Coral

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For other uses, see Coral (disambiguation).

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. Coral species include the
important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard
skeleton.
A coral "group" is a colony of myriad genetically identical polyps. Each polyp is a sac-like animal
typically only a few millimeters in diameter and a few centimeters in height. A set
of tentacles surround a central mouth opening. Each polyp excretes an 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 of polyps. Corals also breed sexually by spawning: polyps of the same species
release gametes simultaneously overnight, often around a 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 and small fish using stinging cells on their
tentacles, most corals obtain the majority of their energy and nutrients
from photosynthetic unicellular dinoflagellates of the genus Symbiodinium that live within their
tissues. These are commonly known as zooxanthellae and gives the coral color. Such corals
require sunlight and grow in clear, shallow water, typically at depths less than 60 metres (200
feet; 33 fathoms). 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 Australia. These corals are increasingly at risk of 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 Lophelia which can survive as deep as 3,300 metres (10,800 feet; 1,800
fathoms).[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.
 Contents

 1Taxonomy
 2Anatomy
o 2.1Soft corals
o 2.2Stony corals
 3Ecology
o 3.1Feeding
o 3.2Intracellular symbionts
 4Reproduction
o 4.1Sexual
o 4.2Asexual
o 4.3Colony division
 5Coral microbiome
o 5.1Coral holobiont
 6Reefs
 7Evolution
 8Status
o 8.1Threats
o 8.2Protection
o 8.3Coral Health
 9Relation to humans
o 9.1Jewelry
o 9.2Medicine
o 9.3Construction
o 9.4Shoreline protection
o 9.5Local economies
o 9.6Climate research
o 9.7Aquaria
o 9.8Aquaculture
 10Gallery
 11See also
 12References
 13Sources
 14External links

Taxonomy[edit]
The classification of corals has been discussed for millennia, owing to having similarities to both
plants and animals. 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 (tertia natura)".
[3]
 Petrus Gyllius copied Pliny, introducing the term zoophyta for this third group in his 1535
book On the French and Latin Names of the Fishes of the Marseilles Region; it is popularly but
wrongly supposed that Aristotle created the term. [3] Gyllius further noted, following Aristotle, how
hard it was to define what was a plant and what was an animal. [3] The Babylonian Talmud refers
to coral among a list of types of trees, and the 11th century French
commentator Rashi describes it as "a type of tree (‫ )מין עץ‬that grows underwater that goes by the
(French) name "coral."[4]
The Persian polymath Al-Biruni (d.1048) classified sponges and corals as animals, arguing that
they respond to touch.[5] 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.[6]
Presently, corals are classified as species of animals within the sub-
classes Hexacorallia and Octocorallia of the class Anthozoa in the phylum Cnidaria.
[7]
 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.
The group of corals is paraphyletic because the sea anemones are also in the sub-class
Hexacorallia.

Anatomy[edit]

Anatomy of a stony coral polyp

For most of their life corals are sessile animals of colonies of genetically identical polyps. Each
polyp varies from millimeters to centimeters in diameter, and colonies can be formed from many
million individual polyps. Stony coral, also known as hard coral, 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]
The body of the polyp may be roughly compared in a structure to a sac, the wall of which is
composed of two layers of cells. The outer layer is known technically as the ectoderm, the inner
layer as the endoderm. Between ectoderm and endoderm is a supporting layer of gelatinous
substance termed mesoglea, secreted by the cell layers of the body wall.[9] The mesoglea can
contain skeletal elements derived from cells migrated 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 corallites. At the center of the upper end of the
sac lies the only opening called the mouth, surrounded by a circle of tentacles which resemble
glove fingers. The tentacles are organs which serve both for the tactile sense and for the capture
of food.[9] Polyps extend their tentacles, particularly at night, often containing coiled stinging cells
(cnidocytes) which pierce, poison and firmly hold living prey paralysing or killing them. Polyp
prey includes plankton such as copepods 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.[9] 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 toxins to ward off predators.[8]
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.[10]
The external form of the polyp varies greatly. The column may be long and slender, or may be
so short in the vertical 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. [9]
Soft corals[edit]
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 stolonate, 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 chem a central axial skeleton
embedded at its base in the matrix of the supporting branch. [11] These branches are composed
either of a fibrous protein called gorgonin or of a calcified material.
Stony corals[edit]
Montastraea cavernosa polyps with tentacles extended

The polyps of stony corals have six-fold symmetry. In stony corals the polyps 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.[12]
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.[13][14] The organic matrices extracted from diverse species are acidic, and comprise proteins,
sulphated sugars and lipids; they are species specific. [15] The soluble organic matrices of the
skeletons allow to differentiate zooxanthellae and non-zooxanthellae specimens.[16]

Ecology[edit]

Discharge mechanism of a stinging cell (nematocyst)

Feeding[edit]
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. [17]:24
Intracellular symbionts[edit]
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. [17]:23–24 Typically,
each polyp harbors one species of alga, and coral species show a preference for Symbiodinium.
[18]
 Young corals are not born with zooxanthellae, but acquire the algae from the surrounding
environment, including the water column and local sediment. [19] 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.[20] Zooxanthellae also benefit corals by aiding in calcification, for the coral skeleton, and
waste removal.[21][22] 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. [23]
The zooxanthellae benefit from a safe place to live and consume the polyp's 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 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. [24] 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.[25]

Reproduction[edit]
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[edit]

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.[26]
Broadcasters[edit]
About 75% of all hermatypic corals "broadcast spawn" by releasing gametes—eggs and sperm
—into the water to spread offspring. The gametes fertilize at the water's surface 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.
[27]

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.[26] Synchronous
spawning may form hybrids and is perhaps involved in coral speciation.[28] The immediate cue is
most often sunset, which cues the release.[26] The spawning event can be visually dramatic,
clouding the usually clear water with gametes.
Brooders[edit]
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.[26] After fertilization, the corals release planula that are ready to settle. [21]

Generalized life cycle of corals via sexual reproduction: Colonies release gametes in clusters (1) which float to the surface
(2) then disperse and fertilize eggs (3). Embryos become planulae (4) and can settle onto a surface (5). They then
metamorphose into a juvenile polyp (6) which then matures and reproduces asexually to form a colony (7, 8).

Planulae[edit]
The time from spawning to larval settlement is usually two to three days, but can occur
immediately or up to two months.[29] Broadcast-spawned 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.[30] The larvae often need a biological cue to induce settlement such as
specific crustose coralline algae species or microbial biofilms. [31][32] 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,
[33]
 as well as chemical (allelopathic) barriers. [34] The larvae metamorphose into a single polyp and
eventually develops into a juvenile and then adult by asexual budding and growth.
Asexual[edit]

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. [27] 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. [35]
Colony division[edit]
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.
[36]
Coral microbiome[edit]

Relationships between corals and their microbial symbionts [37]

Coral holobiont[edit]
Reef-building corals are well-studied holobionts that include the coral itself together with its
symbiont zooxanthellae (photosynthetic dinoflagellates), as well as its associated bacteria and
viruses.[38] Co-evolutionary patterns exist for coral microbial communities and coral phylogeny. [39]