Coral

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Corals
Brain Coral, Diploria labyrinthiformis
Scientific classification
Kingdom: Animalia Phylum: Cnidaria Class: Anthozoa Ehrenberg, 1831
Subclasses
Alcyonaria Zoantharia See text for orders

Corals are marine animals of the class Anthozoa, which also includes the sea anemones (order Actiniaria). Corals from the phylum Cnidaria and exist as small sea anemone-like polyps, typically in 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. The latter are also known as stony corals since the living tissue thinly covers a skeleton composed of calcium carbonate.

A coral "head", commonly perceived to be a single organism, is formed of thousands of individual polyps, each polyp only a few millimeters in diameter. The colony of polyps function as a single organism by sharing nutrients via a well-developed gastrovascular network. Genetically, the polyps are clones, each having exactly the same genome. Each polyp generation grows on the skeletal remains of previous generations, forming a structure that has a shape characteristic of the species, but also subject to environmental influences.

Although sea anemones can catch fish and other prey items and corals can catch plankton, these animals obtain much of their nutrients from symbiotic unicellular dinoflagellates (a type of photosynthetic algae) called symbiodiniume. Consequently, most corals are dependent upon sunlight and for that reason are usually found not far beneath the surface, in clear waters corals can growing at depths of up to 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^[1]. A head of coral grows by asexual reproduction of individual polyps, however corals breed sexually 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 in tropical and subtropical waters. Some corals exist in cold waters, such as off the coast of Norway (north to at least 69° 14.25' 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. Although Indonesia is home to 581 of the world's 793 known coral reef-building coral species.

[edit] Phylogeny

Pillar coral

Orange cup coral, Balanophyllia elegans

Close photo of Montastrea cavernosa polyps

The Anthozoa is a class within the phylum Cnidaria and contains the Sea anemones and corals. Corals are divided into two subclasses^[2] and a series of orders^[3][4][5]. In the list below, extinct orders from the Paleozoic (570-245 m.y.a.)^[6] are italicised:

• Subclass Alcyonaria (= Octocorallia; eight tentacles)
  • Order Alcyonacea (soft corals)
  • Order Gorgonacea (sea fans, sea feathers)
  • Order Helioporacea (Indo Pacific blue coral)
  • Order Pennatulacea (sea pens and sea pansies)
  • Order Stolonifera (organ pipe coral)
• Subclass Zoantharia (= Hexacorallia; more than 8 tentacles - typically 12)
  • Order Antipatharia (black corals, thorny corals)
  • Order Scleractinia (=Madreporaria; stony corals)
  • Order Corallimorpharia
  • Order Ptychodactiaria
  • Order Rugosa (tetracoralla)
  • Order Kilbuchophyllida
  • Order Cothoniida
  • Order Tabulata (tabulate corals)
  • Order Tabulacondia
  • Order Heliolitida
  • Order Heterocorallida
  • Order Numidiaphyllida

[edit] Anatomy

Anatomy of a coral polyp. Click to enlarge.

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 millimetres 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 allowing it to retreat into the exoskeleton for protection.

The polyp grows by extension of vertical calices which are occasionally septated to form a new, higher, basal plate. Over many generations this extension forms the large calciferous (Calcium containing) structures of corals and ultimately coral reefs.

Formation of the calciferous exoskeleton involves deposition of calcium carbonate by the polyps from calcium ions they accumulate 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 however dependent on light, with production reduced by 90% at night compared to the middle of the day^[7].

Nematocyst discharge: A dormant nematocyst (1) discharges its stinging aparatus in response to nearby prey (2-3), leaving a barbed stinging filament (4) with which to draw in the prey.

The polyp's tentacles trap prey using stinging cells called nematocysts. These are cells modified to capture and immobilise prey such as plankton, by injecting poisons, firing very rapidly in response to contact. In fire corals these poisons are harmful to humans, however in most other cases it is harmless. Nematocysts can also be found in jellyfish and sea anemones. The toxins injected by nematocysts immobilise 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.

Aside from feeding on plankton, many corals belong in a symbiotic relationship with a class of algae, zooxanthellae. Typically a polyp will harbour particular species of algae, which will photosynthesise and thereby provide energy for the coral and aid in calcification^[8], while living in a safe environment and using 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 gives coral colour. This allows the polyp to live longer during stressful periods, and to regain the algae at a later time; however if the conditions persist the polyps and corals die without the photosynthetic algae^[9].

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 have been found to range in size from 50-500 μm in diameter and to allow transport of both metabolites and cellular components^[10].

[edit] Reproduction

[edit] Sexual

Life cycles of broadcasters and brooders.

Corals reproduce predominantly sexually, with 25% of hermatypic corals (stony corals) forming single sex (gonochoristic) colonies and the rest hermaphroditic^[11]. About 75% of all hermatypic corals release gametes - eggs and sperm - into the water to spread colonies over large distances in what is called broadcast spawning. The gametes fuse during fertilisation to form a microscopic larva called a planula, typically pink and elliptical in shape; a moderately sized coral colony can form several thousands of these larva per year to overcome the huge hazards that prevent formation of a new colony^[12].

The planula swims towards light, positive phototaxis, to surface waters where they drift for a time and then swim back down to locate a surface on which it can attach and establish a new colony. The time from spawning to settling is often 2-3 days but can be up to 2 months^[13]. The larva grows into a coral polyp and eventually becomes a coral head by asexual budding and growth to create 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^[8]. The larva grows into a coral polyp and eventually becomes a coral head by asexual budding and growth to create new polyps.

Calices (basal plates) of Orbicella annularis showing two methods of multiplication - gemmation (small central calicle) and division (large double calicle).

Synchronous spawning is very typical on a coral reef, 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 and time of sunset, although chemical signalling has not been ruled out^[11]. Synchronous spawning may have the result of forming coral hybrids, perhaps involved in coral speciation^[14]. In some places the coral spawn can be dramatic, usually occurring at night, where the usually clear water becomes cloudy with gametes.

[edit] 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 on the right. Budding involves a new polyp growing from an adult, whereas division forms two polyps each as large as the original^[12].

Whole colonies can reproduce asexually through fragmentation where a piece broken off a coral head and moved by wave action can continue to grow in a new location.

[edit] 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, 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 may be either solitary or colonial, and like the Tabulate corals their skeletons are also composed of calcite. 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 of 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.

[edit] Environmental effects on coral

A coral reef can be an oasis for marine life.

Coral section; dyed to determine growth rate

Corals 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 then become colorless as they reveal the white of their calcium carbonate skeletons, an event known as coral bleaching^[15].

Scientists have predicted that over 50% of the coral reefs in the world may be destroyed by the year 2030^[16].

Many governments now prohibit removal of coral from reefs to prevent damage by divers taking pieces of coral. However this does not stop damage done by anchors dropped by dive boats or fishermen. In places where local fishing causes reef damage, such as the island of Rodrigues, education schemes have been run to educate the population about reef protection and ecology.

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 subject of study. Climatic variations, such as El Niño-Southern Oscillation (ENSO), can cause the temperature changes that destroy corals. For example the hydrocoral Millepora boschmai, located on the north shore of Uva Island (named Lazarus Cove), Gulf of Chiriquí, Panamá, survived the 1982-83 ENSO warming event, but during the 1997-98 ENSO all the surviving colonies bleached and died six years later ^[17].

[edit] Uses

Living corals underwater are more colorful than dead coral

Local economies near major coral reefs benefit from recreational scuba diving and snorkeling tourism, however this also has deleterious implications such as removal or accidental destruction of coral.

Ancient coral reefs on land are often mined for limestone or building blocks ("coral rag"). An example of the former is the quarrying of Portland limestone from the Isle of Portland. Coral rag is an important local building material in places such as the east African coast.

Red shades of coral are sometimes used as a gemstone, especially in Tibet. In vedic astrology, red coral represents Mars. Pure red coral is known as 'fire coral' and it is very rare because of the demand for perfect fire coral for jewellery-making purposes.

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 paleoenvironmental change^[18].

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^[19].

[edit] References

[edit] See also

• Coral reef
• Ocean acidification

[edit] External links

Wikispecies has information related to:

Coral

Wikimedia Commons has media related to (category):

Coral or Anthozoa

• Coral Reefs and Hard Grounds information from from the Florida Fish and Wildlife Conservation Commission Fish and Wildlife Research Institute
• Coral Reefs of the Tropics: facts, photos and movies from The Nature Conservancy
• Australian Coral Records Research Group
• Coral Identification by Classification and Morphology
• International Council for the Exploration of the Sea: deep-water corals in the northeast Atlantic
• Institute of Marine Research: Coral reefs in Norway
• Reef Corals of the Indo-Malayan Seas: provides an identification key to the black corals, stony corals, soft corals, fire corals and lace corals in the waters of Indonesia and adjacent territories
• Corals can alter their skeleton to match the changing chemistry of seawater - LiveScience.com
• Biomineralisation in modern and fossil corals