Editing Coral (section)

==Anatomy==
[[File:Coral polyp.jpg|thumb|Anatomy of a stony coral polyp]]
For most of their life corals are [[Sessility (motility)|sessile]] animals of [[Colony (biology)|colonies]] of genetically identical [[polyp (zoology)|polyp]]s. 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]] 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 [[corallite]]s. Colonies of stony coral are markedly 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.<ref name=Ruppert>{{cite book |title=Invertebrate Zoology, 7th edition |last1=Ruppert |first1=Edward E. |last2=Fox |first2=Richard S. |last3=Barnes |first3=Robert D. |year=2004 |publisher=Cengage Learning |isbn=978-81-315-0104-7 |pages=132–48 }}</ref>

The body of the polyp may be roughly compared in a structure to a [[wikt:sac|sac]], the wall of which is composed of two layers of [[cell (biology)|cell]]s. The outer layer is known technically as the [[germ layer|ectoderm]], the inner layer as the [[germ layer|endoderm]]. Between ectoderm and endoderm is a supporting layer of gelatinous substance termed [[mesogloea|mesoglea]], secreted by the cell layers of the body wall.<ref name=EB1911>{{EB1911 |wstitle=Polyp |volume=22 |page=37 |first=Edward Alfred |last=Minchin |inline=1}}</ref> The mesoglea can contain [[endoskeleton|skeletal]] elements derived from cells [[cell migration|migrated]] from the 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]]s. At the center of the upper end of the sac lies the only opening called the mouth, surrounded by a circle of [[tentacle]]s which resemble glove fingers. The tentacles are [[organ (anatomy)|organ]]s which serve both for tactile sense and for the capture of food.<ref name=EB1911/> Polyps extend their tentacles, particularly at night, often containing coiled stinging cells ([[cnidocyte]]s) which pierce, poison and firmly hold living prey paralyzing 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.<ref name=EB1911/> 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 defense. Soft corals generally secrete [[terpenoid]] toxins to ward off predators.<ref name="Ruppert" />

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]]te, the corals supplementing their plankton diet with the products of photosynthesis produced by these [[Symbiosis|symbionts]].<ref name=Ruppert/> The polyps interconnect by a complex and well-developed system of [[gastrovascular]] canals, allowing significant sharing of nutrients and symbionts.<ref name=Gateno>{{cite journal |author1=D. Gateno |author2=A. Israel |author3=Y. Barki |author4=B. Rinkevich | title=Gastrovascular Circulation in an Octocoral: Evidence of Significant Transport of Coral and Symbiont Cells | journal=The Biological Bulletin | year=1998 | pages=178–86 | volume=194 | issue=2 | url=http://www.biolbull.org/cgi/reprint/194/2/178 | doi=10.2307/1543048 |pmid=28570841 | jstor=1543048 |s2cid=19530967 }}</ref>

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.<ref name=EB1911/>

===Soft corals===
{{see also|Octocorallia}} 
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, which is reflected in the ''Octo'' in Octocorallia.<ref>{{cite journal|doi=10.1017/jpa.2017.49|title=A problematic cnidarian (Cambroctoconus; Octocorallia?) from the Cambrian (Series 2–3) of Laurentia |year=2017 |last1=Peel |first1=John S. |journal=Journal of Paleontology |volume=91 |issue=5 |pages=871–882 |bibcode=2017JPal...91..871P |s2cid=134826884 |doi-access=free }}</ref>

Soft corals vary considerably in form, and most are colonial. A few soft corals are [[stolon]]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 their base in the matrix of the supporting branch.<ref>{{cite web|url=https://coralreef.noaa.gov/aboutcorals/coral101/anatomy/|title=existing and potential value of coral ecosystems with respect to income and other economic values|last=Administration|first=US Department of Commerce, National Oceanic and Atmospheric|website=coralreef.noaa.gov|language=EN-US|access-date=2018-02-04|url-status=dead|archive-date=2018-02-05|archive-url=https://web.archive.org/web/20180205184335/https://coralreef.noaa.gov/aboutcorals/coral101/anatomy/}}</ref> These branches are composed of a fibrous protein called [[gorgonin]] or of a calcified material.

===Stony corals===
{{main|Scleractinia}}{{see also|Hexacorallia}} 
[[File:Montastrea cavernosa.jpg|thumb|''[[Montastraea cavernosa]]'' polyps with tentacles extended]]
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.<ref name=Sprung>{{cite book |title=Corals: A quick reference guide |last=Sprung |first=Julian |year=1999 |publisher=Ricordea Publishing |isbn=978-1-883693-09-1 |page=145 }}</ref>

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.<ref>{{Cite journal|last1=Cuif|first1=J.P.|last2=Dauphin|first2=Y.|date=1998|title=Microstructural and physico-chemical characterization of 'centers of calcification' in septa of some Recent scleractinian corals|journal=Paläontologische Zeitschrift|volume=72|issue=3–4|pages=257–269|doi=10.1007/bf02988357|bibcode=1998PalZ...72..257C |s2cid=129021387|issn=0031-0220}}</ref><ref>{{Cite journal|last1=Cuif|first1=J.P.|last2=Dauphin|first2=Y.|last3=Doucet|first3=J.|last4=Salomé|first4=M.|last5=Susini|first5=J.|date=2003|title=XANES mapping of organic sulfate in three scleractinian coral skeletons|journal=Geochimica et Cosmochimica Acta|volume=67|issue=1|pages=75–83|doi=10.1016/s0016-7037(02)01041-4|issn=0016-7037|bibcode=2003GeCoA..67...75C}}</ref> The organic matrices extracted from diverse species are acidic, and comprise proteins, sulphated sugars and lipids; they are species specific.<ref>{{Cite journal|last1=Dauphin|first1=Y.|last2=Cuif|first2=J.P.|last3=Williams|first3=C. T.|date=2008|title=Soluble organic matrices of aragonitic skeletons of Merulinidae (Cnidaria, Anthozoa)|journal=Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology|volume=150|issue=1|pages=10–22|doi=10.1016/j.cbpb.2008.01.002|pmid=18325807|issn=1096-4959}}</ref> The soluble organic matrices of the skeletons allow to differentiate [[zooxanthellae]] and non-zooxanthellae specimens.<ref>{{Cite journal|last1=Cuif|first1=J.P.|last2=Dauphin|first2=Y.|last3=Freiwald|first3=A.|last4=Gautret|first4=P.|last5=Zibrowius|first5=H.|date=1999|title=Biochemical markers of zooxanthellae symbiosis in soluble matrices of skeleton of 24 Scleractinia species|journal=Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology|volume=123|issue=3|pages=269–278|doi=10.1016/s1095-6433(99)00059-8|issn=1095-6433}}</ref>