Coral

Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically live in compact colonies of many identical individual polyps. Corals species include the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.

A coral "group" is a colony of myriad genetically identical polyps. Each polyp is a sac-like animal typically only a few millimeters in diameter and a few centimeters in length. A set of tentacles surround a central mouth opening. An exoskeleton is excreted near the base. Over many generations, the colony thus creates a large skeleton characteristic of the species. Individual heads grow by asexual reproduction of polyps. Corals also breed sexually by spawning: polyps of the same species release gametes simultaneously over a period of one to several nights around a full moon.

Although some corals are able to catch small fish and plankton using stinging cells on their tentacles, most corals obtain the majority of their energy and nutrients from photosynthetic unicellular dinoflagellates in the genus Symbiodinium that live within their tissues. These are commonly known as zooxanthellae. Such corals require sunlight and grow in clear, shallow water, typically at depths less than 60 metres (200 ft). Corals are major contributors to the physical structure of the coral reefs that develop in tropical and subtropical waters, such as the enormous Great Barrier Reef off the coast of Queensland, Australia.

Other corals do not rely on zooxanthellae and can live in much deeper water, with the cold-water genus Lophelia surviving as deep as 3,300 metres (10,800 ft).[1] Some have been found on the Darwin Mounds, northwest of Cape Wrath, Scotland, and others as far north as off the coast of Washington State and the Aleutian Islands.

Coral Outcrop Flynn Reef
A coral outcrop on the Great Barrier Reef, Australia

Taxonomy

Aristotle's pupil Theophrastus described the red coral, korallion, in his book on stones, implying it was a mineral, but he described it as a deep-sea plant in his Enquiries on Plants, where he also mentions large stony plants that reveal bright flowers when under water in the Gulf of Heroes.[2] Pliny the Elder stated boldly that several sea creatures including sea nettles and sponges "are neither animals nor plants, but are possessed of a third nature (tertius natura)".[3] Petrus Gyllius copied Pliny, introducing the term zoophyta for this third group in his 1535 book On the French and Latin Names of the Fishes of the Marseilles Region; it is popularly but wrongly supposed that Aristotle created the term.[3] Gyllius further noted, following Aristotle, how hard it was to define what was a plant and what was an animal.[3]

The Persian polymath Al-Biruni (d. 1048) classified sponges and corals as animals, arguing that they respond to touch.[4] Nevertheless, people believed corals to be plants until the eighteenth century, when William Herschel used a microscope to establish that coral had the characteristic thin cell membranes of an animal.[5]

Presently, corals are classified as certain species of animals within the sub-classes Hexacorallia and Octocorallia of the class Anthozoa in the phylum Cnidaria.[6] Hexacorallia includes the stony corals and these groups have polyps that generally have a 6-fold symmetry. Octocorallia includes blue coral and soft corals and species of Octocorallia have polyps with an eightfold symmetry, each polyp having eight tentacles and eight mesenteries.

Fire corals are not true corals, being in the order Anthomedusae (sometimes known as Anthoathecata) of the class Hydrozoa.[7]

Anatomy

Coral polyp
Anatomy of a stony coral polyp

Corals are sessile animals and differ from most other cnidarians in not having a medusa stage in their life cycle. The body unit of the animal is a polyp. Most corals are colonial, the initial polyp budding to produce another and the colony gradually developing from this small start. In stony corals, also known as hard corals, the polyps produce a skeleton composed of calcium carbonate to strengthen and protect the organism. This is deposited by the polyps and by the coenosarc, the living tissue that connects them. The polyps sit in cup-shaped depressions in the skeleton known as corallites. Colonies of stony coral are very variable in appearance; a single species may adopt an encrusting, plate-like, bushy, columnar or massive solid structure, the various forms often being linked to different types of habitat, with variations in light level and water movement being significant.[8]

Soft corals

In soft corals, there is no stony skeleton but the tissues are often toughened by the presence of tiny skeletal elements known as sclerites, which are made from calcium carbonate. Soft corals are very variable in form and most are colonial. A few soft corals are stolonate, but the polyps of most are connected by sheets of coenosarc. In some species this is thick and the polyps are deeply embedded. Some soft corals are encrusting or form lobes. Others are tree-like or whip-like and have a central axial skeleton embedded in the tissue matrix.[9] This is composed either of a fibrous protein called gorgonin or of a calcified material. In both stony and soft corals, the polyps can be retracted, with stony corals relying on their hard skeleton and cnidocytes for defence against predators, and soft corals generally relying on chemical defences in the form of toxic substances present in the tissues known as terpenoids.[8]

Stony corals

Montastrea cavernosa
Montastraea cavernosa polyps with tentacles extended

The polyps of stony corals have six-fold symmetry while those of soft corals have eight. The mouth of each polyp is surrounded by a ring of tentacles. In stony corals these are cylindrical and taper to a point, but in soft corals they are pinnate with side branches known as pinnules. In some tropical species these are reduced to mere stubs and in some they are fused to give a paddle-like appearance.[10] In most corals, the tentacles are retracted by day and spread out at night to catch plankton and other small organisms. Shallow water species of both stony and soft corals can be zooxanthellate, the corals supplementing their plankton diet with the products of photosynthesis produced by these symbionts.[8] The polyps interconnect by a complex and well-developed system of gastrovascular canals, allowing significant sharing of nutrients and symbionts.[11]

Coral skeletons are biocomposites (mineral + organics) Ca carbonate, in the form of calcite or aragonite. In scleractinian corals, "centers of calcification" and fibers are clearly distinct structures differing with respect to both morphology and chemical compositions of the crystalline units.[12] [13][1] [2] The organic matrices extracted from diverse species are acidic, and comprize proteins, sulphated sugars and lipids; they are species specific. [14] The soluble organic matrices of the skeletons allow to differentiate zooxanthellae and non zooxanthellae specimens.[15]

Ecology

Feeding

Polyps feed on a variety of small organisms, from microscopic zooplankton to small fish. The polyp's tentacles immobilize or kill prey using their nematocysts. These cells carry venom which they rapidly release in response to contact with another organism. A dormant nematocyst discharges in response to nearby prey touching the trigger (cnidocil). A flap (operculum) opens and its stinging apparatus fires the barb into the prey. The venom is injected through the hollow filament to immobilise the prey; the tentacles then manoeuvre the prey to the mouth.[16] [17]

The tentacles then contract to bring the prey into the stomach. Once the prey is digested, the stomach reopens, allowing the elimination of waste products and the beginning of the next hunting cycle. They can scavenge drifting organic molecules and dissolved organic molecules.[18]:24

Intracellular symbionts

Many corals, as well as other cnidarian groups such as Aiptasia (a sea anemone) form a symbiotic relationship with a class of dinoflagellate algae, zooxanthellae of the genus Symbiodinium.[18]:24 Aiptasia, a familiar pest among coral reef aquarium hobbyists, serves as a valuable model organism in the study of cnidarian-algal symbiosis.[19] Typically, each polyp harbors one species of alga, and coral species show a preference for Symbiodinium.[20] Young corals are not born with zooxanthellae, but acquire the algae from the surrounding environment, including the water column and local sediment.[21] Via photosynthesis, these provide energy for the coral, and aid in calcification.[22] The main benefit of the zooxanthellae is their ability to photosynthesize. By using this technique, zooxanthellae are able to supply corals with the products of photosynthesis, including glucose, glycerol, and amino acids, which the corals can use for energy.[23] As much as 30% of the tissue of a polyp may be algal material.[18]:23 Zooxanthellae also benefit corals by aiding in waste removal.[24]

The algae benefit from a safe place to live and consume the polyp's carbon dioxide and nitrogenous waste. Due to the strain the algae can put on the polyp, stress on the coral often drives them to eject the algae. Mass ejections are known as coral bleaching, because the algae contribute to coral's brown coloration; other colors, however, are due to host coral pigments, such as green fluorescent proteins (GFPs). Ejection increases the polyp's chance of surviving short-term stress—they can regain algae, possibly of a different species at a later time. If the stressful conditions persist, the polyp eventually dies.[25] Zooxanthellae are located within the corals' cytoplasm and due to the algae's photosynthetic activity, the internal pH of the coral can be raised; this behavior indicates that the zooxanthellae are responsible to some extent for the metabolism of their host corals [26]

Reproduction

Corals can be both gonochoristic (unisexual) and hermaphroditic, each of which can reproduce sexually and asexually. Reproduction also allows coral to settle in new areas. Reproduction is coordinated by chemical communication.

Sexual

Coral Life Cycles ZP
Life cycles of broadcasters and brooders

Corals predominantly reproduce sexually. About 25% of hermatypic corals (stony corals) form single sex (gonochoristic) colonies, while the rest are hermaphroditic.[27]

Broadcasters

About 75% of all hermatypic corals "broadcast spawn" by releasing gameteseggs and sperm—into the water to spread offspring. The gametes fuse during fertilization to form a microscopic larva called a planula, typically pink and elliptical in shape. A typical coral colony forms several thousand larvae per year to overcome the odds against formation of a new colony.[28]

Stony coral spawning 2
A male great star coral, Montastraea cavernosa, releasing sperm into the water.

Synchronous spawning is very typical on the coral reef, and often, even when multiple species are present, all corals spawn on the same night. This synchrony is essential so male and female gametes can meet. Corals rely on environmental cues, varying from species to species, to determine the proper time to release gametes into the water. The cues involve temperature change, lunar cycle, day length, and possibly chemical signalling.[27] Synchronous spawning may form hybrids and is perhaps involved in coral speciation.[29] The immediate cue is most often sunset, which cues the release.[27] The spawning event can be visually dramatic, clouding the usually clear water with gametes.

Brooders

Brooding species are most often ahermatypic (not reef-building) in areas of high current or wave action. Brooders release only sperm, which is negatively buoyant, sinking on to the waiting egg carriers who harbor unfertilized eggs for weeks. Synchronous spawning events sometimes occur even with these species.[27] After fertilization, the corals release planula that are ready to settle.[22]

Planulae

Planula larvae exhibit positive phototaxis, swimming towards light to reach surface waters, where they drift and grow before descending to seek a hard surface to which they can attach and begin a new colony. They also exhibit positive sonotaxis, moving towards sounds that emanate from the reef and away from open water.[30] High failure rates afflict many stages of this process, and even though millions of gametes are released by each colony, few new colonies form. The time from spawning to settling is usually two to three days, but can be up to two months.[31] The larva grows into a polyp and eventually becomes a coral head by asexual budding and growth.

Asexual

Orbicella annularis - calices
Basal plates (calices) of Orbicella annularis showing multiplication by budding (small central plate) and division (large double plate)
AuloporaDevonianSilicaShale
Tabulate coral Aulopora (Devonian) showing initial budding

Within a coral head, the genetically identical polyps reproduce asexually, either by budding (gemmation) or by dividing, whether longitudinally or transversely.

Budding involves splitting a smaller polyp from an adult.[28] As the new polyp grows, it forms its body parts. The distance between the new and adult polyps grows, and with it, the coenosarc (the common body of the colony). Budding can be intratentacular, from its oral discs, producing same-sized polyps within the ring of tentacles, or extratentacular, from its base, producing a smaller polyp.

Division forms two polyps that each become as large as the original. Longitudinal division begins when a polyp broadens and then divides its coelenteron (body), effectively splitting along its length. The mouth divides and new tentacles form. The two polyps thus created then generate their missing body parts and exoskeleton. Transversal division occurs when polyps and the exoskeleton divide transversally into two parts. This means one has the basal disc (bottom) and the other has the oral disc (top); the new polyps must separately generate the missing pieces.

Asexual reproduction offers the benefits of high reproductive rate, delaying senescence, and replacement of dead modules, as well as geographical distribution.[32]

Colony division

Whole colonies can reproduce asexually, forming two colonies with the same genotype. The possible mechanisms include fission, bailout and fragmentation. Fission occurs in some corals, especially among the family Fungiidae, where the colony splits into two or more colonies during early developmental stages. 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.[33]

Reefs

Coral reef locations
Locations of coral reefs around the world

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. 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.[34]

Hertshoon
Staghorn coral (Acropora cervicornis) is an important hermatypic coral from the Caribbean

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 (0.4 in) in height each year. 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.[35] Coral reefs are extremely diverse marine ecosystems hosting over 4,000 species of fish, massive numbers of cnidarians, molluscs, crustaceans, and many other animals.[36]

Evolutionary history

RugosaOrdovician
Solitary rugose coral (Grewingkia) in three views; Ordovician, southeastern Indiana

Although corals first appeared in the Cambrian period,[37] some 535 million years ago, fossils are extremely rare until the Ordovician period, 100 million years later, when rugose and tabulate corals became widespread. Paleozoic corals often contained numerous endobiotic symbionts.[38][39]

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. Their numbers began to decline during the middle of the Silurian period, and they became extinct at the end of the Permian period, 250 million years ago.[40]

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.[41]

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.[42] Scleractinian skeletons are composed of a form of calcium carbonate known as aragonite.[43] Although they are geologically younger than the tabulate and rugose corals, the aragonite of their skeletons is less readily preserved, and their fossil record is accordingly less complete.

RugosaScleractiniaTabulataEdiacaranCambrianCambrianOrdovicianOrdovicianSilurianSilurianDevonianDevonianCarboniferousCarboniferousPermianPermianTriassicTriassicJurassicCretaceousTertiaryPrecambrianPaleozoicMesozoicCenozoicPermian-Triassic extinctionLate Devonian extinctionCothoniidamya (unit)

Timeline of the major coral fossil record and developments from 650 m.y.a. to present.[44][45]

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.[46] Coral fossils are not restricted to reef remnants, and many solitary fossils may be found elsewhere, such as Cyclocyathus, which occurs in England's Gault clay formation.

Status

Threats

Reef0484
A healthy coral reef has a striking level of biodiversity in many forms of marine life.

Coral reefs are under stress around the world.[47] In particular, coral mining, agricultural and urban runoff, pollution (organic and inorganic), overfishing, blast fishing, disease, and the digging of canals and access into islands and bays are localized threats to coral ecosystems. Broader threats are sea temperature rise, sea level rise and pH changes from ocean acidification, all associated with greenhouse gas emissions.[48] In 1998, 16% of the world's reefs died as a result of increased water temperature.[49]

Approximately 10% of the world's coral reefs are dead.[50][51][52] About 60% of the world's reefs are at risk due to human-related activities.[53] The threat to reef health is particularly strong in Southeast Asia, where 80% of reefs are endangered.[54] Over 50% of the world's coral reefs may be destroyed by 2030; as a result, most nations protect them through environmental laws.[55]

In the Caribbean and tropical Pacific, direct contact between ~40–70% of common seaweeds and coral causes bleaching and death to the coral via transfer of lipid-soluble metabolites.[56] Seaweed and algae proliferate given adequate nutrients and limited grazing by herbivores such as parrotfish.

Water temperature changes of more than 1–2 °C (1.8–3.6 °F) or salinity changes can kill some species of coral. Under such environmental stresses, corals expel their Symbiodinium; without them coral tissues reveal the white of their skeletons, an event known as coral bleaching.[57]

Submarine springs found along the coast of Mexico's Yucatán Peninsula produce water with a naturally low pH (relatively high acidity) providing conditions similar to those expected to become widespread as the oceans absorb carbon dioxide.[58] Surveys discovered multiple species of live coral that appeared to tolerate the acidity. 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.[58]

Protection

Marine Protected Areas, Biosphere reserves, marine parks, national monuments world heritage status, fishery management and habitat protection can protect reefs from anthropogenic damage.[59]

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.[48]

To eliminate destruction of corals in their indigenous regions, projects have been started to grow corals in non-tropical countries.[60][61]

Relation to humans

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.[62]

Jewelry

6-Strand Necklace, Navajo (Native American), ca. 1920s, 71.57.1
6-strand necklace, Navajo (Native American), ca. 1920s, Brooklyn Museum

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.[63] In general, it is inadvisable to give coral as gifts since they are in decline from stressors like climate change, pollution, and unsustainable fishing.

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".[64] It reached its height of popularity during the Manchu or Qing Dynasty (1644-1911) when it was almost exclusively reserved for the emperor's use either in the form of coral beads (often combined with pearls) for court jewelry or as decorative Penjing (decorative miniature mineral trees). Coral was known as shanhu in Chinese. The "early-modern 'coral network' [began in] the Mediterranean Sea [and found its way] to Qing China via the English East India Company".[65] There were strict rules regarding its use in a code established by the Qianlong Emperor in 1759.

Medicine

ViennaDioscoridesCoral
Depiction of coral in the Juliana Anicia Codex, a 6th-century copy of Dioscorides' De Materia Medica. The facing page states that coral can be used to treat ulcers.[66]

In medicine, chemical compounds from corals are used to treat cancer, AIDS and pain, and for other uses. Coral skeletons, e.g. Isididae are also used for bone grafting in humans.[67] Coral Calx, known as Praval Bhasma in Sanskrit, is widely used in traditional system of Indian medicine as a supplement in the treatment of a variety of bone metabolic disorders associated with calcium deficiency.[68] In classical times ingestion of pulverized coral, which consists mainly of the weak base calcium carbonate, was recommended for calming stomach ulcers by Galen and Dioscorides.[69]

Construction

Syringoporid
Tabulate coral (a syringoporid); Boone limestone (Lower Carboniferous) near Hiwasse, Arkansas, scale bar is 2.0 cm.

Coral reefs in places such as the East African coast are used as a source of building material.[70] Ancient (fossil) coral limestone, notably including the Coral Rag Formation of the hills around Oxford (England), was once used as a building stone, and can be seen in some of the oldest buildings in that city including the Saxon tower of St Michael at the Northgate, St. George's Tower of Oxford Castle, and the mediaeval walls of the city.[71]

Shoreline protection

Healthy coral reefs absorb 97 percent of a wave’s energy, which buffers shorelines from currents, waves, and storms, helping to prevent loss of life and property damage. Coastlines protected by coral reefs are also more stable in terms of erosion than those without.[72]

Local Economies

Coastal communities near coral reefs rely heavily on them. Worldwide, more than 500 million people depend on coral reefs for food, income, coastal protection, and more.[73] The total economic value of coral reef services in the United States - including fisheries, tourism, and coastal protection - is more than $3.4 billion a year.

Climate research

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.[74] The growth rings allow geologists to construct year-by-year chronologies, a form of incremental dating, which underlie high-resolution records of past climatic and environmental changes using geochemical techniques.[75]

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.[76]

Increasing sea temperatures in tropical regions (~1 degree C) the last century have caused major coral bleaching, death, and therefore shrinking coral populations since although they are able to adapt and acclimate, it is uncertain if this evolutionary process will happen quickly enough to prevent major reduction of their numbers.[77]

Though coral have large sexually-reproducing populations, their evolution can be slowed by abundant asexual reproduction.[78] Gene flow is variable among coral species.[78] According to the biogeography of coral species gene flow cannot be counted on as a dependable source of adaptation as they are very stationary organisms. Also, coral longevity might factor into their adaptivity.[78]

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.[79] Scientists found that a certain scleractinian zooxanthella is becoming more common where sea temperature is high.[80][81] Symbionts able to tolerate warmer water seem to photosynthesise more slowly, implying an evolutionary trade-off.[81]

In the Gulf of Mexico, where sea temperatures are rising, cold-sensitive staghorn and elkhorn coral have shifted in location.[79] Not only have the symbionts and specific species been shown to shift, but there seems to be a certain growth rate favorable to selection. Slower-growing but more heat-tolerant corals have become more common.[82] The changes in temperature and acclimation are complex. 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.[83] This separation of populations by climatic barriers causes a realized niche to shrink greatly in comparison to the old fundamental niche.

Geochemistry

Corals are shallow, colonial organisms that integrate δ18O and trace elements into their skeletal aragonite (polymorph of calcite) crystalline structures, as they grow. 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.[84]

Time can be attributed to coral geochemistry anomalies by correlating strontium/calcium minimums with sea surface temperature (SST) maximums to data collected from NINO 3.4 SSTA.[85]

The comparison of coral strontium/calcium minimums with sea surface temperature maximums, data recorded from NINO 3.4 SSTA, time can be correlated to coral strontium/calcium and δ18O variations. To confirm accuracy of the annual relationship between Sr/Ca and δ18O variations, a perceptible association to annual coral growth rings confirms the age conversion. Geochronology is established by the blending of Sr/Ca data, growth rings, and stable isotope data. El Nino-Southern Oscillation (ENSO) is directly related to climate fluctuations that influence coral δ18O ratio from local salinity variations associated with the position of the South Pacific convergence zone (SPCZ) and can be used for ENSO modeling.[85]

Global Sea Surface Temperature - GPN-2003-00032
Global sea surface temperature (SST)

The global moisture budget is primarily being influenced by tropical sea surface temperatures from the position of the Intertropical Convergence Zone (ITCZ).[86] The Southern Hemisphere has a unique meteorological feature positioned in the southwestern Pacific Basin called the South Pacific Convergence Zone (SPCZ), which contains a perennial position within the Southern Hemisphere. During ENSO warm periods, the SPCZ reverses orientation extending from the equator down south through Solomon Islands, Vanuatu, Fiji and towards the French Polynesian Islands; and due east towards South America affecting geochemistry of corals in tropical regions.[87]

Geochemical analysis of skeletal coral can be linked to sea surface salinity (SSS) and sea surface temperature (SST), from El Nino 3.4 SSTA data, of tropical oceans to seawater δ18O ratio anomalies from corals. ENSO phenomenon can be related to variations in sea surface salinity (SSS) and sea surface temperature (SST) that can help model tropical climate activities.[88]

Porites lutea
Porites lutea

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.[88]

Aquaria

Zoanthus-dragon-eye
This dragon-eye zoanthid is a popular source of color in reef tanks

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.[89] These tanks are either kept in a natural-like state, with algae (sometimes in the form of an algae scrubber) and a deep sand bed providing filtration,[90] or as "show tanks", with the rock kept largely bare of the algae and microfauna that would normally populate it,[91] in order to appear neat and clean.

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.[92] More serious fishkeepers may keep small polyp stony coral, which is from open, brightly lit reef conditions and therefore much more demanding, while large polyp stony coral is a sort of compromise between the two.

Aquaculture

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.[93][94][95] The process bypasses the early growth stages of corals when they are most at risk of dying. Coral fragments known as "seeds" are grown in nurseries then replanted on the reef.[96] Coral is farmed by coral farmers who live locally to the reefs and farm for reef conservation or for income. 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.

Gallery

Further images: commons:Category:Coral reefs and commons:Category:Corals

Mushroom Coral (Fungia) Top Macro 91

Fungia sp. skeleton

Dendrogyra cylindrus (pillar coral) (San Salvador Island, Bahamas) 1 (15513345363)

Pillar coral, Dendrogyra cylindricus

Brain coral spawning

Brain coral spawning

Stony coral spawning 3

Brain coral releasing eggs

EilatFringingReef

Fringing coral reef off the coast of Eilat, Israel.

See also

References

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External links

Atoll

An atoll ( , , , , or ), sometimes called a coral atoll, is a ring-shaped coral reef including a coral rim that encircles a lagoon partially or completely. There may be coral islands or cays on the rim. The coral of the atoll often sits atop the rim of an extinct seamount or volcano which has eroded or subsided partially beneath the water. The lagoon forms over the volcanic crater or caldera while the higher rim remains above water or at shallow depths that permit the coral to grow and form the reefs. For the atoll to persist, continued erosion or subsidence must be at a rate slow enough to permit reef growth upward and outward to replace the lost height.

Battle of the Coral Sea

The Battle of the Coral Sea, fought from 4–8 May 1942, was a major naval battle between the Imperial Japanese Navy (IJN) and naval and air forces from the United States and Australia, taking place in the Pacific Theatre of World War II. The battle is historically significant as the first action in which aircraft carriers engaged each other, as well as the first in which the opposing ships neither sighted nor fired directly upon one another.

In an attempt to strengthen their defensive position in the South Pacific, the Japanese decided to invade and occupy Port Moresby (in New Guinea) and Tulagi (in the southeastern Solomon Islands). The plan to accomplish this was called Operation MO, and involved several major units of Japan's Combined Fleet. These included two fleet carriers and a light carrier to provide air cover for the invasion forces. It was under the overall command of Japanese Admiral Shigeyoshi Inoue.

The U.S. learned of the Japanese plan through signals intelligence, and sent two United States Navy carrier task forces and a joint Australian-U.S. cruiser force to oppose the offensive. These were under the overall command of U.S. Admiral Frank J. Fletcher.

On 3–4 May, Japanese forces successfully invaded and occupied Tulagi, although several of their supporting warships were sunk or damaged in surprise attacks by aircraft from the U.S. fleet carrier Yorktown. Now aware of the presence of U.S. carriers in the area, the Japanese fleet carriers advanced towards the Coral Sea with the intention of locating and destroying the Allied naval forces. On the evening of 6 May, the direction chosen for air searches by the opposing commanders brought the two carrier forces to within 70 nmi (81 mi; 130 km) of each other, unbeknownst to both sides. Beginning on 7 May, the carrier forces from the two sides engaged in airstrikes over two consecutive days. On the first day, both forces mistakenly believed they were attacking their opponent's fleet carriers, but were actually attacking other units, with the U.S. sinking the Japanese light carrier Shōhō while the Japanese sank a U.S. destroyer and heavily damaged a fleet oiler (which was later scuttled). The next day, the fleet carriers found and engaged each other, with the Japanese fleet carrier Shōkaku heavily damaged, the U.S. fleet carrier Lexington critically damaged (and later scuttled), and Yorktown damaged. With both sides having suffered heavy losses in aircraft and carriers damaged or sunk, the two forces disengaged and retired from the battle area. Because of the loss of carrier air cover, Inoue recalled the Port Moresby invasion fleet, intending to try again later.

Although a tactical victory for the Japanese in terms of ships sunk, the battle would prove to be a strategic victory for the Allies for several reasons. The battle marked the first time since the start of the war that a major Japanese advance had been checked by the Allies. More importantly, the Japanese fleet carriers Shōkaku and Zuikaku—the former damaged and the latter with a depleted aircraft complement—were unable to participate in the Battle of Midway the following month, while Yorktown did participate, ensuring a rough parity in aircraft between the two adversaries and contributing significantly to the U.S. victory in that battle. The severe losses in carriers at Midway prevented the Japanese from reattempting to invade Port Moresby from the ocean and helped prompt their ill-fated land offensive over the Kokoda Track. Two months later, the Allies took advantage of Japan's resulting strategic vulnerability in the South Pacific and launched the Guadalcanal Campaign; this, along with the New Guinea Campaign, eventually broke Japanese defenses in the South Pacific and was a significant contributing factor to Japan's ultimate surrender in World War II.

Cape Coral, Florida

Cape Coral is a city located in Lee County, Florida, United States, on the Gulf of Mexico. Founded in 1957 and developed as a planned community, the city grew to a population of 154,305 by the year 2010. The city's population estimate was 165,831 for 2013 and 179,804 for 2016. With an area of 120 square miles (310 km2), Cape Coral is the largest city between Tampa and Miami. It is a principal city in the Cape Coral – Fort Myers, Florida Metropolitan Statistical Area. The population estimate for the statistical area was 679,513 for 2014. The city is known as a "Waterfront Wonderland"; with over 400 mi (640 km) of navigable waterways, Cape Coral has more miles of canals than any other city in the world.

Coral Gables, Florida

Coral Gables, officially the City of Coral Gables, is a city in Miami-Dade County, Florida, United States, located southwest of Downtown Miami. The United States Census Bureau estimates conducted in 2017 yielded the city had a population of 51,095. Coral Gables is home to the University of Miami.

Coral Sea

The Coral Sea (French: Mer de Corail) is a marginal sea of the South Pacific off the northeast coast of Australia, and classified as an interim Australian bioregion. The Coral Sea extends 2,000 kilometres (1,200 mi) down the Australian northeast coast.

It is bounded in the west by the east coast of Queensland, thereby including the Great Barrier Reef, in the east by Vanuatu (formerly the New Hebrides) and by New Caledonia, and in the northeast approximately by the southern extremity of the Solomon Islands. In the northwest, it reaches to the south coast of eastern New Guinea, thereby including the Gulf of Papua. It merges with the Tasman Sea in the south, with the Solomon Sea in the north and with the Pacific Ocean in the east. On the west, it is bounded by the mainland coast of Queensland, and in the northwest, it connects with the Arafura Sea through the Torres Strait.The sea is characterised by its warm and stable climate, with frequent rains and tropical cyclones. It contains numerous islands and reefs, as well as the world's largest reef system, the Great Barrier Reef (GBR), which was declared a World Heritage Site by UNESCO in 1981. All previous oil exploration projects were terminated at the GBR in 1975, and fishing is restricted in many areas. The reefs and islands of the Coral Sea are particularly rich in birds and aquatic life and are a popular tourist destination, both nationally and internationally.

Coral Sea Islands

The Coral Sea Islands Territory is an external territory of Australia which comprises a group of small and mostly uninhabited tropical islands and reefs in the Coral Sea, northeast of Queensland, Australia. The only inhabited island is Willis Island. The territory covers 780,000 km2 (301,160 sq mi), most of which is ocean, extending east and south from the outer edge of the Great Barrier Reef and includes Heralds Beacon Island, Osprey Reef, the Willis Group and fifteen other reef/island groups. Cato Island is the highest point in the Territory.

Coral Springs, Florida

Coral Springs, officially the City of Coral Springs, is a city in Broward County, Florida, United States, approximately 20 miles (32 km) northwest of Fort Lauderdale. As of the 2010 United States Census, the city had a population of 121,096. It is a principal city of the Miami metropolitan area, which was home to an estimated 6,012,331 people at the 2015 census.

The city, officially chartered on July 10, 1963, was master-planned and primarily developed by Coral Ridge Properties, which was acquired by Westinghouse in 1966. The city's name is derived from the company's name, and was selected after several earlier proposals had been considered and rejected. Despite the name, there are no natural springs in the city; Florida's springs are found in the central and northern portions of the state.During the 1970s, 1980s, and 1990s the young city grew rapidly, adding over 35,000 residents each decade. Coral Springs has notably strict building codes, which are designed to maintain the city's distinctive aesthetic appeal. The city government's effective fiscal management has maintained high bond ratings, and the city has won accolades for its overall livability, its low crime rate, and its family-friendly orientation.

Coral bleaching

Coral bleaching occurs when coral polyps expel algae that live inside their tissues. Normally, coral polyps live in an endosymbiotic relationship with this algae crucial for the health of the coral and the reef. The algae provides up to 90% of the coral's energy. Bleached corals continue

to live but begin to starve after bleaching. Some corals recover.

Above-average sea water temperatures caused by global warming is the leading cause of coral bleaching. According to the United Nations Environment Programme, between 2014 and 2016 the longest recorded global bleaching events killed coral on an unprecedented scale. In 2016, bleaching of coral on the Great Barrier Reef killed between 29 and 50 percent of the reef's coral. In 2017, the bleaching extended into the central region of the reef. The average interval between bleaching events has halved between 1980 and 2016.

Coral reef

A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups.

Coral belongs to the class Anthozoa in the animal phylum Cnidaria, which includes sea anemones and jellyfish. Unlike sea anemones, corals secrete hard carbonate exoskeletons that support and protect the coral. Most reefs grow best in warm, shallow, clear, sunny and agitated water.

Often called "rainforests of the sea", shallow coral reefs form some of Earth's most diverse ecosystems. They occupy less than 0.1% of the world's ocean area, about half the area of France, yet they provide a home for at least 25% of all marine species, including fish, mollusks, worms, crustaceans, echinoderms, sponges, tunicates and other cnidarians. Coral reefs flourish in ocean waters that provide few nutrients. They are most commonly found at shallow depths in tropical waters, but deep water and cold water coral reefs exist on smaller scales in other areas.

Coral reefs deliver ecosystem services for tourism, fisheries and shoreline protection. The annual global economic value of coral reefs is estimated between US$30–375 billion and 9.9 trillion USD . Coral reefs are fragile, partly because they are sensitive to water conditions. They are under threat from excess nutrients (nitrogen and phosphorus), rising temperatures, oceanic acidification, overfishing (e.g., from blast fishing, cyanide fishing, spearfishing on scuba), sunscreen use, and harmful land-use practices, including runoff and seeps (e.g., from injection wells and cesspools).

Coral reef fish

Coral reef fish are fish which live amongst or in close relation to coral reefs. Coral reefs form complex ecosystems with tremendous biodiversity. Among the myriad inhabitants, the fish stand out as colourful and interesting to watch. Hundreds of species can exist in a small area of a healthy reef, many of them hidden or well camouflaged. Reef fish have developed many ingenious specialisations adapted to survival on the reefs.

Coral reefs occupy less than one percent of the surface area of the world oceans, but still they provide a home for 25 percent of all marine fish species. Reef habitats are a sharp contrast to the open water habitats that make up the other 99% of the world oceans.

However, loss and degradation of coral reef habitat, increasing pollution, and overfishing including the use of destructive fishing practices, are threatening the survival of the coral reefs and the associated reef fish.

Coral snake

Coral snakes are a large group of elapid snakes that can be subdivided into two distinct groups, Old World coral snakes and New World coral snakes. There are 16 species of Old World coral snake in three genera (Calliophis, Hemibungarus, and Sinomicrurus), and over 65 recognized species of New World coral snakes in three genera (Leptomicrurus, Micruroides, and Micrurus). Genetic studies have found that the most basal lineages are Asian, indicating that the group originated in the Old World.

Corallimorpharia

Corallimorpharia is an order of marine cnidarians closely related to stony or reef building corals (Scleractinia). They are mostly tropical, with a narrow column topped with a wide oral disc. The tentacles are usually short or very short, arranged in rows radiating from the mouth. Many species occur together in large groups. In many respects, they resemble the stony corals, except for the absence of a stony skeleton.

Corallimorpharians occur in a wide range of marine habitats, and are associated with phase shifts in coral reef ecosystems that result in a change from a hard-coral dominated reef to a soft-coral dominated one. Many species are also common invertebrates kept in marine aquaria.

Crab

Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) (Greek: βραχύς, translit. brachys = short, οὐρά / οura = tail), usually entirely hidden under the thorax. They live in all the world's oceans, in fresh water, and on land, are generally covered with a thick exoskeleton and have a single pair of pincers. Many other animals with similar names – such as hermit crabs, king crabs, porcelain crabs, horseshoe crabs, and crab lice – are not true crabs.

Great Barrier Reef

The Great Barrier Reef is the world's largest coral reef system composed of over 2,900 individual reefs and 900 islands stretching for over 2,300 kilometres (1,400 mi) over an area of approximately 344,400 square kilometres (133,000 sq mi). The reef is located in the Coral Sea, off the coast of Queensland, Australia. The Great Barrier Reef can be seen from outer space and is the world's biggest single structure made by living organisms. This reef structure is composed of and built by billions of tiny organisms, known as coral polyps. It supports a wide diversity of life and was selected as a World Heritage Site in 1981. CNN labelled it one of the seven natural wonders of the world. The Queensland National Trust named it a state icon of Queensland.A large part of the reef is protected by the Great Barrier Reef Marine Park, which helps to limit the impact of human use, such as fishing and tourism. Other environmental pressures on the reef and its ecosystem include runoff, climate change accompanied by mass coral bleaching, dumping of dredging sludge and cyclic population outbreaks of the crown-of-thorns starfish. According to a study published in October 2012 by the Proceedings of the National Academy of Sciences, the reef has lost more than half its coral cover since 1985.The Great Barrier Reef has long been known to and used by the Aboriginal Australian and Torres Strait Islander peoples, and is an important part of local groups' cultures and spirituality. The reef is a very popular destination for tourists, especially in the Whitsunday Islands and Cairns regions. Tourism is an important economic activity for the region, generating over AUD$3 billion per year. In November 2014, Google launched Google Underwater Street View in 3D of the Great Barrier Reef.A March 2016 report stated that coral bleaching was more widespread than previously thought, seriously affecting the northern parts of the reef as a result of warming ocean temperatures. In October 2016, Outside published an obituary for the reef; the article was criticized for being premature and hindering efforts to bolster the resilience of the reef. In March 2017, the journal Nature published a paper showing that huge sections of an 800-kilometre (500 mi) stretch in the northern part of the reef had died in the course of 2016 due to high water temperatures, an event that the authors put down to the effects of global climate change. The percentage of baby corals being born on the Great Barrier Reef dropped drastically in 2018 and scientists are describing it as the early stage of a "huge natural selection event unfolding". The reason behind low birth of new corals is that many of the mature breeding adults died in the bleaching events of 2016-17, and thus could not produce offspring. The types of corals that reproduced changed too, which points towards the fact that there will be long-term reorganisation of the reef ecosystem if the trend continues.

Lee County, Florida

Lee County is located in southwest Florida on the Gulf Coast. As of the 2010 census, the population was 618,754. The county seat is Fort Myers (with a 2016 estimated population of 77,146), and the largest city is Cape Coral with an estimated 2016 population of 179,804.

Lee County comprises the Cape Coral–Fort Myers, FL Metropolitan Statistical Area.

Lee County was created in 1887 from Monroe County. Today, Fort Myers is the center of a popular tourist area in Southwest Florida and the seat of Lee County. It is about 120 miles (190 km) south of Tampa at the meeting point of the Gulf of Mexico and the Caloosahatchee River. Currently, Lee County is the spring home of the Boston Red Sox and Minnesota Twins MLB teams for spring training.

Marble

Marble is a metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or dolomite. Marble is typically not foliated, although there are exceptions. In geology, the term "marble" refers to metamorphosed limestone, but its use in stonemasonry more broadly encompasses unmetamorphosed limestone. Marble is commonly used for sculpture and as a building material.

Polyp

A polyp in zoology is one of two forms found in the phylum Cnidaria, the other being the medusa. Polyps are roughly cylindrical in shape and elongated at the axis of the vase-shaped body. In solitary polyps, the aboral end is attached to the substrate by means of a disc-like holdfast called the pedal disc, while in colonies of polyps it is connected to other polyps, either directly or indirectly. The oral end contains the mouth, and is surrounded by a circlet of tentacles.

Reef

A reef is a bar of rock, sand, coral or similar material, lying beneath the surface of water.

Many reefs result from natural, abiotic processes—deposition of sand, wave erosion planing down rock outcrops, etc.—but the best known reefs are the coral reefs of tropical waters developed through biotic processes dominated by corals and coralline algae.

Artificial reefs (e.g. shipwrecks) sometimes have a role in enhancing the physical complexity of featureless sand bottoms, in order to attract a diverse assemblage of organisms, especially algae and fish.

Earth's largest reef system is the Great Barrier Reef in Australia, at a length of over 2,300 kilometres (1,400 miles).

Scleractinia

Scleractinia, also called stony corals or hard corals, are marine animals in the phylum Cnidaria that build themselves a hard skeleton. The individual animals are known as polyps and have a cylindrical body crowned by an oral disc in which a mouth is fringed with tentacles. Although some species are solitary, most are colonial. The founding polyp settles and starts to secrete calcium carbonate to protect its soft body. Solitary corals can be as much as 25 cm (10 in) across but in colonial species the polyps are usually only a few millimetres in diameter. These polyps reproduce asexually by budding, but remain attached to each other, forming a multi-polyp colony of clones with a common skeleton, which may be up to several metres in diameter or height according to species.

The shape and appearance of each coral colony depends not only on the species, but also on its location, depth, the amount of water movement and other factors. Many shallow-water corals contain symbiont unicellular organisms known as zooxanthellae within their tissues. These give their colour to the coral which thus may vary in hue depending on what species of symbiont it contains. Stony corals are closely related to sea anemones, and like them are armed with stinging cells known as cnidocytes. Corals reproduce both sexually and asexually. Most species release gametes into the sea where fertilisation takes place, and the planula larvae drift as part of the plankton, but a few species brood their eggs. Asexual reproduction is mostly by fragmentation, when part of a colony becomes detached and reattaches elsewhere.

Stony corals occur in all the world's oceans. Much of the framework of modern coral reefs is formed by scleractinians. Reef-building or hermatypic corals are mostly colonial; most of these are zooxanthellate and are found in the shallow waters into which sunlight penetrates. Other corals that do not form reefs may be solitary or colonial; some of these occur at abyssal depths where no light reaches.

Stony corals first appeared in the Middle Triassic, but their relationship to the tabulate and rugose corals of the Paleozoic is currently unresolved. Stony coral reefs are dying out worldwide due to human activity, including the effects of global warming and ocean acidification, and may become extinct in the next 100-200 years.

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