Coral

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

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.

Marine invertebrates

Marine invertebrates are the invertebrates that live in marine habitats. Invertebrate is a blanket term that includes all animals apart from the vertebrate members of the chordate phylum. Invertebrates lack a vertebral column, and some have evolved a shell or a hard exoskeleton. As on land and in the air, marine invertebrates have a large variety of body plans, and have been categorised into over 30 phyla. They make up most of the macroscopic life in the oceans.

In biological classification, class is a taxonomic rank, as well as a taxonomic unit, a taxon, in that rank. Other well-known ranks in descending order of size are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order.

Anthozoa class of cnidarians

Anthozoa is a class of marine invertebrates which includes the sea anemones, stony corals and soft corals. Adult anthozoans are almost all attached to the seabed, while their larvae can disperse as part of the plankton. The basic unit of the adult is the polyp; this consists of a cylindrical column topped by a disc with a central mouth surrounded by tentacles. Sea anemones are mostly solitary, but the majority of corals are colonial, being formed by the budding of new polyps from an original, founding individual. Colonies are strengthened by calcium carbonate and other materials and take various massive, plate-like, bushy or leafy forms.

Contents

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.

Cloning process of producing similar populations of genetically identical individuals by Changing Nucleus

Cloning is the process of producing genetically identical individuals of an organism either naturally or artificially. In nature, many organisms produce clones through asexual reproduction. Cloning in biotechnology refers to the process of creating clones of organisms or copies of cells or DNA fragments. Beyond biology, the term refers to the production of multiple copies of digital media or software.

Tentacle varied organ found in many animals and used for palpation and manipulation

In zoology, a tentacle is a flexible, mobile, elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work mainly like muscular hydrostats. Most forms of tentacles are used for grasping and feeding. Many are sensory organs, variously receptive to touch, vision, or to the smell or taste of particular foods or threats. Examples of such tentacles are the eyestalks of various kinds of snails. Some kinds of tentacles have both sensory and manipulatory functions.

Exoskeleton External skeleton of an organism

An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton (endoskeleton) of, for example, a human. In usage, some of the larger kinds of exoskeletons are known as "shells". Examples of animals with exoskeletons include insects such as grasshoppers and cockroaches, and crustaceans such as crabs and lobsters, as well as the shells of certain sponges and the various groups of shelled molluscs, including those of snails, clams, tusk shells, chitons and nautilus. Some animals, such as the tortoise, have both an endoskeleton and an exoskeleton.

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 Great Barrier Reef off the coast of Queensland, Australia.

Fish vertebrate animal that lives in water and (typically) has gills

Fish are gill-bearing aquatic craniate animals that lack limbs with digits. They form a sister group to the tunicates, together forming the olfactores. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Tetrapods emerged within lobe-finned fishes, so cladistically they are fish as well. However, traditionally fish are rendered paraphyletic by excluding the tetrapods. Because in this manner the term "fish" is defined negatively as a paraphyletic group, it is not considered a formal taxonomic grouping in systematic biology, unless it is used in the cladistic sense, including tetrapods. The traditional term pisces is considered a typological, but not a phylogenetic classification.

Plankton Organisms that live in the water column and are incapable of swimming against a current

Plankton are the diverse collection of organisms that live in large bodies of water and are unable to swim against a current. The individual organisms constituting plankton are called plankters. They provide a crucial source of food to many small and large aquatic organisms, such as bivalves, fish and whales.

Cnidocyte explosive cell containing one giant secretory organelle (cnida)

A cnidocyte is an explosive cell containing one giant secretory organelle or cnida that defines the phylum Cnidaria. Cnidae are used for prey capture and defense from predators. Despite being morphologically simple, lacking a skeleton and many species being sessile, cnidarians prey on fish and crustaceans. A cnidocyte fires a structure that contains the toxin, from a characteristic subcellular organelle called a cnidocyst. This is responsible for the stings delivered by a cnidarian.

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 as far north as the Darwin Mounds, northwest of Cape Wrath, Scotland, and others off the coast of Washington State and the Aleutian Islands.

A genus is a taxonomic rank used in the biological classification of living and fossil organisms, as well as viruses, in biology. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclature, the genus name forms the first part of the binomial species name for each species within the genus.

<i>Lophelia</i> Species of cnidarian

Lophelia pertusa, the only species in the genus Lophelia, is a cold-water coral which grows in the deep waters throughout the North Atlantic ocean, as well as parts of the Caribbean Sea and Alboran Sea. L. pertusa reefs are home to a diverse community, however the species is extremely slow growing and may be harmed by destructive fishing practices, or oil exploration and extraction.

Darwin Mounds is a large field of undersea sand mounds situated off the north west coast of Scotland that were first discovered in May 1998. They provide a unique habitat for ancient deep water coral reefs and were found using remote sensing techniques during surveys funded by the oil industry and steered by the joint industry and United Kingdom government group the Atlantic Frontier Environment Network (AFEN). The mounds were named after the research vessel, itself named for the eminent naturalist and evolutionary theorist Charles Darwin.

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]

Aristotle Classical Greek philosopher

Aristotle was a Greek philosopher and polymath during the Classical period in Ancient Greece. He was the founder of the Lyceum and the Peripatetic school of philosophy and Aristotelian tradition. Along with his teacher Plato, he has been called the "Father of Western Philosophy". His writings cover many subjects – including physics, biology, zoology, metaphysics, logic, ethics, aesthetics, poetry, theatre, music, rhetoric, psychology, linguistics, economics, politics and government. Aristotle provided a complex synthesis of the various philosophies existing prior to him, and it was above all from his teachings that the West inherited its intellectual lexicon, as well as problems and methods of inquiry. As a result, his philosophy has exerted a unique influence on almost every form of knowledge in the West and it continues to be a subject of contemporary philosophical discussion.

Theophrastus Ancient greek philosopher

Theophrastus, a Greek native of Eresos in Lesbos, was the successor to Aristotle in the Peripatetic school. He came to Athens at a young age and initially studied in Plato's school. After Plato's death, he attached himself to Aristotle who took to Theophrastus in his writings. When Aristotle fled Athens, Theophrastus took over as head of the Lyceum. Theophrastus presided over the Peripatetic school for thirty-six years, during which time the school flourished greatly. He is often considered the father of botany for his works on plants. After his death, the Athenians honoured him with a public funeral. His successor as head of the school was Strato of Lampsacus.

Red Sea Arm of the Indian Ocean between Arabia and Africa

The Red Sea is a seawater inlet of the Indian Ocean, lying between Africa and Asia. The connection to the ocean is in the south through the Bab el Mandeb strait and the Gulf of Aden. To the north lie the Sinai Peninsula, the Gulf of Aqaba, and the Gulf of Suez. The Red Sea is a Global 200 ecoregion. The sea is underlain by the Red Sea Rift which is part of the Great Rift Valley.

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] [ better source needed ]

Al-Biruni 11th-century Persian scholar and polymath

Abū Rayḥān Muḥammad ibn Aḥmad Al-Bīrūnī, known as Biruni or Al-Biruni in English language, was an Iranian scholar and polymath. He was from Khwarazm – a region which encompasses modern-day western Uzbekistan, and northern Turkmenistan.

William Herschel 18th- and 19th-century German-born British astronomer and composer

Frederick William Herschel, was a German-born British astronomer, composer and brother of fellow astronomer Caroline Herschel, with whom he worked. Born in the Electorate of Hanover, Herschel followed his father into the Military Band of Hanover, before migrating to Great Britain in 1757 at the age of nineteen.

Animal Kingdom of motile multicellular eukaryotic heterotrophic organisms

Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres (110 ft). They have complex interactions with each other and their environments, forming intricate food webs. The kingdom Animalia includes humans, but in colloquial use the term animal often refers only to non-human animals. The study of non-human animals is known as zoology.

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

Anatomy of a stony coral polyp Coral polyp.jpg
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

Soft corals have no solid exoskeleton per se. However, their tissues are often reinforced by small supportive elements known as "sclerites" made of calcium carbonate.

Soft corals vary considerably in form, and most are colonial. A few soft corals are stolonate, but the polyps of most are connected by sheets of coenosarc [def], and in some species these sheets are thick and the polyps deeply embedded in them. Some soft corals encrust other sea objects or form lobes. Others are tree-like or whip-like and chem a central axial skeleton embedded at its base in the matrix of the supporting branch. [9] These branches are 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. Soft corals generally secrete terpenoid toxins to ward off predators. [8]

Stony corals

Montastraea cavernosa polyps with tentacles extended Montastrea cavernosa.jpg
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] The organic matrices extracted from diverse species are acidic, and comprise 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

Discharge mechanism of a stinging cell (nematocyst) Nematocyst discharge.png
Discharge mechanism of a stinging cell (nematocyst)

Feeding

Polyps feed on a variety of small organisms, from microscopic zooplankton to small fish. The polyp's tentacles immobilize or kill prey using stinging cells called 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 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. [16] :24

Intracellular symbionts

Many corals, as well as other cnidarian groups such as sea anemones form a symbiotic relationship with a class of dinoflagellate algae, zooxanthellae of the genus Symbiodinium , which can form as much as 30% of the tissue of a polyp. [16] :23-24 Typically, each polyp harbors one species of alga, and coral species show a preference for Symbiodinium . [17] Young corals are not born with zooxanthellae, but acquire the algae from the surrounding environment, including the water column and local sediment. [18] The main benefit of the zooxanthellae is their ability to photosynthesize which supplies corals with the products of photosynthesis, including glucose, glycerol, and amino acids, which the corals can use for energy. [19] Zooxanthellae also benefit corals by aiding in calcification, for the coral skeleton, and waste removal. [20] [21] In addition to the soft tissue, microbiomes are also found in the coral's mucus and (in stony corals) the skeleton, with the latter showing the greatest microbial richness. [22]

The zooxanthellae benefit from a safe place to live and consume the polyp's carbon dioxide, phosphate 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 coloration; some 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 and if the stress subsides they can regain algae, possibly of a different species, at a later time. If the stressful conditions persist, the polyp eventually dies. [23] Zooxanthellae are located within the coral 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 [24]

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

Life cycles of broadcasters and brooders Coral Life Cycles ZP.svg
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. [25]

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. [26]

A male great star coral, Montastraea cavernosa, releasing sperm into the water. Stony coral spawning 2.jpg
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. [25] Synchronous spawning may form hybrids and is perhaps involved in coral speciation. [27] The immediate cue is most often sunset, which cues the release. [25] 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. [25] After fertilization, the corals release planula that are ready to settle. [20]

Generalized life cycle of corals via sexual reproduction. Life Cycle of Corals.svg
Generalized life cycle of corals via sexual reproduction.

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. [28] 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. [29] The larva grows into a polyp and eventually becomes a coral head by asexual budding and growth.


Asexual

Basal plates (calices) of Orbicella annularis showing multiplication by budding (small central plate) and division (large double plate) Orbicella annularis - calices.jpg
Basal plates (calices) of Orbicella annularis showing multiplication by budding (small central plate) and division (large double plate)

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. [26] 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. [30]

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. [31]

Reefs

Locations of coral reefs around the world Coral reef locations.jpg
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. [32]

Staghorn coral (Acropora cervicornis) is an important hermatypic coral from the Caribbean Hertshoon.jpg
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. [33] Coral reefs are extremely diverse marine ecosystems hosting over 4,000 species of fish, massive numbers of cnidarians, molluscs, crustaceans, and many other animals. [34]

Evolutionary history

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

Although corals first appeared in the Cambrian period, [35] 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. [36] [37]

Tabulate coral (a syringoporid); Boone limestone (Lower Carboniferous) near Hiwasse, Arkansas, scale bar is 2.0 cm Syringoporid.jpg
Tabulate coral (a syringoporid); Boone limestone (Lower Carboniferous) near Hiwasse, Arkansas, scale bar is 2.0 cm
Tabulate coral Aulopora from the Devonian era AuloporaDevonianSilicaShale.jpg
Tabulate coral Aulopora from the Devonian era

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. [38]

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. [39]

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. [40] Scleractinian skeletons are composed of a form of calcium carbonate known as aragonite. [41] 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)Coral

Timeline of the major coral fossil record and developments from 650 m.y.a. to present. [42] [43]

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. [44] 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

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

Coral reefs are under stress around the world. [45] 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. [46] In 1998, 16% of the world's reefs died as a result of increased water temperature. [47]

Approximately 10% of the world's coral reefs are dead. [48] [49] [50] About 60% of the world's reefs are at risk due to human-related activities. [51] The threat to reef health is particularly strong in Southeast Asia, where 80% of reefs are endangered. [52] Over 50% of the world's coral reefs may be destroyed by 2030; as a result, most nations protect them through environmental laws. [53]

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. [54] 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. [55]

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. [56] 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. [56]

Protection

Marine Protected Areas, Biosphere reserves, marine parks, national monuments world heritage status, fishery management and habitat protection can protect reefs from anthropogenic damage. [57]

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. [46]

To eliminate destruction of corals in their indigenous regions, projects have been started to grow corals in non-tropical countries. [58] [59]

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. [60]

Jewelry

6-strand necklace, Navajo (Native American), ca. 1920s, Brooklyn Museum 6-Strand Necklace, Navajo (Native American), ca. 1920s, cropped.jpg
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. [61] 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". [62] 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". [63] There were strict rules regarding its use in a code established by the Qianlong Emperor in 1759.

Medicine

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. ViennaDioscoridesCoral.jpg
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.

In medicine, chemical compounds from corals can potentially be used to treat cancer, AIDS, pain, and for other therapeutic uses. [65] [66] 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

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 medieval 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 oxygen and trace elements into their skeletal aragonite (polymorph of calcite) crystalline structures as they grow. Geochemical 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] The ratio of oxygen-18 to oxygen-1618O), for example, is a proxy for temperature.

Strontium/calcium ratio anomaly

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]

Oxygen isotope anomaly

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]

Sea surface temperature and sea surface salinity
Global sea surface temperature (SST) Global Sea Surface Temperature - GPN-2003-00032.jpg
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]

Limited climate research on current species
Porites lutea Porites lutea.jpg
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

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

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

Related Research Articles

Coral bleaching

Coral bleaching occurs when coral polyps expel algae that live inside their tissues. Normally, coral polyps live in an endosymbiotic relationship with these algae, which are crucial for the health of the coral and the reef. The algae provides up to 90 percent of the coral's energy. Bleached corals continue to live but begin to starve after bleaching. Some corals recover.

Zooxanthellae genus of dinoflagellates

Zooxanthellae is a colloquial term for single-celled dinoflagellates that are able to live in symbiosis with diverse marine invertebrates including corals, jellyfish, and nudibranchs. Most known zooxanthellae are in the genus Symbiodinium, but some are known from the genus Amphidinium, and other taxa, as yet unidentified, may have similar endosymbiont affinities. The true Zooxanthella K.brandt is a mutualist of the radiolarian Collozoum inerme and systematically placed in Peridiniales. Another group of unicellular eukaryotes that partake in similar endosymbiotic relationships in both marine and freshwater habitats are green algae zoochlorellae.

Scleractinia Order of cnidarians

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.

Alcyonacea order of cnidarians

Alcyonacea, or soft corals, are an order of corals that do not produce calcium carbonate skeletons. Formerly known as gorgonians, they are sessile colonial cnidarians found throughout the oceans of the world, especially in the tropics and subtropics. Common names for subset of this order are sea fans and sea whips and are similar to the sea pen, a soft coral. Individual tiny polyps form colonies that are normally erect, flattened, branching, and reminiscent of a fan. Others may be whiplike, bushy, or even encrusting. A colony can be several feet high and across, but only a few inches thick. They may be brightly coloured, often purple, red, or yellow. Photosynthetic gorgonians can be successfully kept in captive aquaria.

Leaf plate montipora species of cnidarian

Leaf plate montipora, also known as vase coral, cap coral, or plating montipora, is a type of small polyp stony (SPS) coral in the family Acroporidae.

<i>Porites</i> genus of cnidarians

Porites is a genus of stony coral; they are SPS corals. They are characterised by a finger-like morphology. Members of this genus have widely spaced calices, a well-developed wall reticulum and are bilaterally symmetrical. Porites, particularly Porites lutea, often form microatolls. Corals of the genus Porites also often serve as hosts for Christmas tree worms.

Ivory bush coral Species of cnidarian

Oculina varicosa, or the ivory bush coral, is a scleractinian deep-water coral primarily found at depths of 70-100m, and ranges from Bermuda and Cape Hatteras to the Gulf of Mexico and the Caribbean. Oculina varicosa flourishes at the Oculina Bank off the east coast of Florida, where coral thickets house a variety of marine organisms. The U.S. National Marine Fisheries Service considers Oculina a genus of concern, due to the threat of rapid ocean warming. Species of concern are those species about which the U.S. Government's National Oceanic and Atmospheric Administration (NOAA), National Marine Fisheries Service, has some concerns regarding status and threats, but for which insufficient information is available to indicate a need to list the species under the U.S. Endangered Species Act (ESA). While Oculina is considered a more robust genus in comparison to tropical corals, rising ocean temperatures continue to threaten coral health across the planet.

<i>Pocillopora</i> genus of cnidarians

Pocillopora is a genus of stony corals in the family Pocilloporidae occurring in the Pacific and Indian Oceans. They are commonly called cauliflower corals and brush corals.

<i>Pavona duerdeni</i> species of cnidarian

Pavona duerdeni, the porkchop coral, is a coral that forms clusters of cream-colored lobes or discs. They grow in large colonies, divided into ridges or hillocks. The coral is considered to be uncommon due to its low confirmed abundance, yet they are more commonly found in Hawaii, the Indo-Pacific, and the Tropical Eastern Pacific. They make up some of the largest colonies of corals, and have a slow growth rate, as indicated by their dense skeletons. Their smooth appearance is due to their small corallites growing on their surface.

The resilience of coral reefs is the biological ability of coral reefs to recover from natural disturbances such as storms and bleaching episodes. Resilience refers to the ability of biological or social systems to overcome pressures and stresses by maintaining key functions through resisting or adapting to change. Reef resistance measures how well coral reefs tolerate changes in ocean chemistry, sea level, and sea surface temperature. Reef resistance and resilience are important factors in coral reef recovery from the effects of ocean acidification. Natural reef resilience can be used as a recovery model for coral reefs and an opportunity for management in marine protected areas (MPAs).

<i>Galaxea fascicularis</i> species of cnidarian

Galaxea fascicularis is a species of colonial stony coral in the family Euphylliidae, commonly known as octopus coral, fluorescence grass coral, galaxy coral among various vernacular names.

Siderastrea radians, also known as the lesser starlet coral or the shallow-water starlet coral, is a stony coral in the family Siderastreidae. It is found in shallow parts of the western Atlantic Ocean as small, solid mounds or encrusting sheets.

<i>Scolymia lacera</i> species of cnidarian

Scolymia lacera, the fleshy disk coral, is a species of stony coral in the family Mussidae. It occurs on reefs in shallow waters in the Caribbean Sea, the Gulf of Mexico, the Bahamas, Bermuda and southern Florida.

<i>Mussa</i> (genus) genus of cnidarians

Mussa is a genus of stony coral in the family Mussidae. It is monotypic, being represented by the species Mussa angulosa, commonly known as the spiny or large flower coral. It is found on reefs in shallow waters in the Caribbean Sea, the Bahamas and the Gulf of Mexico.

<i>Isopora palifera</i> species of Anthozoa

Isopora palifera is a species of stony coral in the family Acroporidae. It is a reef building coral living in shallow water and adopts different forms depending on the water conditions where it is situated. It is found in the Western Indo-Pacific Ocean as far east as Australia.

<i>Pocillopora verrucosa</i> species of cnidarian

Pocillopora verrucosa, commonly known as cauliflower coral, rasp coral, or knob-horned coral, is a species of stony coral in the family Pocilloporidae. It is native to tropical and subtropical parts of the Indian and Pacific Oceans.

<i>Astrangia poculata</i> species of cnidarian

Astrangia poculata, the northern star coral or northern cup coral, is a species of non-reefbuilding stony coral in the family Rhizangiidae. It is native to shallow water in the western Atlantic Ocean and the Caribbean Sea. It is also found on the western coast of Africa. The International Union for Conservation of Nature lists this coral as being of "least concern".

<i>Seriatopora hystrix</i> species of cnidarian

Seriatopora hystrix is a species of colonial stony coral in the family Pocilloporidae. It forms a bushy clump and is commonly known as thin birdsnest coral. It grows in shallow water on fore-reef slopes or in sheltered lagoons, the type locality being the Red Sea. It is native to East Africa, the Red Sea and the western Indo-Pacific region. It is a common species and the International Union for Conservation of Nature has assessed its conservation status as being of "least concern".

Horastrea is a monotypic genus of stony coral in the family Coscinaraeidae. It is represented by the single species Horastrea indica, the blister coral. It is native to the southwestern Indian Ocean where it is found in shallow water sandy reefs. It was first described by M Pichon in 1971. It is an uncommon coral and the International Union for Conservation of Nature has assessed it as being a "vulnerable species".

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