Coral

Individual colonies can often live decades to centuries, and some deep-sea colonies have lived more than years. One way we know this is because corals lay down annual rings, just as trees do. These skeletons can tell us about what conditions were like hundreds or thousands of years ago. The Great Barrier Reef as it exists today began growing about 20, years ago.

There are also deep-sea corals that thrive in cold, dark water at depths of up to 20, feet 6, m. Both stony corals and soft corals can be found in the deep sea. Deep-sea corals do not have the same algae and do not need sunlight or warm water to survive, but they also grow very slowly. One place to find them is on underwater peaks called seamounts. Reefs are the big cities of the sea. They exist because the growth of corals matches or exceeds the death of corals — think of it as a race between the construction cranes new coral skeleton and the wrecking balls the organisms that kill coral and chew their skeletons into sand.

When corals are babies floating in the plankton, they can be eaten by many animals. Population explosions of these predators can result in a reef being covered with tens of thousands of these starfish, with most of the coral killed in less than a year. Corals also have to worry about competitors. They use the same nematocysts that catch their food to sting other encroaching corals and keep them at bay. Seaweeds are a particularly dangerous competitor, as they typically grow much faster than corals and may contain nasty chemicals that injure the coral as well.

Corals do not have to only rely on themselves for their defenses because mutualisms beneficial relationships abound on coral reefs. The partnership between corals and their zooxanthellae is one of many examples of symbiosis, where different species live together and help each other. Some coral colonies have crabs and shrimps that live within their branches and defend their home against coral predators with their pincers.

Parrotfish, in their quest to find seaweed, will often bite off chunks of coral and will later poop out the digested remains as sand. One kind of goby chews up a particularly nasty seaweed, and even benefits by becoming more poisonous itself. The greatest threats to reefs are rising water temperatures and ocean acidification linked to rising carbon dioxide levels. High water temperatures cause corals to lose the microscopic algae that produce the food corals need—a condition known as coral bleaching. Severe or prolonged bleaching can kill coral colonies or leave them vulnerable to other threats.

Meanwhile, ocean acidification means more acidic seawater, which makes it more difficult for corals to build their calcium carbonate skeletons. And if acidification gets severe enough, it could even break apart the existing skeletons that already provide the structure for reefs. Scientists predict that by ocean conditions will be acidic enough for corals around the globe to begin to dissolve.

For one reef in Hawaii this is already a reality. Unfortunately, warming and more acid seas are not the only threats to coral reefs. Overfishing and overharvesting of corals also disrupt reef ecosystems. If care is not taken, boat anchors and divers can scar reefs. Invasive species can also threaten coral reefs. The lionfish , native to Indo-Pacific waters, has a fast-growing population in waters of the Atlantic Ocean. With such large numbers the fish could greatly impact coral reef ecosystems through consumption of, and competition with, native coral reef animals.

Even activities that take place far from reefs can have an impact.

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Runoff from lawns, sewage, cities, and farms feeds algae that can overwhelm reefs. Deforestation hastens soil erosion, which clouds water—smothering corals. Without their zooxanthellae, the living tissues are nearly transparent, and you can see right through to the stony skeleton, which is white, hence the name coral bleaching. Many different kinds of stressors can cause coral bleaching — water that is too cold or too hot, too much or too little light, or the dilution of seawater by lots of fresh water can all cause coral bleaching.

The biggest cause of bleaching today has been rising temperatures caused by global warming. Temperatures more than 2 degrees F or 1 degree C above the normal seasonal maximimum can cause bleaching. Bleached corals do not die right away, but if temperatures are very hot or are too warm for a long time, corals either die from starvation or disease. In , 80 percent of the corals in the Indian Ocean bleached and 20 percent died. There is much that we can do locally to protect coral reefs, by making sure there is a healthy fish community and that the water surrounding the reefs is clean.

Well-protected reefs today typically have much healthier coral populations, and are more resilient better able to recover from natural disasters such as typhoons and hurricanes. Fish play important roles on coral reefs, particularly the fish that eat seaweeds and keep them from smothering corals, which grow more slowly than the seaweeds. Fish also eat the predators of corals, such as crown of thorns starfish. Sclerosponges still assist corals building modern reefs, but like coralline algae are much slower-growing than corals and their contribution is usually minor.

In the northern Pacific Ocean cloud sponges still create deep-water mineral-structures without corals, although the structures are not recognizable from the surface like tropical reefs. They are the only extant organisms known to build reef-like structures in cold water. Recent oceanographic research has brought to light the reality of this paradox by confirming that the oligotrophy of the ocean euphotic zone persists right up to the swell-battered reef crest. When you approach the reef edges and atolls from the quasidesert of the open sea, the near absence of living matter suddenly becomes a plethora of life, without transition.

So why is there something rather than nothing, and more precisely, where do the necessary nutrients for the functioning of this extraordinary coral reef machine come from? In The Structure and Distribution of Coral Reefs , published in , Darwin described how coral reefs were found in some tropical areas but not others, with no obvious cause. The largest and strongest corals grew in parts of the reef exposed to the most violent surf and corals were weakened or absent where loose sediment accumulated.

Tropical waters contain few nutrients [68] yet a coral reef can flourish like an "oasis in the desert". Coral reefs support over one-quarter of all marine species. This diversity results in complex food webs , with large predator fish eating smaller forage fish that eat yet smaller zooplankton and so on. However, all food webs eventually depend on plants , which are the primary producers. One reason for the unusual clarity of tropical waters is their nutrient deficiency and drifting plankton. Further, the sun shines year-round in the tropics, warming the surface layer, making it less dense than subsurface layers.

The warmer water is separated from deeper, cooler water by a stable thermocline , where the temperature makes a rapid change. This keeps the warm surface waters floating above the cooler deeper waters. In most parts of the ocean, there is little exchange between these layers.

Organisms that die in aquatic environments generally sink to the bottom, where they decompose, which releases nutrients in the form of nitrogen N , phosphorus P and potassium K. These nutrients are necessary for plant growth, but in the tropics, they do not directly return to the surface.

Plants form the base of the food chain and need sunlight and nutrients to grow. In the ocean, these plants are mainly microscopic phytoplankton which drift in the water column. They need sunlight for photosynthesis , which powers carbon fixation , so they are found only relatively near the surface, but they also need nutrients. Phytoplankton rapidly use nutrients in the surface waters, and in the tropics, these nutrients are not usually replaced because of the thermocline.

Around coral reefs, lagoons fill in with material eroded from the reef and the island. They become havens for marine life, providing protection from waves and storms. Most importantly, reefs recycle nutrients, which happens much less in the open ocean. In coral reefs and lagoons, producers include phytoplankton, as well as seaweed and coralline algae, especially small types called turf algae, which pass nutrients to corals.

Recycling reduces the nutrient inputs needed overall to support the community. Corals also absorb nutrients, including inorganic nitrogen and phosphorus, directly from water. Many corals extend their tentacles at night to catch zooplankton that pass near.

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Zooplankton provide the polyp with nitrogen, and the polyp shares some of the nitrogen with the zooxanthellae, which also require this element. Sponges live in crevices in the reefs. Sponges eventually excrete nutrients in a form that corals can use. The roughness of coral surfaces is key to coral survival in agitated waters. Normally, a boundary layer of still water surrounds a submerged object, which acts as a barrier.

Waves breaking on the extremely rough edges of corals disrupt the boundary layer, allowing the corals access to passing nutrients. Turbulent water thereby promotes reef growth. Without the access to nutrients brought by rough coral surfaces, even the most effective recycling would not suffice. Deep nutrient-rich water entering coral reefs through isolated events may have significant effects on temperature and nutrient systems. Temperature regimes on coral reefs in the Bahamas and Florida are highly variable with temporal scales of minutes to seasons and spatial scales across depths.

Water can pass through coral reefs in various ways, including current rings, surface waves, internal waves and tidal changes. As tides interact with varying bathymetry and wind mixes with surface water, internal waves are created. An internal wave is a gravity wave that moves along density stratification within the ocean. When a water parcel encounters a different density it oscillates and creates internal waves. The irregular structure characteristic of coral reef bathymetry may enhance mixing and produce pockets of cooler water and variable nutrient content.

Cyanobacteria provide soluble nitrates via nitrogen fixation. Coral reefs often depend on surrounding habitats, such as seagrass meadows and mangrove forests , for nutrients. Seagrass and mangroves supply dead plants and animals that are rich in nitrogen and serve to feed fish and animals from the reef by supplying wood and vegetation.

Reefs, in turn, protect mangroves and seagrass from waves and produce sediment in which the mangroves and seagrass can root. Coral reefs form some of the world's most productive ecosystems, providing complex and varied marine habitats that support a wide range of other organisms. This level of variety in the environment benefits many coral reef animals, which, for example, may feed in the sea grass and use the reefs for protection or breeding. Reefs are home to a variety of animals, including fish, seabirds , sponges , cnidarians which includes some types of corals and jellyfish , worms , crustaceans including shrimp , cleaner shrimp , spiny lobsters and crabs , mollusks including cephalopods , echinoderms including starfish , sea urchins and sea cucumbers , sea squirts , sea turtles and sea snakes.

Aside from humans, mammals are rare on coral reefs, with visiting cetaceans such as dolphins the main exception. A few species feed directly on corals, while others graze on algae on the reef. The same hideouts in a reef may be regularly inhabited by different species at different times of day. Nighttime predators such as cardinalfish and squirrelfish hide during the day, while damselfish , surgeonfish , triggerfish , wrasses and parrotfish hide from eels and sharks. Reefs are chronically at risk of algal encroachment. Overfishing and excess nutrient supply from onshore can enable algae to outcompete and kill the coral.

Runoff can carry nitrogen and phosphorus which promote excess algae growth. Algae can sometimes out-compete the coral for space.

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The algae can then smother the coral by decreasing the oxygen supply available to the reef. Some sea urchins such as Diadema antillarum eat these algae and could thus decrease the risk of algal encroachment. Sponges are essential for the functioning of the coral reef that system. Algae and corals in coral reefs produce organic material. This is filtered through sponges which convert this organic material into small particles which in turn are absorbed by algae and corals. Over 4, species of fish inhabit coral reefs. Hypotheses include the "lottery", in which the first lucky winner recruit to a territory is typically able to defend it against latecomers, "competition", in which adults compete for territory, and less-competitive species must be able to survive in poorer habitat, and "predation", in which population size is a function of postsettlement piscivore mortality.

Sea urchins, Dotidae and sea slugs eat seaweed. Some species of sea urchins, such as Diadema antillarum , can play a pivotal part in preventing algae from overrunning reefs. A number of invertebrates, collectively called "cryptofauna," inhabit the coral skeletal substrate itself, either boring into the skeletons through the process of bioerosion or living in pre-existing voids and crevices. Animals boring into the rock include sponges, bivalve mollusks, and sipunculans. Those settling on the reef include many other species, particularly crustaceans and polychaete worms.

Coral reef systems provide important habitats for seabird species, some endangered. For example, Midway Atoll in Hawaii supports nearly three million seabirds, including two-thirds 1. Altogether, 17 species of seabirds live on Midway. The short-tailed albatross is the rarest, with fewer than 2, surviving after excessive feather hunting in the late 19th century. Sea snakes feed exclusively on fish and their eggs.

Some land-based reptiles intermittently associate with reefs, such as monitor lizards , the marine crocodile and semiaquatic snakes, such as Laticauda colubrina. Sea turtles , particularly hawksbill sea turtles , feed on sponges. The shell of Latiaxis wormaldi , a coral snail.

Coral reefs deliver ecosystem services to tourism, fisheries and coastline protection. To improve the management of coastal coral reefs, the World Resources Institute WRI developed and published tools for calculating the value of coral reef-related tourism, shoreline protection and fisheries, partnering with five Caribbean countries. As of April , published working papers covered St. Lucia, Tobago, Belize, and the Dominican Republic. The WRI was "making sure that the study results support improved coastal policies and management planning".

Coral reefs protect shorelines by absorbing wave energy, and many small islands would not exist without reefs. Reefs can attenuate waves as well as or better than artificial structures designed for coastal defence such as breakwaters. Restoring reefs is significantly cheaper than building artificial breakwaters in tropical environments. Expected damages from flooding would double, and costs from frequent storms would triple without the topmost meter of reefs.

About six million tons of fish are taken each year from coral reefs. Well-managed reefs have an average annual yield of 15 tons of seafood per square kilometer. Coral reefs are dying around the world. Broader threats are sea temperature rise, sea level rise and ocean acidification , all associated with greenhouse gas emissions. Air pollution can stunt the growth of coral reefs; including coal-burning and volcanic eruptions. In , researchers suggested that "extant marine invertebrates face the same synergistic effects of multiple stressors" that occurred during the end-Permian extinction , and that genera "with poorly buffered respiratory physiology and calcareous shells", such as corals, were particularly vulnerable.

Rock coral on seamounts are threatened by bottom trawling. These ecosystems take years to regrow, destroying coral communities faster than they can rebuild. Another cause for the death of coral reefs is bioerosion.

Various fishes graze corals and change the morphology of coral reefs making them more susceptible to other threats. Only the algae growing on dead corals is eaten and the live ones are not. However, this act still destroys the top layer of coral substrate and makes it harder for the reefs to sustain. Bleaching leaves corals vulnerable to disease, stunts their growth, and affects their reproduction, while severe bleaching kills them. Coral reefs with one type of zooxanthellae are more prone to bleaching than are reefs with another, more hardy, species.

Ecotourism in the Great Barrier Reef contributes to coral disease. In a large-scale systematic study of Jarvis Island coral community, scientists have observed ten coral bleaching events from to []. They are intended to promote responsible fishery management and habitat protection.

MPAs can encompass both social and biological objectives, including reef restoration, aesthetics, biodiversity and economic benefits. According to the Caribbean Coral Reefs - Status Report , states that; stop overfishing especially fishes key to coral reef like parrotfish , coastal zone management that reduce human pressure on reef, for example restricting coastal settlement, development and tourism and control pollution specially sewage , may reduce coral decline or even reverse it.

The report shows that healthier reefs in the Caribbean are those with large populations of parrotfish in countries that protect these key fishes and sea urchins , banning fish trap ping and spearfishing , creating "resilient reefs". To help combat ocean acidification, some laws are in place to reduce greenhouse gases such as carbon dioxide. Many land use laws aim to reduce CO 2 emissions by limiting deforestation. Deforestation can release significant amounts of CO 2 absent sequestration via active follow-up forestry programs. Deforestation can also cause erosion, which flows into the ocean, contributing to ocean acidification.

Incentives are used to reduce miles traveled by vehicles, which reduces carbon emissions into the atmosphere, thereby reducing the amount of dissolved CO 2 in the ocean. State and federal governments also regulate land activities that affect coastal erosion. Designating a reef as a biosphere reserve , marine park , national monument or world heritage site can offer protections.

This plan consists of adaptive management strategies, including reducing carbon footprint. A public awareness plan providezs education on the "rainforests of the sea" and how people can reduce carbon emissions. Inhabitants of Ahus Island, Manus Province , Papua New Guinea , have followed a generations-old practice of restricting fishing in six areas of their reef lagoon.

Their cultural traditions allow line fishing, but no net or spear fishing. Both biomass and individual fish sizes are significantly larger than in places where fishing is unrestricted. Coral aquaculture , also known as coral farming or coral gardening, is showing promise as a potentially effective tool for restoring coral reefs. The "gardening" 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.

Efforts to expand the size and number of coral reefs generally involve supplying substrate to allow more corals to find a home. Substrate materials include discarded vehicle tires, scuttled ships, subway cars and formed concrete, such as reef balls. Reefs grow unaided on marine structures such as oil rigs.

Biorock is a substrate produced by a patented process [] that runs low voltage electrical currents through seawater to cause dissolved minerals to precipitate onto steel structures. The resultant white carbonate aragonite is the same mineral that makes up natural coral reefs. Corals rapidly colonize and grow at accelerated rates on these coated structures. The electrical currents also accelerate formation and growth of both chemical limestone rock and the skeletons of corals and other shell-bearing organisms, such as oysters. The vicinity of the anode and cathode provides a high- pH environment which inhibits the growth of competitive filamentous and fleshy algae.

The increased growth rates fully depend on the accretion activity. Under the influence of the electric field, corals display an increased growth rate, size and density. One case study with coral reef restoration was conducted on the island of Oahu in Hawaii.

Many areas of coral reef patches in the channel had been damaged from past dredging in the channel. Dredging covers corals with sand. Coral larvae cannot settle on sand; they can only build on existing reefs or compatible hard surfaces, such as rock or concrete. Because of this, the University decided to relocate some of the coral. They transplanted them with the help of United States Army divers, to a site relatively close to the channel. They observed little if any damage to any of the colonies during transport and no mortality of coral reefs was observed on the transplant site.

While attaching the coral to the transplant site, they found that coral placed on hard rock grew well, including on the wires that attached the corals to the site. No environmental effects were seen from the transplantation process, recreational activities were not decreased, and no scenic areas were affected. Another possibility for coral restoration is gene therapy: Hawaiian coral reefs smothered by the spread of invasive algae were managed with a two-prong approach: Grazing pressure on invasive algae needed to be increased to prevent the regrowth of the algae. Researchers found that native collector urchins were reasonable candidate grazers for algae biocontrol, to extirpate the remaining invasive algae from the reef.

The times of maximum reef development were in the Middle Cambrian — Ma , Devonian — Ma and Carboniferous — Ma , owing to order Rugosa extinct corals and Late Cretaceous —66 Ma and all Neogene 23 Ma—present , owing to order Scleractinia corals. Not all reefs in the past were formed by corals: Measurements of the oxygen isotopic composition of the aragonitic skeleton of coral reefs, such as Porites , can indicate changes in sea surface temperature and sea surface salinity conditions during the growth of the coral.

This technique is often used by climate scientists to infer a region's paleoclimate. 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 disruptions.

The separated individuals can start new colonies. Many corals in the order Scleractinia are hermatypic , meaning that they are involved in building reefs. Most such corals obtain some of their energy from zooxanthellae in the genus Symbiodinium. These are symbiotic photosynthetic dinoflagellates which require sunlight; reef-forming corals are therefore found mainly in shallow water.

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They secrete calcium carbonate to form hard skeletons that become the framework of the reef. However, not all reef-building corals in shallow water contain zooxanthellae, and some deep water species, living at depths to which light cannot penetrate, form reefs but do not harbour the symbionts. There are various types of shallow-water coral reef, including fringing reefs, barrier reefs and atolls; most occur in tropical and subtropical seas.

They are very slow-growing, adding perhaps one centimetre 0. The Great Barrier Reef is thought to have been laid down about two million years ago. Over time, corals fragment and die, sand and rubble accumulates between the corals, and the shells of clams and other molluscs decay to form a gradually evolving calcium carbonate structure. Paleozoic corals often contained numerous endobiotic symbionts. Tabulate corals occur in limestones and calcareous shales of the Ordovician and Silurian periods, and often form low cushions or branching masses of calcite alongside rugose corals.

Rugose or horn 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 were also composed of calcite. The scleractinian 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 became common in the Jurassic and later periods. Timeline of the major coral fossil record and developments from m.

At certain times in the geological past, corals were very abundant. Like modern corals, these ancestors built reefs, some of which ended as great structures in sedimentary rocks. Fossils of fellow reef-dwellers algae, sponges, and the remains of many echinoids , brachiopods , bivalves , gastropods , and trilobites appear along with coral fossils. This makes some corals useful index fossils.

Coral reefs are under stress around the world. Broader threats are sea temperature rise, sea level rise and pH changes from ocean acidification , all associated with greenhouse gas emissions. Under such environmental stresses, corals expel their Symbiodinium ; without them coral tissues reveal the white of their skeletons, an event known as coral bleaching.

The colonies were small and patchily distributed, and had not formed structurally complex reefs such as those that compose the nearby Mesoamerican Barrier Reef System. Marine Protected Areas MPAs , Biosphere reserves , marine parks , national monuments world heritage status, fishery management and habitat protection can protect reefs from anthropogenic damage. Many governments now prohibit removal of coral from reefs, and inform coastal residents about reef protection and ecology.

While local action such as habitat restoration and herbivore protection can reduce local damage, the longer-term threats of acidification, temperature change and sea-level rise remain a challenge. To eliminate destruction of corals in their indigenous regions, projects have been started to grow corals in non-tropical countries. 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 and snorkeling tourism.

These activities can damage coral but international projects such as Green Fins that encourage dive and snorkel centres to follow a Code of Conduct have been proven to mitigate these risks. Corals' many colors give it appeal for necklaces and other jewelry. Intensely red coral is prized as a gemstone. Sometimes called fire coral, it is not the same as fire coral. Red coral is very rare because of overharvesting.

Always considered a precious mineral, "the Chinese have long associated red coral with auspiciousness and longevity because of its color and its resemblance to deer antlers so by association, virtue, long life, and high rank". Coral was known as shanhu in Chinese. In medicine, chemical compounds from corals are used to treat cancer, AIDS and pain, and for other uses. Isididae are also used for bone grafting in humans. Coral reefs in places such as the East African coast are used as a source of building material.

George's Tower of Oxford Castle , and the mediaeval walls of the city. Annual growth bands in some corals, such as the deep sea bamboo corals Isididae , may be among the first signs of the effects of ocean acidification on marine life. Certain species form communities called microatolls , which are colonies whose top is dead and mostly above the water line, but whose perimeter is mostly submerged and alive. Average tide 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 Radiocarbon dating. Such methods can help to reconstruct Holocene sea levels. Though coral have large sexually-reproducing populations, their evolution can be slowed by abundant asexual reproduction.

Also, coral longevity might factor into their adaptivity. However, adaptation to climate change has been demonstrated in many cases. These are usually due to a shift in coral and zooxanthellae genotypes. These shifts in allele frequency have progressed toward more tolerant types of zooxanthellae. In the Gulf of Mexico, where sea temperatures are rising, cold-sensitive staghorn and elkhorn coral have shifted in location.

Slower-growing but more heat-tolerant corals have become more common. Some reefs in current shadows represent a refugium location that will help them adjust to the disparity in the environment even if eventually the temperatures may rise more quickly there than in other locations.

Geochemistry anomalies within the crystalline structures of corals represent functions of temperature, salinity and oxygen isotopic composition. Such geochemical analysis can help with climate modeling. The global moisture budget is primarily being influenced by tropical sea surface temperatures from the position of the Intertropical Convergence Zone ITCZ. Climate research on live coral species is limited to a few studied species. Studying Porites coral provides a stable foundation for geochemical interpretations that is much simpler to physically extract data in comparison to Platygyra species where the complexity of Platygyra species skeletal structure creates difficulty when physically sampled, which happens to be one of the only multidecadal living coral records used for coral paleoclimate modeling.

The saltwater fishkeeping hobby has expanded, over recent years, to include reef tanks , fish tanks that include large amounts of live rock on which coral is allowed to grow and spread.

Tasmanian researchers discover new corals in dense 'underwater garden' on ocean mountains

The most popular kind of coral kept is soft coral , especially zoanthids and mushroom corals, which are especially easy to grow and propagate in a wide variety of conditions, because they originate in enclosed parts of reefs where water conditions vary and lighting may be less reliable and direct.

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 reefs , which have been declining around the world. Coral fragments known as "seeds" are grown in nurseries then replanted on the reef. It is also farmed by scientists for research, by businesses for the supply of the live and ornamental coral trade and by private aquarium hobbyists. Coral reefs and commons: Polyps of Eusmilia fastigiata. Pillar coral , Dendrogyra cylindricus.

Brain coral , Diploria labyrinthiformis. Fringing coral reef off the coast of Eilat , Israel. From Wikipedia, the free encyclopedia. Marine invertebrates of the class Anthozoa. For other uses, see Coral disambiguation.