hook. 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. "Handmade Lace & Patterns" by Annette Feldman. 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. Crochet: History & Technique by Lis Paludan. Most other anthozoans would be treated under the common name of "sea anemone". A living mystery : the international art & history of crochet by Annie Louise Potter.

These two groups went extinct at the end of the Paleozoic. Irish crochet. Extinct corals include rugose corals and tabulate coral. Cro-hook. 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). Hairpin lace. . Broomstick lace.

Indonesia is home to 581 of the world's 793 known coral reef-building coral species. Tunisian crochet. The most extensive development of extant coral reef is the Great Barrier Reef off the coast of Queensland, Australia. Filet crochet. 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. Corals are major contributors to the physical structure of coral reefs that develop only in tropical and subtropical waters.

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. 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. 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). The hermatypic corals obtain much of their nutrient requirement from symbiotic unicellular algae called zooxanthellae, and so are dependent upon growing in sunlight.

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 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. A coral "head" is formed of many individual polyps, each polyp only a few millimetres in diameter. The latter are also known as stony corals in as much as the living tissue thinly covers a skeleton composed of calcium carbonate.

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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