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Corals are
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 ...
within the
class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently f ...
Anthozoa Anthozoa is a class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical co ...

Anthozoa
of the
phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plural of a noun typically denotes a q ...
Cnidaria Pacific sea nettles, ''Chrysaora fuscescens'' Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, ...

Cnidaria
. They typically form compact
colonies In political science, a colony is a territory subject to a form of foreign rule. Though dominated by the foreign colonizers, colonies remain separate from the administration of the original country of the colonizers, the metropole, metropolitan ...
of many identical individual polyps. Coral species include the important
reef A reef is a ridge or shoal A tidal sandbar connecting the islands of Waya and Wayasewa of the Yasawa Islands, Fiji ">Fiji.html" ;"title="Yasawa Islands, Fiji">Yasawa Islands, Fiji In oceanography, geomorphology, and earth sciences, a s ...

reef
builders that inhabit tropical oceans and secrete
calcium carbonate Calcium carbonate is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held togethe ...

calcium carbonate
to form a hard skeleton. A coral "group" is a colony of myriad
genetically identical
genetically identical
polyps. Each polyp is a sac-like animal typically only a few millimeters in diameter and a few centimeters in height. A set of
tentacle with 2 tentacles and 8 arms In zoology Zoology ()The pronunciation of zoology as is typically regarded as nonstandard, though it is not uncommon. is the branch of biology that studies the animal kingdom, including the anatomy, structure, emb ...

tentacle
s surround a central mouth opening. Each polyp excretes an
exoskeleton An exoskeleton (from Greek έξω, ''éxō'' "outer" and σκελετός, ''skeletós'' "skeleton") is the external skeleton A skeleton is a structural frame that supports an animal Animals (also called Metazoa) are multicellular ...

exoskeleton
near the base. Over many generations, the colony thus creates a skeleton characteristic of the species which can measure up to several meters in size. Individual colonies grow by
asexual reproduction Asexual reproduction is a type of reproduction Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism In biology, an organism (from Ancient Gree ...
of polyps. Corals also breed sexually by
spawning Spawn is the eggs and sperm Sperm is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, "female" reproductive cell and a smaller, "male" one). Animals produce ...
: polyps of the same species release
gamete A gamete ( /ˈɡæmiːt/; from Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply ...
s simultaneously overnight, often around a
full moon The full moon is the lunar phase s in 2022 as viewed from the Southern Hemisphere The Southern Hemisphere is the half (hemisphere Hemisphere may refer to: * A half of a sphere As half of the Earth * A hemispheres of Earth, hemisphere o ...

full moon
. Fertilized eggs form planulae, a mobile early form of the coral polyp which when mature settles to form a new colony. Although some corals are able to catch
plankton Plankton are the diverse collection of organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by tax ...

plankton
and small
fish Fish are aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the ...

fish
using
stinging cells
stinging cells
on their tentacles, most corals obtain the majority of their energy and nutrients from
photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Doctor Who'' audio), an episode of the audio drama ' ...

photosynthetic
unicellular A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Or ...
dinoflagellate The dinoflagellates (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 m ...
s of the genus ''
Symbiodinium : ''This is about the genus sometimes called Zoox. For the company, see Zoox (company)'' ''Symbiodinium'' is a genus of dinoflagellates that encompasses the largest and most prevalent group of endosymbiotic dinoflagellates known. These unicellula ...

Symbiodinium
'' that live within their tissues. These are commonly known as
zooxanthellae Zooxanthellae is a colloquial term for single-celled dinoflagellate The dinoflagellates (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a count ...

zooxanthellae
and give the coral color. Such corals require sunlight and grow in clear, shallow water, typically at depths less than . Corals are major contributors to the physical structure of the
coral reef A coral reef is an underwater ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient c ...

coral reef
s that develop in tropical and subtropical waters, such as the
Great Barrier Reef The Great Barrier Reef is the world's largest coral reef A coral reef is an underwater ecosystems, ecosystem characterized by reef-building corals. Reefs are formed of Colony (biology), colonies of coral polyp (zoology), polyps held tog ...

Great Barrier Reef
off the coast of
Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

Australia
. These corals are increasingly at risk of
bleaching Bleach is the generic name for any chemical product which is used industrially and domestically to remove color from a fabric or fiber or to clean or to remove stains in a process called bleaching. It often refers, specifically, to a dilute s ...

bleaching
events where polyps expel the zooxanthellae in response to stress such as high water temperature or toxins. Other corals do not rely on zooxanthellae and can live globally in much deeper water, such as the cold-water
genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumscribing) and classifying gr ...
''
Lophelia ''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 ...
'' which can survive as deep as . Some have been found as far north as the
Darwin Mounds Darwin Mounds is a large field of undersea sand mounds situated off the north west coast of Scotland Scotland ( sco, Scotland, gd, Alba ) is a Countries of the United Kingdom, country that is part of the United Kingdom. Covering the norther ...
, northwest of
Cape Wrath Cape Wrath ( gd, Am Parbh, known as ' in Lewis) is a cape A cape is a sleeveless outer garment, which drapes the wearer's back, arms, and chest, and connects at the neck. History Capes were common in medieval Europe, especially when comb ...
, Scotland, and others off the coast of
Washington state Washington (), officially the State of Washington, is a state State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', a monthly magazine published by the U.S. Department of State * The State (newspaper), ''The S ...
and the
Aleutian Islands The Aleutian Islands (; ; ale, Unangam Tanaa, literally "Land of the s", possibly from ''aliat'', "island"), also called the Aleut Islands or Aleutic Islands and known before 1867 as the Catherine Archipelago, are a chain of 14 large volcanic ...
.


Taxonomy

The classification of corals has been discussed for millennia, owing to having similarities to both plants and animals.
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and fundamental questio ...

Aristotle
's pupil
Theophrastus Theophrastus (; grc-gre, Θεόφραστος ; c. 371c. 287 BC), a Greek native of Eresos Eresos (; el, Ερεσός; grc, Ἔρεσος) and its twin beach village Skala Eresou are located in the southwest part of the Greek island of Le ...

Theophrastus
described the
red coral
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
Gulf of Heroes
.
Pliny the Elder #REDIRECT Pliny the Elder #REDIRECT Pliny the Elder#REDIRECT Pliny the Elder Gaius Plinius Secundus (AD 23/2479), called Pliny the Elder (), was a Roman author, a naturalist Natural history is a domain of inquiry involving organisms, includi ...

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 (''tertia natura'')". 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. Gyllius further noted, following Aristotle, how hard it was to define what was a plant and what was an animal. The
Babylonian Talmud The Talmud (; he, תַּלְמוּד ''Tálmūḏ'') is the central text of Rabbinic Judaism Rabbinic Judaism ( he, יהדות רבנית, Yahadut Rabanit), also called Rabbinism, Rabbinicism, or Judaism espoused by the Rabbanites, has ...
refers to coral among a list of types of trees, and the 11th century French commentator
Rashi Shlomo Yitzchaki ( he, רבי שלמה יצחקי; la, Salomon Isaacides; french: Salomon de Troyes, 22 February 1040 – 13 July 1105), today generally known by the acronym Rashi (see below), was a medieval French rabbi A rabbi is a spi ...
describes it as "a type of tree (מין עץ) that grows underwater that goes by the (French) name "coral." The Persian polymath
Al-Biruni Abu Rayhan al-Biruni (973 – after 1050) was an Iranian Iranian may refer to: * Iran Iran ( fa, ایران ), also called Persia and officially the Islamic Republic of Iran ( fa, جمهوری اسلامی ایران ), is a co ...
(d.1048) classified sponges and corals as animals, arguing that they respond to touch. Nevertheless, people believed corals to be plants until the eighteenth century, when
William Herschel Sir Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a spe ...

William Herschel
used a microscope to establish that coral had the characteristic thin cell membranes of an
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

animal
. Presently, corals are classified as species of animals within the sub-classes
Hexacorallia Hexacorallia is a class of Anthozoa comprising approximately 4,300 species of aquatic organisms formed of polyps, generally with 6-fold symmetry. It includes all of the stony coral Corals are marine invertebrates within the class (biology), ...
and
Octocorallia Octocorallia (also known as Alcyonaria) is a subclass of Anthozoa comprising around 3,000 species of water-based organisms formed of colonial polyps with 8-fold symmetry. It includes the blue coral, soft corals, sea pens, and gorgonians (sea ...
of the
class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently f ...
Anthozoa Anthozoa is a class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical co ...

Anthozoa
in the
phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plural of a noun typically denotes a q ...
Cnidaria Pacific sea nettles, ''Chrysaora fuscescens'' Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, ...

Cnidaria
. Hexacorallia includes the stony corals and these groups have polyps that generally have a 6-fold symmetry. Octocorallia includes and
soft coral Alcyonacea, or soft corals, are an order of corals. In addition to the fleshy soft corals, the order Alcyonacea now contains all species previously known as "gorgonian corals", that produce a more or less hard skeleton, though quite different fr ...
s and species of Octocorallia have polyps with an eightfold symmetry, each polyp having eight tentacles and eight mesenteries. The group of corals is
paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and all descendants of that ancestor excluding a few—typically only one or two—Monophyly, monophyletic subgroups. The group is said to be paraphyleti ...

paraphyletic
because the
sea anemone Sea anemones are the marine, predatory Predation is a biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms ...

sea anemone
s are also in the sub-class Hexacorallia.


Anatomy

For most of their life corals are sessile animals of
colonies In political science, a colony is a territory subject to a form of foreign rule. Though dominated by the foreign colonizers, colonies remain separate from the administration of the original country of the colonizers, the metropole, metropolitan ...
of genetically identical polyps. Each polyp varies from millimeters to centimeters in diameter, and colonies can be formed from many millions of individual polyps. Stony coral, also known as hard coral, polyps produce a skeleton composed of
calcium carbonate Calcium carbonate is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held togethe ...

calcium carbonate
to strengthen and protect the organism. This is deposited by the polyps and by the
coenosarcIn coral Corals are marine invertebrates within the class (biology), class Anthozoa of the phylum Cnidaria. They typically live in compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the im ...
, the living tissue that connects them. The polyps sit in cup-shaped depressions in the skeleton known as
corallite A corallite is the skeletal cup, formed by an individual stony coral polyp (zoology), polyp, in which the polyp sits and into which it can retract. The cup is composed of aragonite, a crystalline form of calcium carbonate, and is secreted by the ...
s. 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. The body of the polyp may be roughly compared in a structure to a
sac SAC or Sac may refer to: Organizations Education * Santa Ana College, California, US * San Antonio College, Texas, US * St. Andrew's College, Aurora, Canada * Students' Administrative Council, University of Toronto, Canada * SISD Student Activitie ...
, the wall of which is composed of two layers of cells. The outer layer is known technically as the
ectoderm The ectoderm is one of the three primary germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans ( ...
, the inner layer as the
endoderm Endoderm is the innermost of the three primary germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazo ...
. Between ectoderm and endoderm is a supporting layer of gelatinous substance termed
mesoglea Mesoglea refers to the tissue found in cnidaria Image:Sea nettles.jpg, Chrysaora fuscescens, Pacific sea nettles, ''Chrysaora fuscescens'' Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found bot ...
, secreted by the cell layers of the body wall. The mesoglea can contain
skeletal The skeleton refers to the frames of support of animal bodies. There are several different skeletal types: the exoskeleton An exoskeleton (from Greek έξω, ''éxō'' "outer" and σκελετός, ''skeletós'' "skeleton") is the external sk ...
elements derived from cells from ectoderm. The sac-like body built up in this way is attached to a hard surface, which in hard corals are cup-shaped depressions in the skeleton known as
corallite A corallite is the skeletal cup, formed by an individual stony coral polyp (zoology), polyp, in which the polyp sits and into which it can retract. The cup is composed of aragonite, a crystalline form of calcium carbonate, and is secreted by the ...
s. At the center of the upper end of the sac lies the only opening called the mouth, surrounded by a circle of
tentacle with 2 tentacles and 8 arms In zoology Zoology ()The pronunciation of zoology as is typically regarded as nonstandard, though it is not uncommon. is the branch of biology that studies the animal kingdom, including the anatomy, structure, emb ...

tentacle
s which resemble glove fingers. The tentacles are
organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly cate ...
s which serve both for the tactile sense and for the capture of food. Polyps extend their tentacles, particularly at night, often containing coiled stinging cells (
cnidocyte A cnidocyte (also known as a cnidoblast or nematocyte) is an explosive cell containing one giant secretory organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural scien ...

cnidocyte
s) which pierce, poison and firmly hold living prey paralysing or killing them. Polyp prey includes plankton such as
copepods Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (inhabiting sea waters), some are benthos, benthic (living on the ocean floor) ...
and fish larvae. Longitudinal muscular fibers formed from the cells of the ectoderm allow tentacles to contract to convey the food to the mouth. Similarly, circularly disposed muscular fibres formed from the endoderm permit tentacles to be protracted or thrust out once they are contracted. In both stony and soft corals, the polyps can be retracted by contracting muscle fibres, with stony corals relying on their hard skeleton and cnidocytes for defence. Soft corals generally secrete
terpenoid The terpenoids, also known as isoprenoids, are a large and diverse class of naturally occurring organic compound, organic chemicals derived from the 5-carbon compound isoprene, and the isoprene polymers called terpenes. While sometimes used interch ...
toxins to ward off predators. 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
zooxanthella Zooxanthellae is a colloquial term for single-celled dinoflagellate The dinoflagellates (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a count ...

zooxanthella
te, the corals supplementing their plankton diet with the products of photosynthesis produced by these
symbionts Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, ...

symbionts
. The polyps interconnect by a complex and well-developed system of gastrovascular canals, allowing significant sharing of nutrients and symbionts. The external form of the polyp varies greatly. The column may be long and slender, or may be so short in the axial direction that the body becomes disk-like. The tentacles may number many hundreds or may be very few, in rare cases only one or two. They may be simple and unbranched, or feathery in pattern. The mouth may be level with the surface of the peristome, or may be projecting and trumpet-shaped.


Soft corals

Soft corals have no solid exoskeleton as such. However, their tissues are often reinforced by small supportive elements known as "sclerites" made of calcium carbonate. The polyps of soft corals have eight-fold symmetry. Soft corals vary considerably in form, and most are colonial. A few soft corals are
stolon In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
ate, but the polyps of most are connected by sheets of tissue called coenosarc, 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 have a central axial skeleton embedded at its base in the matrix of the supporting branch. These branches are composed of a fibrous protein called gorgonin or of a calcified material.


Stony corals

The polyps of stony corals have six-fold symmetry. In stony corals the tentacles 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. Coral skeletons are biocomposites (mineral + organics) of calcium 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. The organic matrices extracted from diverse species are acidic, and comprise proteins, sulphated sugars and lipids; they are species specific. The soluble organic matrices of the skeletons allow to differentiate
zooxanthellae Zooxanthellae is a colloquial term for single-celled dinoflagellate The dinoflagellates (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a count ...

zooxanthellae
and non-zooxanthellae specimens.


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 stinging cells called
nematocysts A cnidocyte (also known as a cnidoblast or nematocyte) is an explosive Cell (biology), cell containing one giant secretory organelle called a cnidocyst (also known as a cnida (plural ''cnidae'') or nematocyst) that can deliver a sting to other or ...
. These cells carry
venom Venom is a type of poison In biology, poisons are Chemical substance, substances that can cause death, injury or harm to organs, Tissue (biology), tissues, Cell (biology), cells, and DNA usually by chemical reactions or other activity (chemi ...

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.


Intracellular symbionts

Many corals, as well as other
cnidaria Pacific sea nettles, ''Chrysaora fuscescens'' Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, ...

cnidaria
n groups such as
sea anemone Sea anemones are the marine, predatory Predation is a biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms ...

sea anemone
s form a
symbiotic Symbiosis (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approx ...
relationship with a class of
dinoflagellate The dinoflagellates (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 m ...
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

algae
,
zooxanthellae Zooxanthellae is a colloquial term for single-celled dinoflagellate The dinoflagellates (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a count ...

zooxanthellae
of the genus ''
Symbiodinium : ''This is about the genus sometimes called Zoox. For the company, see Zoox (company)'' ''Symbiodinium'' is a genus of dinoflagellates that encompasses the largest and most prevalent group of endosymbiotic dinoflagellates known. These unicellula ...

Symbiodinium
'', which can form as much as 30% of the tissue of a polyp. Typically, each polyp harbors one species of alga, and coral species show a preference for ''
Symbiodinium : ''This is about the genus sometimes called Zoox. For the company, see Zoox (company)'' ''Symbiodinium'' is a genus of dinoflagellates that encompasses the largest and most prevalent group of endosymbiotic dinoflagellates known. These unicellula ...

Symbiodinium
''. Young corals are not born with zooxanthellae, but acquire the algae from the surrounding environment, including the water column and local sediment. 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. Zooxanthellae also benefit corals by aiding in
calcification Calcification is the accumulation of calcium Calcium is a chemical element with the Symbol (chemistry), symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when expose ...
, for the coral skeleton, and waste removal. In addition to the soft tissue,
microbiome A microbiome () is the community (ecology), community of microorganisms that can usually be found living together in any given habitat. It was defined more precisely in 1988 by Whipps ''et al.'' as "a characteristic microbial community occupyi ...
s are also found in the coral's mucus and (in stony corals) the skeleton, with the latter showing the greatest microbial richness. The zooxanthellae benefit from a safe place to live and consume the polyp's
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

carbon dioxide
, phosphate and nitrogenous waste. Stressed corals will eject their zooxanthellae, a process that is becoming increasingly common due to strain placed on coral by rising ocean temperatures. Mass ejections are known as
coral bleaching Coral bleaching occurs when coral polyp (zoology), polyps expel algae that live inside their tissues. Normally, coral polyps live in an Endosymbiont, endosymbiotic relationship with these algae, which are crucial for the health of the coral and the ...

coral bleaching
because the algae contribute to coral coloration; some colors, however, are due to host coral pigments, such as
green fluorescent protein The green fluorescent protein (GFP) is a protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within orga ...
s (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.
Zooxanthellae Zooxanthellae is a colloquial term for single-celled dinoflagellate The dinoflagellates (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a count ...

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 Zooxanthellae is a colloquial term for single-celled dinoflagellate The dinoflagellates (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a count ...

zooxanthellae
are responsible to some extent for the metabolism of their host corals.


Reproduction

Corals can be both
gonochoristic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...
(unisexual) and
hermaphroditic In reproductive biology, a hermaphrodite () is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life ...
, each of which can reproduce sexually and asexually. Reproduction also allows coral to settle in new areas. Reproduction is coordinated by chemical communication.


Sexual

Corals predominantly reproduce sexually. About 25% of
hermatypic coral Hermatypic corals are those corals in the order 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 polyp (zoolog ...
s (reef building stony corals) form single sex (
gonochoristic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...
) colonies, while the rest are
hermaphroditic In reproductive biology, a hermaphrodite () is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life ...
. It is estimated more than 67% of coral are simultaneous hermaphrodites.


Broadcasters

About 75% of all hermatypic corals "broadcast spawn" by releasing
gamete A gamete ( /ˈɡæmiːt/; from Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply ...
s—
eggs Egg An egg is the organic vessel containing the in which an develops until it can survive on its own, at which point the animal hatches. An egg results from of an . Most s, (excluding s), and lay eggs, although some, such as s, do ...
and
sperm Sperm is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as ...

sperm
—into the water to spread offspring. The gametes fertilize at the water's surface to form a microscopic
larva A larva (plural larvae ) is a distinct juvenile form many animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that f ...
called a
planula A planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidaria Image:Sea nettles.jpg, Chrysaora fuscescens, Pacific sea nettles, ''Chrysaora fuscescens'' Cnidaria () is a phylum under kingdom Animal ...

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. Synchronous spawning is very typical on the coral reef, and often, even when multiple
species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individu ...

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 The lunar phase or Moon phase is the shape of the Moon's directly sunlit portion as viewed from Earth. The lunar phases gradually change over a synodic month (about 29.53 days) as the Moon's orbital positions around Earth and Earth around th ...
,
day length Daytime as observed on Earth is the period of the day during which a given location experiences Daylight, natural illumination from direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the globe's ...
, and possibly chemical signalling. Synchronous spawning may form hybrids and is perhaps involved in coral
speciation Speciation is the evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes that are ...

speciation
. The immediate cue is most often sunset, which cues the release. 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. After fertilization, the corals release planula that are ready to settle.


Planulae

The time from spawning to larval settlement is usually two to three days, but can occur immediately or up to two months. Broadcast-spawned
planula A planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidaria Image:Sea nettles.jpg, Chrysaora fuscescens, Pacific sea nettles, ''Chrysaora fuscescens'' Cnidaria () is a phylum under kingdom Animal ...

planula
larvae develop at the water's surface before descending to seek a hard surface on the benthos to which they can attach and begin a new colony. The larvae often need a biological cue to induce settlement such as specific crustose coralline algae species or microbial biofilms. High failure rates afflict many stages of this process, and even though thousands of eggs are released by each colony, few new colonies form. During settlement, larvae are inhibited by physical barriers such as sediment, as well as chemical (allelopathic) barriers. The larvae metamorphose into a single polyp and eventually develops into a juvenile and then adult by asexual budding and growth.


Asexual

Within a coral head, the genetically identical polyps reproduce asexually, either by
budding Budding is a type of asexual reproduction Asexual reproduction is a type of reproduction Reproduction (or procreation or breeding) is the biological process by which new individual organism In biology, an organism (from Ancient Gr ...

budding
(gemmation) or by dividing, whether longitudinally or transversely. Budding involves splitting a smaller polyp from an adult. 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.


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 The Fungiidae () are a family of Cnidaria, often known as plate corals. The family contains thirteen extant taxon, extant genera. They range from solitary corals to Colony (biology), colonial species. Some genera such as ''Cycloseris'' and ''Fung ...
, 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.


Coral microbiome

Corals are one of the more common examples of an animal host whose symbiosis with microalgae can turn to dysbiosis, and is visibly detected as bleaching. Coral
microbiome A microbiome () is the community (ecology), community of microorganisms that can usually be found living together in any given habitat. It was defined more precisely in 1988 by Whipps ''et al.'' as "a characteristic microbial community occupyi ...
s have been examined in a variety of studies, which demonstrate how variations in the ocean environment, most notably temperature, light, and inorganic nutrients, affect the abundance and performance of the microalgal symbionts, as well as
calcification Calcification is the accumulation of calcium Calcium is a chemical element with the Symbol (chemistry), symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when expose ...
and physiology of the host.Apprill, A. (2017) "Marine animal microbiomes: toward understanding host–microbiome interactions in a changing ocean". ''Frontiers in Marine Science'', 4: 222. . Material was copied from this source, which is available under
Creative Commons Attribution 4.0 International License
Studies have also suggested that resident bacteria, archaea, and fungi additionally contribute to nutrient and organic matter cycling within the coral, with viruses also possibly playing a role in structuring the composition of these members, thus providing one of the first glimpses at a multi-domain marine animal symbiosis. The gammaproteobacterium ''Endozoicomonas'' is emerging as a central member of the coral's microbiome, with flexibility in its lifestyle.Neave, M.J., Apprill, A., Ferrier-Pagès, C. and Voolstra, C.R. (2016) "Diversity and function of prevalent symbiotic marine bacteria in the genus ''Endozoicomonas''". ''Applied Microbiology and Biotechnology'', 100(19): 8315–8324. . Given the recent mass bleaching occurring on reefs, corals will likely continue to be a useful and popular system for symbiosis and dysbiosis research. ''Astrangia poculata'', the northern star coral, is a temperate stony coral, widely documented along the eastern coast of the United States. The coral can live with and without
zooxanthellae Zooxanthellae is a colloquial term for single-celled dinoflagellate The dinoflagellates (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a count ...

zooxanthellae
(algal symbionts), making it an ideal model organism to study microbial community interactions associated with symbiotic state. However, the ability to develop Primer (molecular biology), primers and Molecular probe, probes to more specifically target key microbial groups has been hindered by the lack of full length 16S rRNA sequences, since sequences produced by the Illumina platform are of insufficient length (approximately 250 base pairs) for the design of primers and probes. In 2019, Goldsmith et al demonstrated Sanger sequencing was capable of reproducing the biologically-relevant diversity detected by deeper next-generation sequencing, while also producing longer sequences useful to the research community for probe and primer design (see diagram on right). Material was copied from this source, which is available under
Creative Commons Attribution 4.0 International License


Holobiont

Reef-building corals are well-studied holobionts that include the coral itself together with its symbiont
zooxanthellae Zooxanthellae is a colloquial term for single-celled dinoflagellate The dinoflagellates (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a count ...

zooxanthellae
(photosynthetic dinoflagellates), as well as its associated bacteria and viruses.Knowlton, N. and Rohwer, F. (2003) "Multispecies microbial mutualisms on coral reefs: the host as a habitat". ''The American Naturalist'', 162(S4): S51-S62. . Co-evolutionary patterns exist for coral microbial communities and coral phylogeny.


Cophylogeny and Phylosymbiosis

It is known that the coral's
microbiome A microbiome () is the community (ecology), community of microorganisms that can usually be found living together in any given habitat. It was defined more precisely in 1988 by Whipps ''et al.'' as "a characteristic microbial community occupyi ...
and symbiont influence host health, however the historic influence of each member on others is not well understood. Scleractinian corals have been diversifying for longer than many other symbiotic systems, and their microbiomes are known to be partially species-specific. It has been suggested that ''Endozoicomonas'', a commonly highly abundant bacterium in corals, has exhibited codiversification with its host. This hints at an intricate set of relationships between the members of the coral holobiont that have been developing as evolution of these members occurs. A study published in 2018 revealed evidence of phylosymbiosis between corals and their tissue and skeleton microbiomes. The coral skeleton, which represents the most diverse of the three coral microbiomes, showed the strongest evidence of phylosymbiosis. Coral microbiome composition and Species richness, richness were found to reflect coral phylogeny. For example, interactions between bacterial and eukaryotic coral phylogeny influence the abundance of ''Endozoicomonas'', a highly abundant bacterium in the coral holobiont. However, host-microbial cophylogeny appears to influence only a subset of coral-associated bacteria.


Reefs

Many corals in the order Scleractinia are Hermatypic coral, hermatypic, meaning that they are involved in building reefs. Most such corals obtain some of their energy from
zooxanthellae Zooxanthellae is a colloquial term for single-celled dinoflagellate The dinoflagellates (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a count ...

zooxanthellae
in the genus ''Symbiodinium''. These are symbiosis, symbiotic photosynthetic
dinoflagellate The dinoflagellates (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 m ...
s 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. 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 The Great Barrier Reef is the world's largest coral reef A coral reef is an underwater ecosystems, ecosystem characterized by reef-building corals. Reefs are formed of Colony (biology), colonies of coral polyp (zoology), polyps held tog ...

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. Coral reefs are extremely diverse marine ecosystems hosting over 4,000 species of fish, massive numbers of cnidarians, Mollusca, molluscs, crustaceans, and many other animals.


Evolution

Corals first appeared in the Cambrian about . Fossils are extremely rare until the Ordovician period, 100 million years later, when Rugosa, rugose and tabulate corals became widespread. Paleozoic corals often contained numerous endobiotic symbionts. Tabulate corals occur in limestones and calcareous shales of the Ordovician and Silurian periods, and often form low cushions or branching masses of calcite alongside rugose corals. Their numbers began to decline during the middle of the Silurian period, and they became extinct at the end of the Permian period, . 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. File:Syringoporid.jpg, Tabulate coral (a syringoporid); Boone limestone (Lower Carboniferous) near Hiwasse, Arkansas, scale bar is 2.0 cm File:AuloporaDevonianSilicaShale.jpg, Tabulate coral ''Aulopora'' from the Devonian period File:RugosaOrdovician.jpg, Solitary rugose coral (''Grewingkia'') in three views; Ordovician, southeastern Indiana The currently ubiquitous Scleractinia, stony corals filled the niche vacated by the extinct rugose and tabulate species. Their fossils are found in small numbers in rocks from the Triassic period, and become common in the Jurassic and later periods. The skeletons of stony corals are composed of a form of calcium carbonate known as aragonite. Although they are geologically younger than the tabulate and rugose corals, the aragonite of their skeletons is less readily preserved, and their fossil record is accordingly less complete. 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 Echinoderm, echinoids, brachiopods, bivalves, gastropods, and trilobites appear along with coral fossils. This makes some corals useful index fossils. Coral fossils are not restricted to reef remnants, and many solitary fossils are found elsewhere, such as ''Cyclocyathus'', which occurs in England's Gault clay formation.


Status


Threats

Coral reefs are under stress around the world. In particular, coral mining, agricultural runoff, 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. In 1998, 16% of the world's reefs died as a result of increased water temperature. Approximately 10% of the world's coral reefs are dead. About 60% of the world's reefs are at risk due to human-related activities. The threat to reef health is particularly strong in Southeast Asia, where 80% of reefs are endangered species, endangered. Over 50% of the world's
coral reef A coral reef is an underwater ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient c ...

coral reef
s may be destroyed by 2030; as a result, most nations protect them through environmental laws. 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. 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 : ''This is about the genus sometimes called Zoox. For the company, see Zoox (company)'' ''Symbiodinium'' is a genus of dinoflagellates that encompasses the largest and most prevalent group of endosymbiotic dinoflagellates known. These unicellula ...

Symbiodinium
; without them coral tissues reveal the white of their skeletons, an event known as
coral bleaching Coral bleaching occurs when coral polyp (zoology), polyps expel algae that live inside their tissues. Normally, coral polyps live in an Endosymbiont, endosymbiotic relationship with these algae, which are crucial for the health of the coral and the ...

coral bleaching
. 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. 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.


Protection

Marine Protected Areas, Biosphere reserves, marine parks, national monuments world heritage status, Fisheries management, fishery management and habitat (ecology), habitat protection can protect reefs from anthropogenic damage. Many governments now prohibit removal of coral from reefs, and inform coastal residents about reef protection and ecology. While local action such as habitat restoration and herbivore protection can reduce local damage, the longer-term threats of acidification, temperature change and sea-level rise remain a challenge. Protecting networks of diverse and healthy reefs, not only climate Refugium (population biology), refugia, helps ensure the greatest chance of genetic diversity, which is critical for coral to adapt to new climates. A variety of conservation methods applied across marine and terrestrial threatened ecosystems makes coral adaption more likely and effective. To eliminate destruction of corals in their indigenous regions, projects have been started to grow corals in non-tropical countries.


Coral Health

To assess the threat level of coral, scientists developed a coral imbalance ratio, Log (Average abundance of disease associated taxa / Average abundance of healthy associated taxa). The lower the ratio the healthier the microbial community is. This ratio was developed after the microbial mucus of coral was collected and studied.


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.


Jewelry

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. 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". 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". There were strict rules regarding its use in a code established by the Qianlong Emperor in 1759.


Medicine

In medicine, chemical compounds from corals can potentially be used to treat cancer, AIDS, pain, and for other therapeutic uses. Coral skeletons, e.g. ''Isididae'' are also used for bone grafting in humans. Coral Calx, known as Praval Bhasma in Sanskrit, is widely used in traditional system of Ayurveda, Indian medicine as a supplement in the treatment of a variety of bone metabolic disorders associated with calcium deficiency. In classical times ingestion of pulverized coral, which consists mainly of the weak base
calcium carbonate Calcium carbonate is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held togethe ...

calcium carbonate
, was recommended for calming stomach ulcers by Galen and Dioscorides.


Construction

Coral reefs in places such as the East African coast are used as a source of building material. 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.


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.


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. 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. The growth rings allow geologists to construct year-by-year chronologies, a form of incremental dating, which underlie high-resolution records of past paleoclimatology, climatic and paleoecology, environmental changes using geochemistry, geochemical techniques. 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. 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. Though coral have large sexually-reproducing populations, their evolution can be slowed by abundant
asexual reproduction Asexual reproduction is a type of reproduction Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism In biology, an organism (from Ancient Gree ...
. Gene flow is variable among coral species. 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. 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. Scientists found that a certain scleractinian zooxanthella is becoming more common where sea temperature is high. Symbionts able to tolerate warmer water seem to photosynthesise more slowly, implying an evolutionary trade-off. In the Gulf of Mexico, where sea temperatures are rising, cold-sensitive Staghorn coral, staghorn and elkhorn coral have shifted in location. 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. 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. This vicariance, 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 (Polymorphism (materials science), 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. The δ18O, ratio of oxygen-18 to oxygen-16 (δ18O), 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, sea surface temperature (SST) maximums to data collected fro
NINO 3.4 SSTA


= Oxygen isotope anomaly

= The comparison of coral strontium/calcium minimums with sea surface temperature maximums, data recorded fro
NINO 3.4 SSTA
time can be correlated to coral strontium/calcium and Δ18O, δ18O variations. To confirm accuracy of the annual relationship between Sr/Ca and Δ18O, 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 ratio, stable isotope data. El Nino-Southern Oscillation, El Nino-Southern Oscillation (ENSO) is directly related to climate fluctuations that influence coral Δ18O, δ18O ratio from local salinity variations associated with the position of the South Pacific convergence zone, South Pacific convergence zone (SPCZ) and can be used for El Niño Southern Oscillation, ENSO modeling.


= Sea surface temperature and sea surface salinity

= The global moisture budget is primarily being influenced by tropical sea surface temperatures from the position of the Intertropical Convergence Zone (ITCZ). The Southern Hemisphere has a unique meteorological feature positioned in the southwestern Pacific Basin called the South Pacific convergence zone, South Pacific Convergence Zone (SPCZ), which contains a perennial position within the Southern Hemisphere. During El Niño Southern Oscillation, ENSO warm periods, the South Pacific convergence zone, SPCZ reverses orientation extending from the equator down south through Solomon Islands, Vanuatu, Fiji and towards the French Polynesian islands, Polynesian Islands; and due east towards South America affecting geochemistry of corals in tropical regions. Geochemical analysis of skeletal coral can be linked to sea surface salinity (SSS) and sea surface temperature (SST), fro
El Nino 3.4 SSTA
data, of tropical oceans to seawater Δ18O, δ18O ratio anomalies from corals. El Niño Southern Oscillation, ENSO phenomenon can be related to variations in sea surface salinity (SSS) and sea surface temperature, sea surface temperature (SST) that can help model tropical climate activities.


= Limited climate research on current species

= 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.


Aquaria

The saltwater fishkeeping hobby has expanded, over recent years, to include Reef aquarium, reef tanks, fish tanks that include large amounts of live rock on which coral is allowed to grow and spread. 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, or as "show tanks", with the rock kept largely bare of the algae and microfauna that would normally populate it, in order to appear neat and clean. The most popular kind of coral kept is
soft coral Alcyonacea, or soft corals, are an order of corals. In addition to the fleshy soft corals, the order Alcyonacea now contains all species previously known as "gorgonian corals", that produce a more or less hard skeleton, though quite different fr ...
, especially zoanthids and mushroom corals, which are especially easy to grow and propagate in a wide variety of conditions, because they originate in enclosed parts of reefs where water conditions vary and lighting may be less reliable and direct.Coral Reefs
. Marinebio.org. Retrieved on 2016-06-13.
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

Aquaculture of coral, 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 reef A coral reef is an underwater ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient c ...

coral reef
s, which have been declining around the world. 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. Coral is farmed by coral farmers who live locally to the reefs and farm for reef Conservation movement, 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:Coral reefs and commons:Corals'' File:Mushroom Coral (Fungia) Top Macro 91.JPG, ''Fungia'' sp. skeleton File:Eusmilia fastigiata large.jpg, Polyps of ''Eusmilia fastigiata'' File:Dendrogyra cylindrus (pillar coral) (San Salvador Island, Bahamas) 1 (15513345363).jpg, Pillar coral, ''Dendrogyra cylindricus'' File:Brain coral.jpg, Brain coral, ''Diploria labyrinthiformis'' File:Brain coral spawning.jpg, Brain coral spawning File:Stony coral spawning 3.jpg, Brain coral releasing eggs File:EilatFringingReef.jpg, Fringing
coral reef A coral reef is an underwater ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient c ...

coral reef
off the coast of Eilat, Israel. File:000324-Corals-IMG 0418-2.jpg, Corals, Tis Beach, Chabahar, Iran File:000407-Corals-IMG 0693-2.jpg, Corals, Tis Beach, Chabahar, Iran


See also

*Keystone species


References


Sources

* * * * * * * * * * *


External links


Coral Reefs
The Ocean Portal by the Smithsonian Institution * NOAA
Coral Reef Conservation Program
* NOAA CoRIS
Coral Reef Biology
* NOAA Office for Coastal Management
Fast Facts - Coral Reefs
* NOAA Ocean Service Education
Corals
* {{Authority control Anthozoa Coral reefs,