View source for Cnidaria

{{Short description|Aquatic animal phylum having cnydocytes}}
{{Good article}}
{{Automatic taxobox
| fossil_range = {{Fossil range|earliest=635|600|0}} [[Ediacaran]]–Present
| image = Cnidaria.png
| image_upright = 1.3
| image_caption = Four examples of cnidaria (''clockwise, from top left''): 
* A jellyfish ''[[Chrysaora melanaster]]'' 
* A gorgonian ''[[Annella mollis]]'' 
* A sea anemone ''[[Nemanthus annamensis]]''
* A stony coral ''[[Acropora cervicornis]]''
| display_parents = 2
| taxon = Cnidaria
| authority = [[Berthold Hatschek|Hatschek]], 1888
| subdivision_ranks = Subphyla and classes
| subdivision_ref = <ref>Subphyla Anthozoa, Endocnidozoa and Medusozoa based on {{cite web |url=https://www.marinespecies.org/aphia.php?p=taxdetails&id=1267 |title=WoRMS - Cnidaria |access-date=2025-08-10 |publisher=World Register of Marine Species |archive-url=https://web.archive.org/web/20250718021244/https://www.marinespecies.org/aphia.php?p=taxdetails&id=1267 |archive-date=2025-07-18}}</ref>
| subdivision = *Subphylum [[Anthozoa]]—[[coral]]s and [[sea anemone]]
** Class [[Octocorallia]]
** Class [[Hexacorallia]]
*Subphylum [[Medusozoa]]—[[jellyfish]] and [[hydrozoans]]:<ref>Classes in Medusozoa based on {{cite web|url=https://itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=718920#null| title=ITIS Report – Taxon: Subphylum Medusozoa| access-date=2018-03-18| publisher=Universal Taxonomic Services}}</ref>
** Class [[Cubozoa]]—[[box jellyfish]], sea wasps
** Class [[Hydrozoa]]—[[hydroid (zoology)|hydroids]], [[hydra (genus)|hydra]]-like animals
** Class [[Scyphozoa]]—[[true jellyfish]]
** Class [[Staurozoa]]—[[stalked jellyfish]]
* Subphylum '''Endocnidozoa'''—[[Parasitism|parasites]]
**Class [[Myxozoa]]
**Class [[Polypodiozoa]]|
}}
[[File:Sea nettles.jpg|thumb|[[Chrysaora fuscescens|Pacific sea nettles]], ''Chrysaora fuscescens'']]

'''Cnidaria''' ({{IPAc-en|n|ᵻ|ˈ|d|ɛər|i|ə|,_|n|aɪ|-}} {{respell|nih|DAIR|ee|ə|,_|ny|-}})<ref>{{OED|cnidaria}}</ref> is a [[phylum]] under kingdom [[Animal]]ia containing over 11,000 [[species]]<ref>{{cite web|url=http://www.marinespecies.org/aphia.php?p=browser&id%5B%5D=2&id%5B%5D=1267#focus|title=WoRMS - World Register of Marine Species|website=www.marinespecies.org|access-date=2018-12-17}}</ref> of [[aquatic invertebrate]]s found both in [[fresh water|freshwater]] and [[marine environment]]s (predominantly the latter), including [[jellyfish]], [[hydroid (zoology)|hydroid]]s, [[sea anemone]]s, [[coral]]s and some of the smallest marine [[parasite]]s. Their distinguishing features are an uncentralized nervous system distributed throughout a gelatinous body and the presence of [[cnidocyte]]s or cnidoblasts, specialized cells with ejectable [[organelles]] used mainly for [[envenomation]] and capturing [[prey]]. Their bodies consist of [[mesoglea]], a non-living, jelly-like substance, sandwiched between two layers of [[epithelium]] that are mostly one [[cell (biology)|cell]] thick. Many cnidarian species can reproduce both sexually and asexually.

Cnidarians mostly have two basic body forms: swimming [[medusa (biology)|medusa]]e and [[sessility (motility)|sessile]] [[polyp (zoology)|polyp]]s, both of which are [[radially symmetrical]] with mouths surrounded by [[tentacle]]s that bear cnidocytes, which are specialized stinging cells used to capture prey. Both forms have a single [[Body orifice|orifice]] and body cavity that are used for [[digestion]] and [[respiration (physiology)|respiration]]. Many cnidarian species produce [[Colony (biology)|colonies]] that are single organisms composed of medusa-like or polyp-like [[zooid]]s, or both (hence they are [[trimorphic]]). Cnidarians' activities are coordinated by a decentralized [[nerve net]] and [[sensory neuron|simple receptors]]. Cnidarians also have [[Rhopalium|rhopalia]], which are involved in gravity sensing and sometimes chemoreception. Several free-swimming species of [[Cubozoa]] and [[Scyphozoa]] possess balance-sensing [[statocyst]]s, and some have [[Simple eye in invertebrates|simple eyes]]. Not all cnidarians [[sexual reproduction|reproduce sexually]], but many species have complex life cycles of [[asexual reproduction|asexual]] polyp stages and sexual medusae stages. Some, however, omit either the polyp or the medusa stage, and the parasitic classes evolved to have neither form.

Cnidarians were formerly grouped with [[ctenophore]]s, also known as comb jellies, in the phylum [[Coelenterata]], but increasing awareness of their differences caused them to be placed in separate phyla.<ref>{{cite journal|last1=Dunn|first1=Casey W.|last2=Leys|first2=Sally P.|last3=Haddock|first3=Steven H.D.|title=The hidden biology of sponges and ctenophores|journal=Trends in Ecology & Evolution|date=May 2015|volume=30|issue=5|pages=282–291|doi=10.1016/j.tree.2015.03.003|pmid=25840473|doi-access=free|bibcode=2015TEcoE..30..282D }}</ref> Most cnidarians are classified into four main groups: the almost wholly [[Sessility (zoology)|sessile]] [[Anthozoa]] ([[sea anemone]]s, [[coral]]s, [[sea pen]]s); swimming [[Scyphozoa]] ([[jellyfish]]); [[Cubozoa]] (box jellies); and [[Hydrozoa]] (a diverse group that includes all the freshwater cnidarians as well as many marine forms, and which has both sessile members, such as ''[[Hydra (genus)|Hydra]]'', and colonial swimmers (such as the [[Portuguese man o' war]])). [[Staurozoa]] have recently been recognised as a [[class (biology)|class]] in their own right rather than a sub-group of Scyphozoa, and the highly derived parasitic [[Myxozoa]] and [[Polypodiozoa]] were firmly recognized as cnidarians only in 2007.<ref name="E.">{{cite journal|author=E. Jímenez-Guri|date=July 2007|title=''Buddenbrockia'' is a cnidarian worm|journal=[[Science (journal)|Science]]|volume=317|issue=116|pages=116–118|doi=10.1126/science.1142024|pmid=17615357|display-authors=etal|bibcode=2007Sci...317..116J|s2cid=5170702}}</ref>

Most cnidarians prey on [[organism]]s ranging in size from [[plankton]] to animals several times larger than themselves, but many obtain much of their nutrition from symbiotic [[dinoflagellate]]s, and a few are [[Parasitism|parasites]]. Many are preyed on by other animals including [[starfish]], [[Nudibranch|sea slugs]], [[fish]], [[turtle]]s, and even other cnidarians. Many [[scleractinia]]n corals—which form the structural foundation for [[coral reef]]s—possess polyps that are filled with symbiotic photo-synthetic [[zooxanthellae]]. While reef-forming corals are almost entirely restricted to warm and shallow marine waters, other cnidarians can be found at great depths, in [[polar region]]s, and in freshwater.

Cnidarians are a very ancient phylum, with fossils having been found in rocks formed about {{ma|580}} during the [[Ediacaran]] [[geologic period|period]], preceding the [[Cambrian Explosion]]. Other fossils show that corals may have been present shortly before {{ma|490}} and diversified a few million years later. [[Molecular clock]] analysis of [[mitochondria]]l genes suggests an even older age for the [[crown group]] of cnidarians, estimated around {{ma|741}}, almost 200 million years before the [[Cambrian]] period, as well as before any fossils.<ref>{{cite journal |vauthors=Park E, Hwang D, Lee J, Song J, Seo T, Won Y |display-authors=3| title=Estimation of divergence times in cnidarian evolution based on mitochondrial protein-coding genes and the fossil record. | journal= Molecular Phylogenetics & Evolution | volume=62| issue=1| date=January 2012| pages=329–45 | doi=10.1016/j.ympev.2011.10.008 | pmid = 22040765 |bibcode=2012MolPE..62..329P}}</ref> Recent [[phylogenetic]] analyses support [[monophyly]] of cnidarians, as well as the position of cnidarians as the [[sister group]] of [[bilateria]]ns.<ref>{{cite journal |vauthors=Zapata F, Goetz FE, Smith SA, Howison M, Siebert S, Church SH |display-authors=3| title=Phylogenomic Analyses Support Traditional Relationships within Cnidaria. | journal= PLOS ONE | volume=10| issue=10| year=2015| article-number=e0139068| doi=10.1371/journal.pone.0139068 | pmid=26465609 | pmc=4605497| bibcode=2015PLoSO..1039068Z| doi-access=free }}</ref>

==Etymology==
The term ''cnidaria'' derives from the [[Ancient Greek]] word ''knídē'' ([[wikt:κνίδη|κνίδη]] "nettle"), signifying the coiled thread reminiscent of cnidocytes.<ref>{{cite web |title=Cnidaria - etymonline.com |url=https://www.etymonline.com/word/Cnidaria}}</ref>

==Distinguishing features==
{{further|Porifera|Ctenophora|Bilateria|Placozoa}}
Cnidarians form a [[phylum]] of [[animal]]s that are more complex than [[sponge]]s, about as complex as [[ctenophore]]s (comb jellies), and less complex than [[bilateria]]ns, which include almost all other animals. Both cnidarians and ctenophores are more complex than sponges as they have: cells bound by inter-cell connections and carpet-like [[basement membrane]]s; [[muscle]]s; [[nervous system]]s; and '''some''' have [[sensory receptor|sensory]] organs. Cnidarians are distinguished from all other animals by having [[cnidocyte]]s that fire [[harpoon]]-like structures that are mainly used to capture prey. In some species, cnidocytes can also be used as anchors.<ref name="Hinde2001">{{cite book| author=Hinde, R.T.| year=1998| chapter=The Cnidaria and Ctenophora| pages=28–57| editor=Anderson, D.T.| title=Invertebrate Zoology| publisher=Oxford University Press| isbn=978-0-19-551368-4}}</ref> Cnidarians are also distinguished by the fact that they have only one opening in their body for ingestion and excretion i.e. they do not have a separate mouth and anus.

Like sponges and ctenophores, cnidarians have two main layers of cells that sandwich a middle layer of jelly-like material, which is called the [[mesoglea]] in cnidarians; more complex [[animal]]s have three main cell layers and no intermediate jelly-like layer. Hence, cnidarians and ctenophores have traditionally been labelled [[diploblastic]], along with sponges.<ref name="Hinde2001" /><ref name="Ruppert" /> However, both cnidarians and ctenophores have a type of [[muscle]] that, in more complex animals, arises from the [[mesoderm|middle cell layer]].<ref name="Seipel">{{cite journal|author1=Seipel, K. |author2=Schmid, V.| title=Evolution of striated muscle: Jellyfish and the origin of triploblasty| journal=Developmental Biology| volume=282| issue=1| date=June 2005| pages=14–26| doi=10.1016/j.ydbio.2005.03.032| pmid=15936326| doi-access=free}}</ref> As a result, some recent text books classify ctenophores as [[triploblastic]],<ref name="Ruppert" />{{rp|182–195}} and it has been suggested that cnidarians evolved from triploblastic ancestors.<ref name="Seipel"/>

{| class="wikitable" style="margin-left:4px"
! &nbsp;!![[Sponge]]s<ref name="Ruppert" />{{rp|76–97}}<ref>{{cite book| author=Bergquist, P.R.| year=1998| chapter=Porifera| pages=10–27| editor=Anderson, D.T.| title=Invertebrate Zoology| publisher=Oxford University Press| isbn=978-0-19-551368-4}}</ref>!!Cnidarians<ref name="Hinde2001" /><ref name="Ruppert" />!![[Ctenophore]]s<ref name="Hinde2001" /><ref name="Ruppert" />{{rp|182–195}}!![[Bilateria]]<ref name="Hinde2001" />
|- style="text-align:center;"
! [[Cnidocyte]]s
| No||Yes||colspan=2| No
|- style="text-align:center;"
! [[Colloblast]]s
| colspan=2| No||Yes||No
|- style="text-align:center;"
! [[Digestive system|Digestive]] and [[circulatory system|circulatory]] [[organ (anatomy)|organ]]s
| colspan=3| No||Yes
|- style="text-align:center;"
! Number of main cell layers
| Two, with jelly-like layer between them
|Two||Two||Three
|- style="text-align:center;"
! Cells in each layer bound together
| cell-adhesion molecules, but no basement membranes except [[Homoscleromorpha]].<ref>{{cite journal|author1=Exposito, J-Y. |author2=Cluzel, C. |author3=Garrone, R. |author4=Lethias, C. |name-list-style=amp | title=Evolution of collagens| journal=The Anatomical Record Part A: Discoveries in Molecular, Cellular, and Evolutionary Biology| volume=268| pages=302–316| doi=10.1002/ar.10162| year=2002|issue=3| pmid=12382326|s2cid=12376172 |doi-access=free}}</ref>||colspan=3|inter-cell connections; basement membranes
|- style="text-align:center;"
! [[Sensory receptor|Sensory]] organs
| No||colspan=3| Yes
|- style="text-align:center;"
! Number of cells in middle "jelly" layer
| Many||colspan=2| Few||(Not applicable)
|- style="text-align:center;"
! Cells in outer layers can move inwards and change functions
| Yes||colspan=2| No||(Not applicable)
|- style="text-align:center;"
! Nervous system
| No||colspan=2| Yes, simple||Simple to complex
|- style="text-align:center;"
! [[Muscle]]s
| None||Mostly epitheliomuscular||Mostly myoepithelial||Mostly [[myocyte]]s
|}
{{clear}}

==Description==

===Basic body forms===
[[File:Cnidiria medusa and polip svg hariadhi.svg|thumb|250px]] 
[[File:Actinodiscus macro.JPG|thumb|right|250px|Oral end of [[actinodiscus]] polyp]]
Most adult cnidarians appear as either free-swimming [[Medusa (biology)|medusa]]e or [[Sessility (zoology)|sessile]] [[polyp (zoology)|polyp]]s, and many [[hydrozoa]]ns species are known to alternate between the two forms.

Both are [[Symmetry (biology)#Radial symmetry|radially symmetrical]], like a wheel and a tube respectively. Since these animals have no heads, their ends are described as "oral" (nearest the mouth) and "aboral" (furthest from the mouth).

Most have fringes of tentacles equipped with [[cnidocyte]]s around their edges, and medusae generally have an inner ring of tentacles around the mouth. Some hydroids may consist of colonies of [[zooid]]s that serve different purposes, such as defence, reproduction and catching prey. The [[mesoglea]] of polyps is usually thin and often soft, but that of medusae is usually thick and springy, so that it returns to its original shape after muscles around the edge have contracted to squeeze water out, enabling medusae to swim by a sort of [[jet propulsion]].<ref name="Ruppert" />

===Skeletons===
In medusae, the only supporting structure is the [[mesoglea]]. ''[[hydra (genus)|Hydra]]'' and most [[sea anemone]]s close their mouths when they are not feeding, and the [[water]] in the digestive cavity then acts as a [[hydrostatic skeleton]], rather like a water-filled balloon. Other polyps such as ''[[Tubularia]]'' use columns of water-filled cells for support. [[Sea pen]]s stiffen the mesoglea with [[calcium carbonate]] [[wikt:spicule|spicule]]s and tough fibrous [[protein]]s, rather like [[sponge]]s.<ref name="Ruppert" />

In some colonial polyps, a [[chitin]]ous [[Epidermis (zoology)|epidermis]] gives support and some protection to the connecting sections and to the lower parts of individual polyps. A few polyps collect materials such as sand grains and shell fragments, which they attach to their outsides. Some colonial sea anemones stiffen the mesoglea with [[sediment]] particles.<ref name="Ruppert" />

A mineralized [[exoskeleton]] made of calcium carbonate is found in subphylum Anthozoa in the order [[Scleractinia]] (stony corals; class Hexacorallia) and the class [[Octocorallia]],<ref>[https://academic.oup.com/gbe/article/12/9/1623/5882021?login=false Comparative Proteomics of Octocoral and Scleractinian Skeletomes and the Evolution of Coral Calcification]</ref> and in subphylum Medusozoa in three [[hydrozoa]]n families in order [[Anthoathecata]]; [[Fire coral|Milleporidae]], [[Stylasteridae]] and [[Hydractiniidae]] (the latter with a mix of calcified and uncalcified species).<ref>[https://academic.oup.com/icb/article/50/3/428/621249?login=false Evolution of Calcium-carbonate Skeletons in the Hydractiniidae]</ref>

===Main cell layers===
Cnidaria are [[diploblastic]] animals; in other words, they have two main cell layers, while more complex animals are [[triploblast]]s having three main layers. The two main cell layers of cnidarians form [[epithelia]] that are mostly one cell thick, and are attached to a fibrous [[basement membrane]], which they [[secrete]]. They also secrete the jelly-like [[mesoglea]] that separates the layers. The layer that faces outwards, known as the [[ectoderm]] ("outside skin"), generally contains the following types of cells:<ref name="Hinde2001" />
*Epitheliomuscular cells whose bodies form part of the epithelium but whose bases extend to form [[muscle]] fibers in parallel rows.<ref name="Ruppert" />{{rp| 103–104}} The fibers of the outward-facing cell layer generally run at right angles to the fibers of the inward-facing one. In [[Anthozoa]] (anemones, corals, etc.) and [[Scyphozoa]] (jellyfish), the [[mesoglea]] also contains some muscle cells.<ref name="Ruppert">{{cite book| author1=Ruppert, E.E.| author2=Fox, R.S.| author3=Barnes, R.D.| name-list-style=amp| title=Invertebrate Zoology| publisher=Brooks / Cole| edition=7| isbn=978-0-03-025982-1| year=2004| pages=[https://archive.org/details/isbn_9780030259821/page/111 111–124]| url=https://archive.org/details/isbn_9780030259821/page/111}}</ref>
*[[Cnidocyte]]s, the harpoon-like "nettle cells" that give the [[phylum]] Cnidaria its name. These appear between or sometimes on top of the muscle cells.<ref name="Hinde2001" />
*[[Nerve]] cells. [[Sensory neuron|Sensory]] cells appear between or sometimes on top of the muscle cells,<ref name="Hinde2001" /> and communicate via [[synapse]]s (gaps across which chemical signals flow) with [[motor nerve]] cells, which lie mostly between the bases of the muscle cells.<ref name="Ruppert" /> Some form a simple [[nerve net]].
*Interstitial cells, which are unspecialized and can replace lost or damaged cells by transforming into the appropriate types. These are found between the bases of muscle cells.<ref name="Hinde2001" />

In addition to epitheliomuscular, nerve and interstitial cells, the inward-facing [[gastrodermis|gastroderm]] ("stomach skin") contains [[gland]] cells that secrete digestive [[enzyme]]s. In some species it also contains low concentrations of cnidocytes, which are used to subdue prey that is still struggling.<ref name="Hinde2001" /><ref name="Ruppert" />

The mesoglea contains small numbers of [[amoeba (genus)|amoeba]]-like cells,<ref name="Ruppert" /> and muscle cells in some species.<ref name="Hinde2001" /> However, the number of middle-layer cells and types are much lower than in sponges.<ref name="Ruppert" />

===Polymorphism===
[[Polymorphism (biology)|Polymorphism]] refers to the occurrence of structurally and functionally more than two different types of individuals within the same organism. It is a characteristic feature of cnidarians, particularly the [[polyp (zoology)|polyp]] and [[medusae|medusa]] forms, or of [[zooids]] within colonial organisms like those in [[Hydrozoa]].<ref>{{cite journal |doi=10.1098/rspb.1966.0037 |title=Genetic polymorphism |journal=Proceedings of the Royal Society of London. Series B. Biological Sciences |date=1966 |volume=164 |issue=995 |pages=350–361 |pmid=4379524 |bibcode=1966RSPSB.164..350F |last1=Ford |first1=E. B. }}</ref> In [[Hydrozoa]]ns, colonial individuals arising from individual zooids will take on separate tasks.<ref>{{cite journal|last=Dunn|first=Casey W.|author2=Wagner, Günter P.|title=The evolution of colony-level development in the Siphonophora (Cnidaria:Hydrozoa)|journal=Development Genes and Evolution|date=16 September 2006|volume=216|issue=12|pages=743–754|doi=10.1007/s00427-006-0101-8|pmid=16983540|s2cid=278540}}</ref> For example, in ''[[Obelia]]'' there are feeding individuals, the [[gastrozooid]]s; the individuals capable of asexual reproduction only, the gonozooids, blastostyles and free-living or sexually reproducing individuals, the [[medusae]].

===Cnidocytes===
These "nettle cells" function as [[harpoon]]s, since their [[Wikt:payload|payload]]s remain connected to the bodies of the cells by threads. Three types of [[cnidocyte]]s are known:<ref name="Hinde2001" /><ref name="Ruppert" />

[[File:Hydra nematocyst firing 01.png|thumb|right|200px| Firing sequence of the cnida in a hydra's nematocyst<ref name="Ruppert" /><br />{{Color box|blue|border=silver}} Operculum (lid)<br />{{Color box|red|border=silver}} "Finger" that turns inside out<br />/ / / Barbs<br />{{Color box|yellow|border=silver}} Venom<br />{{Color box|silver|border=silver}} Victim's skin<br />{{Color box|#ffcad0|border=silver}} Victim's tissues]]
*[[Nematocyst]]s inject [[venom]] into prey, and usually have barbs to keep them embedded in the victims. Most species have nematocysts.<ref name="Hinde2001" />
*[[Spirocyst]]s do not penetrate the victim or inject venom, but entangle it by means of small sticky hairs on the thread.
*[[Ptychocyst]]s are not used for prey capture&nbsp;— instead the threads of discharged ptychocysts are used for building protective tubes in which their owners live. Ptychocysts are found only in the [[order (biology)|order]] [[Ceriantharia]], [[Tube-dwelling anemone|tube anemones]].<ref name="Ruppert" />

The main components of a cnidocyte are:<ref name="Hinde2001" /><ref name="Ruppert" />
[[File:Hydra nematocyst 01.png|thumb|200px| A [[hydra (genus)|hydra]]'s nematocyst, before firing.<br />{{Color box|#00b900|border=silver}} "trigger" cilium<ref name="Ruppert" />]]

*A [[cilium]] (fine hair) which projects above the surface and acts as a trigger. Spirocysts do not have cilia.
*A tough capsule, the [[cnida]], which houses the thread, its payload and a mixture of chemicals that may include venom or [[adhesive]]s or both. ("cnida" is derived from the Greek word κνίδη, which means "nettle"<ref>{{Cite book| title=Shorter Oxford English Dictionary| contribution=Cnida|author1=Trumble, W. |author2=Brown, L.| publisher=Oxford University Press| year=2002}}</ref>)
*A tube-like extension of the wall of the cnida that points into the cnida, like the finger of a rubber glove pushed inwards. When a cnidocyte fires, the finger pops out. If the cell is a venomous nematocyte, the "finger"'s tip reveals a set of barbs that anchor it in the prey.
*The thread, which is an extension of the "finger" and coils round it until the cnidocyte fires. The thread is usually hollow and delivers chemicals from the cnida to the target.
*An [[Operculum (animal)|operculum]] (lid) over the end of the cnida. The lid may be a single hinged flap or three flaps arranged like slices of pie.
*The cell body, which produces all the other parts.

It is difficult to study the firing mechanisms of cnidocytes as these structures are small but very complex. At least four hypotheses have been proposed:<ref name="Hinde2001" />
*Rapid contraction of fibers round the cnida may increase its internal pressure.
*The thread may be like a coiled spring that extends rapidly when released.
*In the case of ''[[Chironex]]'' (the "sea wasp"), chemical changes in the cnida's contents may cause them to expand rapidly by [[polymerization]].
*Chemical changes in the liquid in the cnida make it a much more [[concentration|concentrated]] solution, so that [[osmotic pressure]] forces water in very rapidly to dilute it. This mechanism has been observed in nematocysts of the class [[Hydrozoa]], sometimes producing pressures as high as 140 [[Atmosphere (unit)|atmospheres]], similar to that of [[Scuba set|scuba]] air tanks, and fully extending the thread in as little as 2&nbsp;milliseconds (0.002&nbsp;second).<ref name="Ruppert" />

Cnidocytes can only fire once, and about 25% of a hydra's nematocysts are lost from its tentacles when capturing a [[brine shrimp]]. Used cnidocytes have to be replaced, which takes about 48&nbsp;hours. To minimise wasteful firing, two types of stimulus are generally required to trigger cnidocytes: nearby [[sensory neuron|sensory]] cells detect chemicals in the water, and their cilia respond to contact. This combination prevents them from firing at distant or non-living objects. Groups of cnidocytes are usually connected by nerves and, if one fires, the rest of the group requires a weaker minimum stimulus than the cells that fire first.<ref name="Hinde2001" /><ref name="Ruppert" />

===Locomotion===
[[File:Chrysaora quinquecirrha-Sea nettle (jellyfish).ogg|thumb| right| 200px| A swimming sea nettle known as the purple-striped jelly (''[[Chrysaora colorata]]'')]]
Medusae swim by a form of jet propulsion: muscles, especially inside the rim of the bell, squeeze water out of the cavity inside the bell, and the springiness of the mesoglea powers the recovery stroke. Since the tissue layers are very thin, they provide too little power to swim against currents and just enough to control movement within currents.<ref name="Ruppert" />

[[hydra (genus)|Hydra]]s and some [[sea anemone]]s can move slowly over rocks and sea or stream beds by various means: creeping like snails, crawling like [[Geometer moth|inchworm]]s, or by [[Cartwheel (gymnastics)|somersault]]ing. A few can swim clumsily by waggling their bases.<ref name="Ruppert" />

===Nervous system and senses===
Cnidarians are generally thought to have no brains or even central nervous systems. However, they do have integrative areas of neural tissue that could be considered some form of centralization. Most of their bodies are innervated by decentralized nerve nets that control their swimming musculature and connect with sensory structures, though each clade has slightly different structures.<ref name=Central>{{Cite journal|last=Satterlie|first=Richard A.|date=15 April 2011|title=Do jellyfish have central nervous systems?|journal=Journal of Experimental Biology|language=en|volume=214|issue=8|pages=1215–1223|doi=10.1242/jeb.043687|issn=0022-0949|pmid=21430196|doi-access=free|bibcode=2011JExpB.214.1215S }}</ref> These sensory structures, usually called rhopalia, can generate signals in response to various types of stimuli such as light, pressure, chemical changes, and much more. Medusa usually have several of them around the margin of the bell that work together to control the motor nerve net, that directly innervates the swimming muscles. Most cnidarians also have a parallel system. In scyphozoans, this takes the form of a diffuse nerve net, which has modulatory effects on the nervous system.<ref name=Control>{{Cite journal|last=Satterlie|first=Richard A|s2cid=18244609|date=2002-10-01|title=Neuronal control of swimming in jellyfish: a comparative story|journal=Canadian Journal of Zoology|volume=80|issue=10|pages=1654–1669|doi=10.1139/z02-132|bibcode=2002CaJZ...80.1654S |issn=0008-4301}}</ref> As well as forming the "signal cables" between sensory neurons and motoneurons, intermediate neurons in the nerve net can also form ganglia that act as local coordination centers. Communication between nerve cells can occur by chemical synapses or gap junctions in hydrozoans, though gap junctions are not present in all groups. Cnidarians have many of the same neurotransmitters as bilaterians, including chemicals such as glutamate, GABA, and glycine.<ref>{{Cite journal|last1=Kass-Simon|first1=G.|last2=Pierobon|first2=Paola|date=1 January 2007|title=Cnidarian chemical neurotransmission, an updated overview|journal=Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology|volume=146|issue=1|pages=9–25|doi=10.1016/j.cbpa.2006.09.008|pmid=17101286|url=https://digitalcommons.uri.edu/bio_facpubs/380 }}</ref> Serotonin, dopamine, noradrenaline, octopamine, histamine, and acetylcholine, on the other hand, are absent.<ref>[https://www.frontiersin.org/articles/10.3389/fcell.2022.1071961/full What is a neuron? (Ctenophores, sponges and placozoans)]</ref>

This structure ensures that the musculature is excited rapidly and simultaneously, and can be directly stimulated from any point on the body, and it also is better able to recover after injury.<ref name=Central/><ref name=Control />

Medusae and complex swimming colonies such as [[siphonophore]]s and [[chondrophore]]s sense tilt and acceleration by means of [[statocyst]]s, chambers lined with hairs which detect the movements of internal mineral grains called statoliths. If the body tilts in the wrong direction, the animal rights itself by increasing the strength of the swimming movements on the side that is too low. Most species have [[ocelli]] ("simple eyes"), which can detect sources of light. However, the agile [[box jellyfish]] are unique among Medusae because they possess four kinds of true eyes that have [[retinas]], [[cornea]]s and [[lens (anatomy)|lenses]].<ref name="live">{{cite web|url=http://www.livescience.com/7243-jellyfish-human-eyes.html|title=Jellyfish Have Human-Like Eyes|publisher=www.livescience.com|date=April 1, 2007|access-date=2012-06-12}}</ref> Although the eyes probably do not form images, Cubozoa can clearly distinguish the direction from which light is coming as well as negotiate around solid-colored objects.<ref name="Hinde2001" /><ref name="live" />

===Feeding and excretion===
Cnidarians feed in several ways: [[predation]], absorbing dissolved [[organic matter|organic]] chemicals, [[filter feeding|filtering]] food particles out of the water, obtaining [[nutrients]] from [[symbiotic]] [[algae]] within their cells, and parasitism. Most obtain the majority of their food from predation but some, including the [[coral]]s ''[[Hetroxenia]]'' and ''[[Leptogorgia]]'', depend almost completely on their [[endosymbiont]]s and on absorbing dissolved nutrients.<ref name="Hinde2001" /> Cnidaria give their symbiotic algae [[carbon dioxide]], some nutrients, and protection against predators.<ref name="Ruppert" />

Predatory species use their [[cnidocyte]]s to poison or entangle prey, and those with venomous [[nematocyst]]s may start digestion by injecting digestive [[enzyme]]s. The "smell" of fluids from wounded prey makes the tentacles fold inwards and wipe the prey off into the mouth. In medusae, the tentacles around the edge of the bell are often short and most of the prey capture is done by "oral arms", which are extensions of the edge of the mouth and are often frilled and sometimes branched to increase their surface area. These "oral arms" aid in cnidarians' ability to move prey towards their mouth once it has been poisoned and entangled. Medusae often trap prey or suspended food particles by swimming upwards, spreading their tentacles and oral arms and then sinking. In species for which suspended food particles are important, the tentacles and oral arms often have rows of [[cilia]] whose beating creates currents that flow towards the mouth, and some produce nets of [[mucus]] to trap particles.<ref name="Hinde2001" /> Their digestion is both intra and extracellular.

Once the food is in the digestive cavity, [[gland]] cells in the [[gastrodermis|gastroderm]] release enzymes that reduce the prey to slurry, usually within a few hours. This circulates through the digestive cavity and, in colonial cnidarians, through the connecting tunnels, so that gastroderm cells can absorb the nutrients. Absorption may take a few hours, and digestion within the cells may take a few days. The circulation of nutrients is driven by water currents produced by cilia in the gastroderm or by muscular movements or both, so that nutrients reach all parts of the digestive cavity.<ref name="Ruppert" /> Nutrients reach the outer cell layer by [[diffusion]] or, for animals or zooids such as medusae which have thick [[mesoglea]]s, are transported by mobile cells in the mesoglea.<ref name="Hinde2001" />

Indigestible remains of prey are expelled through the mouth. The main waste product of cells' internal processes is [[ammonia]], which is removed by the external and internal water currents.<ref name="Ruppert" />

===Respiration===
There are no respiratory organs, and both cell layers absorb oxygen from and expel [[carbon dioxide]] into the surrounding water. When the water in the digestive cavity becomes stale it must be replaced, and nutrients that have not been absorbed will be expelled with it. Some [[Anthozoa]] have ciliated grooves on their tentacles, allowing them to pump water out of and into the digestive cavity without opening the mouth. This improves respiration after feeding and allows these animals, which use the cavity as a [[hydrostatic skeleton]], to control the water pressure in the cavity without expelling undigested food.<ref name="Hinde2001" />

Cnidaria that carry [[photosynthetic]] [[symbiont]]s may have the opposite problem, an excess of oxygen, which may prove [[Oxygen toxicity|toxic]]. The animals produce large quantities of [[antioxidant]]s to neutralize the excess oxygen.<ref name="Hinde2001" />

===Regeneration===
All cnidarians can [[regeneration (biology)|regenerate]], allowing them to recover from injury and to reproduce [[asexual reproduction|asexually]]. Medusae have limited ability to regenerate, but polyps can do so from small pieces or even collections of separated cells. This enables corals to recover even after apparently being destroyed by predators.<ref name="Hinde2001" />

==Reproduction==
{{Annotated image| float=right| caption=Life cycle of a [[jellyfish]]:<ref name="Hinde2001" /><ref name="Ruppert" /><br />'''1–3''' Larva searches for site<br />'''4–8''' Polyp grows<br />'''9–11''' Polyp [[strobilation|strobilates]]<br />'''12–14''' Medusa grows| image=Schleiden-meduse-2.jpg| width=193| image-width=200| height=292| annot-font-weight=bold| annot-background-color=yellow| annot-font-size=10
| annotations=
{{Annotation|100|231|&nbsp;1&nbsp;|background-color=#ffcccc}}
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{{Annotation|161|62|&nbsp;14&nbsp;|background-color=#ffcccc}}
}}

===Sexual===
Cnidarian [[sexual reproduction]] often involves a complex life cycle with both [[polyp (zoology)|polyp]] and [[Medusa (biology)|medusa]] stages. For example, in [[Scyphozoa]] (jellyfish) and [[Cubozoa]] (box jellies), a [[Planula|larva]] swims until it finds a good site, and then becomes a polyp. This grows normally but then absorbs its tentacles and splits horizontally into a series of disks that become juvenile medusae, a process called [[strobilation]]. The juveniles swim off and slowly grow to maturity, while the polyp re-grows and may continue strobilating periodically. The adult medusae have [[gonad]]s in the [[gastrodermis|gastroderm]], and these release [[ovum|ova]] and [[sperm]] into the water in the breeding season.<ref name="Hinde2001" /><ref name="Ruppert" />

This phenomenon of succession of differently organized generations (one asexually reproducing, sessile polyp, followed by a [[free-swimming]] medusa or a sessile polyp that reproduces sexually)<ref>{{cite book|author=Vernon A. Harris |year=1990 |chapter=Hydroids |title=Sessile animals of the sea shore |publisher=Springer |page=223 |isbn=978-0-412-33760-4 |chapter-url=https://books.google.com/books?id=DbaBQm_ZQkIC&q=alternation+of+generation+&pg=PA223}}</ref> is sometimes called "alternation of asexual and sexual phases" or "metagenesis", but should not be confused with the [[alternation of generations]] as found in plants.

Shortened forms of this life cycle are common, for example some oceanic scyphozoans omit the polyp stage completely, and cubozoan polyps produce only one medusa. [[Hydrozoa]] have a variety of life cycles. Some have no polyp stages and some (e.g. ''[[hydra (genus)|hydra]]'') have no medusae. In some species, the medusae remain attached to the polyp and are responsible for sexual reproduction; in extreme cases these reproductive zooids may not look much like medusae. Meanwhile, life cycle reversal, in which polyps are formed directly from medusae without the involvement of sexual reproduction process, was observed in both Hydrozoa (''Turritopsis dohrnii''<ref>{{Cite journal|url = http://www.icm.csic.es/scimar/pdf/56/sm56n2137.pdf|title = Bi-directional conversion in Turritopsis nutricula (Hydrozoa)|last = Bavestrello|date = 1992|journal = Scientia Marina|access-date = 2015-12-31|display-authors = etal|archive-date = 2014-12-14|archive-url = https://web.archive.org/web/20141214014457/http://www.icm.csic.es/scimar/pdf/56/sm56n2137.pdf}}</ref> and ''Laodicea undulata''<ref>{{Cite journal|title = Evidence of reverse development in Leptomedusae (Cnidaria, Hydrozoa): the case of Laodicea undulata (Forbes and Goodsir 1851)|last = De Vito|date = 2006|journal = Marine Biology|doi = 10.1007/s00227-005-0182-3|display-authors=etal|volume=149|issue = 2|pages=339–346| bibcode=2006MarBi.149..339D |s2cid = 84325535}}</ref>) and Scyphozoa (''Aurelia'' sp.1<ref>{{Cite journal|title = Life Cycle Reversal in Aurelia sp.1 (Cnidaria, Scyphozoa)|last = He|date = 21 December 2015|journal = PLOS ONE|doi = 10.1371/journal.pone.0145314|pmid = 26690755|display-authors=etal|pmc=4687044|volume=10|issue = 12|article-number=e0145314|bibcode=2015PLoSO..1045314H|doi-access = free}}</ref>). [[Anthozoa]] have no medusa stage at all and the polyps are responsible for sexual reproduction.<ref name="Hinde2001" />

Spawning is generally driven by environmental factors such as changes in the water temperature, and their release is triggered by lighting conditions such as sunrise, sunset or the [[Lunar phase|phase of the moon]]. Many species of Cnidaria may spawn simultaneously in the same location, so that there are too many ova and sperm for predators to eat more than a tiny percentage&nbsp;— one famous example is the [[Great Barrier Reef]], where at least 110 [[coral]]s and a few non-cnidarian [[invertebrate]]s produce enough gametes to turn the water cloudy. These mass spawnings may produce [[Hybrid (biology)|hybrids]], some of which can settle and form polyps, but it is not known how long these can survive. In some species the ova release chemicals that attract sperm of the same species.<ref name="Hinde2001" />

The fertilized eggs develop into larvae by dividing until there are enough cells to form a hollow sphere ([[blastula]]) and then a depression forms at one end ([[gastrulation]]) and eventually becomes the digestive cavity. However, in cnidarians the depression forms at the end further from the yolk (at the [[animal pole]]), while in [[bilaterian]]s it forms at the other end ([[vegetal pole]]).<ref name="Ruppert" /> The larvae, called [[planula]]e, swim or crawl by means of [[cilia]].<ref name="Hinde2001" /> They are cigar-shaped but slightly broader at the "front" end, which is the aboral, vegetal-pole end and eventually attaches to a substrate if the species has a polyp stage.<ref name="Ruppert" />

Anthozoan larvae either have large [[yolk]]s or are capable of feeding on [[plankton]], and some already have [[endosymbiotic]] [[algae]] that help to feed them. Since the parents are immobile, these feeding capabilities extend the larvae's range and avoid overcrowding of sites. Scyphozoan and hydrozoan larvae have little yolk and most lack endosymbiotic algae, and therefore have to settle quickly and [[metamorphose]] into polyps. Instead, these species rely on their medusae to extend their ranges.<ref name="Ruppert" />

===Asexual===
All known cnidarians can reproduce [[asexual reproduction|asexually]] by various means, in addition to regenerating after being fragmented. [[Hydrozoa]]n polyps only bud, while the medusae of some hydrozoans can divide down the middle. [[Scyphozoa]]n polyps can both bud and split down the middle. In addition to both of these methods, [[Anthozoa]] can split horizontally just above the base. Asexual reproduction makes the daughter cnidarian a clone of the adult. The ability of cnidarians to asexually reproduce ensures a greater number of mature medusa that can mature to reproduce sexually.<ref name="Hinde2001" /><ref name="Ruppert" />

===DNA repair===

Two classical [[DNA repair]] pathways, [[nucleotide excision repair]] and [[base excision repair]], are present in [[hydra (genus)|hydra]],<ref name = Barve2021>{{cite journal|pmid=33995496 |date=2021 |last1=Barve |first1=A. |last2=Galande |first2=A. A. |last3=Ghaskadbi |first3=S. S. |last4=Ghaskadbi |first4=S. |title=DNA Repair Repertoire of the Enigmatic Hydra |journal=Frontiers in Genetics |volume=12 |article-number=670695 |doi=10.3389/fgene.2021.670695 |pmc=8117345 |doi-access=free }}</ref> and these repair pathways facilitate unhindered reproduction.  The identification of these pathways in hydra is based, in part, on the presence in the hydra [[genome]] of genes homologous to genes in other genetically well studied species that have been demonstrated to play key roles in these DNA repair pathways.<ref name = Barve2021/>

==Classification==
Cnidarians were for a long time grouped with [[ctenophore]]s in the phylum [[Coelenterata]], but increasing awareness of their differences caused them to be placed in separate phyla. Modern cnidarians are generally classified into four main [[class (biology)|classes]]:<ref name="Hinde2001" /> sessile [[Anthozoa]] ([[sea anemone]]s, [[coral]]s, [[sea pen]]s); swimming [[Scyphozoa]] (jellyfish) and [[Cubozoa]] (box jellies); and [[Hydrozoa]], a diverse group that includes all the freshwater cnidarians as well as many marine forms, and has both sessile members such as ''[[Hydra (genus)|Hydra]]'' and colonial swimmers such as the [[Portuguese Man o' War]]. [[Staurozoa]] have recently been recognised as a [[class (biology)|class]] in their own right rather than a sub-group of Scyphozoa, and the parasitic [[Myxozoa]] and [[Polypodiozoa]] are now recognized as highly derived cnidarians rather than more closely related to the [[bilateria]]ns.<ref name="E."/><ref name="haaretz">{{cite news|last=Schuster|first=Ruth|date=20 November 2015|url=https://www.haaretz.com/israel-news/science/1.687159|title=Microscopic parasitic jellyfish defy everything we know, astonish scientists|work=[[Haaretz]]|access-date=4 April 2018}}</ref>

{| class="wikitable"
|-
! !![[Hydrozoa]]!![[Scyphozoa]]!![[Cubozoa]]!![[Anthozoa]]!![[Myxozoa]]
|-
! Number of species<ref>{{cite journal|author=Zhang, Z.-Q.|year=2011|title=Animal biodiversity: An introduction to higher-level classification and taxonomic richness|url=http://mapress.com/zootaxa/2011/f/zt03148p012.pdf |archive-url=https://web.archive.org/web/20120112043434/http://mapress.com/zootaxa/2011/f/zt03148p012.pdf |archive-date=2012-01-12 |url-status=live|journal=Zootaxa|volume=3148|pages=7–12|doi=10.11646/zootaxa.3148.1.3}}</ref>
||3,600||228||42||6,100||1300
|-
! Examples
| ''[[Hydra (genus)|Hydra]]'', [[siphonophore]]s||[[Jellyfish]]||[[Box jelly|Box jellies]]||[[Sea anemone]]s, [[coral]]s, [[sea pen]]s||''[[Myxobolus cerebralis]]''
|-
! Cells found in [[mesoglea]]
| No||Yes||Yes||Yes||
|-
! [[Nematocyst]]s in [[exodermis]]
| No||Yes||Yes||Yes||
|-
! Medusa phase in life cycle
| In some species||Yes||Yes||No||
|-
! Number of medusae produced per polyp
| Many||Many||One||(not applicable)||
|}

Stauromedusae, small [[Sessility (zoology)|sessile]] cnidarians with stalks and no medusa stage, have traditionally been classified as members of the Scyphozoa, but recent research suggests they should be regarded as a separate class, Staurozoa.<ref>{{cite journal|author1=Collins, A.G. |author2=Cartwright, P. |author3=McFadden, C.S. |author4=Schierwater, B. |name-list-style=amp | title=Phylogenetic Context and Basal Metazoan Model Systems| journal=Integrative and Comparative Biology| year=2005| volume=45| issue=4| pages=585–594| doi=10.1093/icb/45.4.585| pmid=21676805| doi-access=free}}</ref>

The [[Myxozoa]], microscopic [[parasite]]s, were first classified as [[protozoa]]ns.<ref>{{cite journal|author=Štolc, A.|title=Actinomyxidies, nouveau groupe de Mesozoaires parent des Myxosporidies|year=1899|journal= Bull. Int. l'Acad. Sci. Bohème|volume=12|pages=1–12}}</ref> Research then found that ''[[Polypodium hydriforme]]'', a non-myxozoan parasite ''within'' the egg cells of [[sturgeon]], is closely related to the Myxozoa and suggested that both ''Polypodium'' and the Myxozoa were intermediate between cnidarians and [[bilateria]]n animals.<ref name="ZrzavýHypša2003MyxozoaPolypodium">{{cite journal|author1=Zrzavý, J. |author2=Hypša, V.| date=April 2003| title=Myxozoa, ''Polypodium'', and the origin of the Bilateria: The phylogenetic position of "Endocnidozoa" in light of the rediscovery of ''Buddenbrockia''| journal=Cladistics| volume=19| issue=2| pages=164–169| doi=10.1111/j.1096-0031.2003.tb00305.x
|s2cid=221583517}}</ref> More recent research demonstrates that the previous identification of bilaterian genes reflected contamination of the myxozoan samples by material from their host organism, and they are now firmly identified as heavily derived cnidarians, and more closely related to Hydrozoa and Scyphozoa than to Anthozoa.<ref name="E."/><ref name="haaretz"/><ref>{{cite journal|author=E. Jímenez-Guri|date=July 2007|title=''Buddenbrockia'' is a cnidarian worm|journal=[[Science (journal)|Science]]|volume=317|issue=116|pages=116–118|doi=10.1126/science.1142024|pmid=17615357|last2=Philippe|first2=H|last3=Okamura|first3=B|last4=Holland|first4=PW|bibcode = 2007Sci...317..116J |s2cid=5170702}}</ref><ref>{{cite journal|last1=Chang|first1=E. Sally|last2=Neuhof|first2=Moran|last3=Rubinstein|first3=Nimrod D.|last4=Diamant|first4=Arik|last5=Philippe|first5=Hervé|last6=Huchon|first6=Dorothée|last7=Cartwright|first7=Paulyn|display-authors=3|title=Genomic insights into the evolutionary origin of Myxozoa within Cnidaria|journal=Proceedings of the National Academy of Sciences|date=1 December 2015|volume=112|issue=48|pages=14912–14917|doi=10.1073/pnas.1511468112|pmid=26627241|pmc=4672818|bibcode=2015PNAS..11214912C|doi-access=free}}</ref>

Some researchers classify the extinct [[Conulariida|conulariid]]s as cnidarians, while others propose that they form a completely separate [[phylum]].<ref>{{cite web| url=http://www.ucmp.berkeley.edu/cnidaria/conulariida.html| access-date=2008-11-27| title=The Conulariida| publisher=University of California Museum of Paleontology}}</ref>

Current classification according to the [[World Register of Marine Species]]: 
* subphylum [[Anthozoa]] <small>Ehrenberg, 1834</small>
** class [[Hexacorallia]] <small>Haeckel, 1896</small> — stony corals
** class [[Octocorallia]] <small>Haeckel, 1866</small> — soft corals and sea fans
* subphylum Endocnidozoa <small> Zrzavý & Hypša, 2003</small> — parasites
** class [[Myxozoa]] <small>Grassé, 1970</small>
** class [[Polypodiozoa]] <small>Raikova, 1994</small>
* subphylum [[Medusozoa]] <small>Petersen, 1979</small>
** class [[Cubozoa]] <small>Werner, 1973</small> — box jellies
** class [[Hydrozoa]] <small>Owen, 1843</small> — hydrozoans (fire corals, hydroids, hydroid jellyfishes, siphonophores...)
** class [[Scyphozoa]] <small>Goette, 1887</small> — "true" jellyfishes
** class [[Staurozoa]] <small>Marques & Collins, 2004</small> — stalked jellyfishes

<gallery style="text-align:center;" mode="packed">
Image:Cerianthus filiformis.jpg|''[[Cerianthus filiformis]]'' ([[Ceriantharia]])
Image:Haeckel Actiniae.jpg|Sea anemones ([[Actiniaria]], part of [[Hexacorallia]])
Image:Hertshoon.jpg|Coral ''[[Acropora muricata]]'' ([[Scleractinia]], part of [[Hexacorallia]])
Image:Gorgonia ventalina, Bahamas.jpg|Sea fan ''[[Gorgonia ventalina]]'' ([[Octocorallia]], part of [[Octocorallia]])
Image:Carybdea branchi9.jpg|Box jellyfish ''[[Carybdea branchi]]'' ([[Cubozoa]])
Image:Portuguese Man-O-War (Physalia physalis).jpg|Siphonophore ''[[Physalia physalis]]'' ([[Hydrozoa]])
Image:Fdl17-9-grey.jpg|''[[Myxobolus cerebralis]]'' ([[Myxozoa]])
Image:Polypodium hydriforme.jpg|''[[Polypodium hydriforme]]'' ([[Polypodiozoa]])
Image:Phyllorhiza punctata macro II.jpg|Jellyfish ''[[Phyllorhiza punctata]]'' ([[Scyphozoa]])
Image:Haliclystus antarcticus 1B.jpg|Stalked jelly ''[[Haliclystus antarcticus]]'' ([[Staurozoa]])
</gallery>

==Ecology==
Many cnidarians are limited to shallow waters because they depend on [[endosymbiotic]] [[algae]] for much of their nutrients. The life cycles of most have polyp stages, which are limited to locations that offer stable substrates. Nevertheless, major cnidarian groups contain species that have escaped these limitations. [[Hydrozoan]]s have a worldwide range: some, such as ''[[Hydra (genus)|Hydra]]'', live in freshwater; ''[[Obelia]]'' appears in the coastal waters of all the oceans; and ''[[Liriope (cnidarian)|Liriope]]'' can form large shoals near the surface in mid-ocean. Among [[anthozoa]]ns, a few [[scleractinia]]n [[coral]]s, [[sea pen]]s and [[gorgonia|sea fan]]s live in deep, cold waters, and some sea anemones inhabit polar seabeds while others live near [[hydrothermal vent]]s over {{convert|10|km|ft|abbr=on}} below sea-level. [[Reef]]-building corals are limited to tropical seas between 30°N and 30°S with a maximum depth of {{convert|46|m|ft|abbr=on}}, temperatures between {{convert|20|and|28|C}}, high [[salinity]], and low [[carbon dioxide]] levels. [[Stauromedusae]], although usually classified as jellyfish, are stalked, [[Sessility (zoology)|sessile]] animals that live in cool to [[Arctic]] waters.<ref name="Shostak">{{Cite book| author=Shostak, S.| contribution=Cnidaria (Coelenterates)| title=Encyclopedia of Life Sciences| publisher= John Wiley & Sons| year=2006| doi=10.1038/npg.els.0004117| isbn=978-0-470-01617-6}}</ref> Cnidarians range in size from a mere handful of cells for the parasitic myxozoans<ref name="haaretz"/> through ''Hydra'''s length of {{convert|5|-|20|mm|in|frac=8|abbr=on}},<ref>{{cite book| title=Small-scale Freshwater Toxicity Investigations: Toxicity Test Methods|author1=Blaise, C. |author2=Férard, J-F.| publisher=Springer| year=2005|isbn=978-1-4020-3119-9| page=398| url=https://books.google.com/books?id=Ibew5SLx2oMC&q=hydra+size+length| access-date=2008-11-21}}</ref> to the [[lion's mane jellyfish]], which may exceed {{convert|2|m|ftin|abbr=on}} in diameter and {{convert|75|m|ft|abbr=on}} in length.<ref name="Safina" />

Prey of cnidarians ranges from plankton to animals several times larger than themselves.<ref name="Shostak" /><ref>{{cite book| author=Cowen, R.| title=History of Life| publisher=Blackwell| year=2000| edition=3| isbn=978-0-632-04444-3| page=54| url=https://books.google.com/books?id=qvyBS4gwPF4C&q=cnidaria+prey&pg=PA54| access-date=2008-11-21}}</ref> Some cnidarians are [[parasite]]s, mainly on jellyfish but a few are major pests of fish.<ref name="Shostak" /> Others obtain most of their nourishment from endosymbiotic algae or dissolved nutrients.<ref name="Hinde2001" /> Predators of cnidarians include: [[nudibranch|sea slugs]], [[flatworm]]s and [[Ctenophora|comb jellies]], which can incorporate [[nematocyst]]s into their own bodies for self-defense (nematocysts used by cnidarian predators are referred to as kleptocnidae);<ref>{{cite journal | url=https://www.sciencedirect.com/science/article/abs/pii/S0171933518300268 | doi=10.1016/j.ejcb.2018.04.002 | title=Organelle survival in a foreign organism: Hydra nematocysts in the flatworm Microstomum lineare | year=2018 | last1=Krohne | first1=Georg | journal=European Journal of Cell Biology | volume=97 | issue=4 | pages=289–299 | pmid=29661512 | url-access=subscription }}</ref><ref>{{cite journal | pmc=2783962 | year=2009 | last1=Greenwood | first1=P. G. | title=Acquisition and Use of Nematocysts by Cnidarian Predators | journal=Toxicon | volume=54 | issue=8 | pages=1065–1070 | doi=10.1016/j.toxicon.2009.02.029 | pmid=19269306 | bibcode=2009Txcn...54.1065G }}</ref><ref>{{cite journal|author=Frick, K|title=Predator Suites and Flabellinid Nudibranch Nematocyst Complements in the Gulf of Maine|journal=In: SF Norton (Ed). Diving for Science...2003.|volume=Proceedings of the American Academy of Underwater Sciences|issue=22nd Annual Scientific Diving Symposium|year=2003|url=http://archive.rubicon-foundation.org/4744|access-date=2008-07-03|archive-date=2011-01-06|archive-url=https://web.archive.org/web/20110106062453/http://archive.rubicon-foundation.org/4744|url-status=usurped}}</ref> [[starfish]], notably the [[crown of thorns starfish]], which can devastate corals;<ref name="Shostak" /> [[butterfly fish]] and [[parrot fish]], which eat corals;<ref>{{cite book|editor=Paxton, J.R. |editor2=Eschmeyer, W.N. |author1=Choat, J.H. |author2=Bellwood, D.R.|year=1998|title=Encyclopedia of Fishes|publisher= Academic Press|location=San Diego|pages= 209–211|isbn= 978-0-12-547665-2}}</ref> and marine [[turtle]]s, which eat jellyfish.<ref name="Safina">{{cite book| author=Safina, C.| title=Voyage of the Turtle: In Pursuit of the Earth's Last Dinosaur| publisher=Macmillan| year=2007| isbn=978-0-8050-8318-7| page=154| url=https://books.google.com/books?id=dQD883dAv6YC&q=cnidaria+turtle&pg=PA154| access-date=2008-11-21}}</ref> Some sea anemones and jellyfish have a [[symbiotic]] relationship with some fish; for example [[clownfish]] live among the tentacles of sea anemones, and each partner protects the other against predators.<ref name="Shostak" />

[[Coral reef]]s form some of the world's most productive ecosystems. Common coral reef cnidarians include both anthozoans (hard corals, octocorals, anemones) and hydrozoans (fire corals, lace corals). The endosymbiotic algae of many cnidarian species are very effective [[primary productivity|primary producers]], in other words converters of [[inorganic]] chemicals into [[Organic compound|organic]] ones that other organisms can use, and their coral hosts use these organic chemicals very efficiently. In addition, reefs provide complex and varied habitats that support a wide range of other organisms.<ref name="Barnes">{{cite book| title=Fundamentals of Aquatic Ecology|author1=Barnes, R.S.K. |author2=Mann, K.H.| publisher=Blackwell Publishing| year=1991| isbn=978-0-632-02983-9| pages=217–227| url=https://books.google.com/books?id=mOZZlzgdTrwC&q=%22Coral+Reef%22+productivity&pg=PA227| access-date=2008-11-26}}</ref> [[Fringing reef]]s just below low-[[tide]] level also have a mutually beneficial relationship with [[mangrove]] forests at high-tide level and [[seagrass meadow]]s in between: the reefs protect the mangroves and seagrass from strong currents and waves that would damage them or [[erosion|erode]] the sediments in which they are rooted, while the mangroves and seagrass protect the coral from large influxes of [[silt]], fresh water and [[pollution|pollutants]]. This additional level of variety in the environment is beneficial to many types of coral reef animals, which for example may feed in the sea grass and use the reefs for protection or breeding.<ref>{{cite book|author=Hatcher, B.G. |author2=Johannes, R.E. |author3=Robertson, A.J. |name-list-style=amp| chapter=Conservation of Shallow-water Marine Ecosystems| title=Oceanography and Marine Biology: An Annual Review: Volume 27| publisher=Routledge| year=1989| isbn=978-0-08-037718-6| chapter-url=https://books.google.com/books?id=XpmNqFaDZ7cC&q=%22Coral+Reef%22+mangrove+%22seagrass%22&pg=PA320| access-date=2008-11-21|page=320}}</ref>

==Evolutionary history==
[[File:Stranded Cambrian scyphozoans.jpg|right|thumb|Stranded scyphozoans on a [[Cambrian]] tidal flat in [[Blackberry Hill]], Wisconsin.]]
[[File:Cladocora.jpg|right|thumb|The fossil coral ''[[Cladocora]]'' from [[Pliocene]] rocks in [[Cyprus]]]]

===Fossil record===
The earliest widely accepted animal fossils are rather modern-looking cnidarians, possibly from around {{ma|580}}, although fossils from the [[Doushantuo Formation]] can only be dated approximately.<ref>{{cite journal|last1=Chen|first1=J-Y. |date=25 April 2000 |title=Putative phosphatized embryos from the Doushantuo Formation of China| journal=Proceedings of the National Academy of Sciences |volume=97|pages=4457–4462|doi=10.1073/pnas.97.9.4457|pmid=10781044|pmc=18256|issue=9|last2=Oliveri|first2=P|last3=Li|first3=CW|last4=Zhou|first4=GQ|last5=Gao|first5=F|last6=Hagadorn|first6=JW|last7=Peterson|first7=KJ|last8=Davidson|first8=EH|display-authors=3|bibcode = 2000PNAS...97.4457C |doi-access=free}}</ref> The identification of some of these as embryos of animals has been contested, but other fossils from these rocks strongly resemble tubes and other [[biomineralization|mineralized]] structures made by [[coral]]s.<ref>{{cite journal|author1=Xiao, S. |author2=Yuan, X. |author3=Knoll, A.H. |name-list-style=amp | date=5 December 2000| title=Eumetazoan fossils in terminal Proterozoic phosphorites?| journal=Proceedings of the National Academy of Sciences| volume=97| pages=13684–13689| doi=10.1073/pnas.250491697| pmid=11095754| issue=25| pmc=17636|bibcode = 2000PNAS...9713684X |doi-access=free}}</ref> Their presence implies that the cnidarian and [[bilateria]]n lineages had already diverged.<ref>{{cite journal |author=Chen, J.-Y. |author2=Oliveri, P. |author3=Gao, F. |author4=Dornbos, S.Q. |author5=Li, C-W. |author6=Bottjer, D.J. |author7=Davidson, E.H. |display-authors=3|name-list-style=amp |title=Precambrian Animal Life: Probable Developmental and Adult Cnidarian Forms from Southwest China |journal=Developmental Biology |volume=248 |issue=1 |date=August 2002 |doi=10.1006/dbio.2002.0714 |url=http://www.uwm.edu/~sdornbos/PDF's/Chen%20et%20al.%202002.pdf |access-date=2008-09-03 |pages=182–196 |pmid=12142030 |archive-url=https://web.archive.org/web/20080911075353/http://www.uwm.edu/~sdornbos/PDF%27s/Chen%20et%20al.%202002.pdf |archive-date=2008-09-11 }}</ref> Although the Ediacaran fossil ''[[Charnia]]'' used to be classified as a [[jellyfish]] or [[sea pen]],<ref>{{cite journal|author1=Donovan, Stephen K. |author2=Lewis, David N. | year = 2001| title = Fossils explained 35. The Ediacaran biota| journal = Geology Today| volume =17| issue =3| pages =115–120| doi =10.1046/j.0266-6979.2001.00285.x|s2cid=128395097 | type = abstract}}</ref> more recent study of growth patterns in ''Charnia'' and modern cnidarians has cast doubt on this hypothesis,<ref>{{cite journal| author = Antcliffe, J.B.| year = 2007| title = Charnia and sea pens are poles apart| journal = Journal of the Geological Society| volume = 164| issue = 1| pages = 49–51| doi = 10.1144/0016-76492006-080| last2 = Brasier| first2 = M. D.| bibcode = 2007JGSoc.164...49A| s2cid = 130602154}}</ref><ref>{{cite journal| author = Antcliffe, J.B.| last2 = Brasier| year = 2007| first2 = Martin D.| title = Charnia At 50: Developmental Models For Ediacaran Fronds| journal = Palaeontology| volume = 51| issue = 1| pages = 11–26| doi = 10.1111/j.1475-4983.2007.00738.x| s2cid = 83486435| doi-access = free}}</ref> leaving the Canadian polyp ''[[Haootia]]'' and the British ''[[Auroralumina]]'' as the only recognized cnidarian body fossils from the Ediacaran. ''Auroralumina'' is the earliest known animal [[predation|predator]].<ref>{{cite web |url=https://www.bbc.co.uk/news/science-environment-62291954 |title=Ancient fossil is earliest known animal predator |last=Amos |first=Jonathan |date=25 July 2022 |website=bbc.co.uk |publisher=[[BBC News]] |access-date=25 July 2022}}</ref> Few fossils of cnidarians without mineralized [[skeleton]]s are known from more recent rocks, except in [[Lagerstätte]]n that preserved soft-bodied animals.<ref name="UCMP">{{cite web| url=http://www.ucmp.berkeley.edu/cnidaria/cnidariafr.html| access-date=2008-11-27| title=Cnidaria: Fossil Record| publisher=University of California Museum of Paleontology}}</ref>

A few mineralized fossils that resemble [[coral]]s have been found in rocks from the [[Cambrian]] period, and corals diversified in the Early [[Ordovician]].<ref name="UCMP" /> These corals, which were wiped out in the [[Permian–Triassic extinction event]] about {{ma|252}},<ref name="UCMP" /> did not dominate reef construction since [[sponge]]s and [[alga]]e also played a major part.<ref>{{cite journal| author=Copper, P.| title=Ancient reef ecosystem expansion and collapse| journal=Coral Reefs| volume=13| issue=1|date=January 1994| pages=3–11| doi=10.1007/BF00426428| bibcode=1994CorRe..13....3C| s2cid=42938715}}</ref> During the [[Mesozoic]] era, [[rudist]] bivalves were the main reef-builders, but they were wiped out in the [[Cretaceous–Paleogene extinction event]] {{ma|66}},<ref>{{cite web| url=http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/rudists.php| access-date=2008-11-27| title=The Rudists| publisher=University of California Museum of Paleontology}}</ref> and since then the main reef-builders have been [[scleractinia]]n corals.<ref name="UCMP" />

[[Hydroconozoa]] is an extinct class of cnidarians, established by K.B. Korde in 1964 based on Lower Cambrian fossils from Tuva, USSR.  These conical and cylindrical organisms, including genera like [[Hydroconus]] and [[Tuvaeconus]], possessed external skeletons with features resembling both [[scyphozoa]]ns and [[Tetracoralla|tetracorals]].  Their unique skeletal structures suggest a distinct lineage within early cnidarian evolution.<ref>{{cite journal |last=Korde |first=K.B. |title=Hydroconoz oa – A New Class of Coelenterates |journal=International Geology Review |volume=6 |issue=12 |pages=2229–2234 |year=1964 |doi=10.1080/00206816409474709|bibcode=1964IGRv....6.2229K }}</ref>

===Phylogeny===

It is difficult to reconstruct the early stages in the [[evolution]]ary "family tree" of animals using only [[morphology (biology)|morphology]] (their shapes and structures) because of the large differences between the major groups of animals. Hence, reconstructions now rely almost entirely on [[molecular phylogenetics]], which groups organisms based on their [[biochemistry]], most commonly by analyzing [[DNA]] or [[RNA]] sequences.<ref>{{cite journal| author=Halanych, K.M.| title=The New View of Animal Phylogeny| journal=Annual Review of Ecology, Evolution, and Systematics| volume=35| pages=229–256| date=December 2004| doi=10.1146/annurev.ecolsys.35.112202.130124| url=http://gump.auburn.edu/halanych/lab/Pub.pdfs/Halanych2004.pdf| access-date=2008-11-27| archive-url=https://web.archive.org/web/20081007035514/http://gump.auburn.edu/halanych/lab/Pub.pdfs/Halanych2004.pdf| archive-date=2008-10-07}}</ref>

In 1866, it was proposed that Cnidaria and Ctenophora were more closely related to each other than to Bilateria and formed a group called [[Coelenterata]] ("hollow guts") because both rely on the flow of water in and out of a single cavity for feeding, excretion and respiration. In 1881, it was proposed that Ctenophora and Bilateria were more closely related to each other, since they shared features that Cnidaria lack, such as a middle layer of cells ([[mesoglea]] in Ctenophora, [[mesoderm]] in Bilateria) between the outer and inner layer found in other animals. However, more recent analyses indicate that these similarities were evolved independently in both lineages, instead of being present in their common ancestor. The current view is that Cnidaria and Bilateria are more closely related to each other than either is to Ctenophora. This grouping of Cnidaria and Bilateria has been labelled "[[Planulozoa]]",{{efn|This group also probably contains [[Placozoa]]}} named so because the earliest Bilateria were probably similar to the [[planula]] larvae of Cnidaria.<ref name="Collins">{{cite journal| author=Collins, A.G.| date=May 2002| title=Phylogeny of Medusozoa and the Evolution of Cnidarian Life Cycles| journal=Journal of Evolutionary Biology| volume=15| issue=3| pages=418–432| doi=10.1046/j.1420-9101.2002.00403.x| s2cid=11108911| doi-access=free}}</ref><ref>{{cite journal| author=Wallberg, A. | author2=Thollesson, M. | author3=Farris, J.S. | author4=Jondelius, U. |name-list-style=amp| title=The phylogenetic position of the comb jellies (Ctenophora) and the importance of taxonomic sampling| journal=Cladistics| volume=20| year=2004| pages=558–578| doi=10.1111/j.1096-0031.2004.00041.x| issue=6| pmid=34892961 |s2cid=86185156| doi-access=free}}</ref>

In 2005, Katja Seipel and Volker Schmid suggested that cnidarians and ctenophores are simplified descendants of [[triploblastic]] animals, since ctenophores and the medusa stage of some cnidarians have [[striated muscle]], which in bilaterians arises from the [[mesoderm]]. They did not commit themselves on whether bilaterians evolved from early cnidarians or from the hypothesized triploblastic ancestors of cnidarians.<ref name="Seipel" />

Resolving the evolutionary relationships within Cnidaria has also been challenging, with almost every possible combination of clades being proposed. As time went on though, a semi-consensus has started to emerge. The enigmatic ''[[Polypodium hydriforme]]'' and subphylum [[Myxozoa]] have been firmly placed within the Cnidaria and have been shown to be closely related to the [[Medusozoa]]. In addition, these two groups have been found to likely be each other's closest relatives which, if true, would form the clade "Endocnidozoa". The relationships within the Medusozoa are currently probably the most contentious part of the tree. Traditionally, the class [[Scyphozoa]] also included [[Staurozoa]] and [[Cubozoa]], but significant morphological differences eventually lead to the split of the three.<ref>Daly, Brugler, Cartwright, Collins, Dawson, Fautin, France, McFadden, Opresko, Rodriguez, Romano & Stake (2007). ''[http://www.mapress.com/zootaxa/2007f/zt01668p182.pdf The phylum Cnidaria: A review of phylogenetic patterns and diversity 300 years after Linnaeus.]'' Zootaxa 1668: 127–182</ref> The group containing them has since been named "Acraspeda". The relationships between these three and [[Hydrozoa]] have since and still are debated. A relationship between Scyphozoa and Cubozoa with Staurozoa as its sister has seen support in nearly all studies, but the position of the remaining class, Hydrozoa, is not understood. Several studies have found that Acraspeda is paraphyletic, with Hydrozoa being more closely related to Scyphozoa than to the other classes.<ref name="Kayal2013">{{cite journal |last1=Kayal |first1=Ehsan |last2=Roure |first2=Béatrice |last3=Philippe |first3=Hervé |last4=Collins |first4=Allen |last5=Lavrov |first5=Dennis |title=Cnidarian phylogenetic relationships as revealed by mitogenomics |journal=  BMC Evolutionary Biology|date=9 January 2013 |volume=13 |issue=1 |page=5 |doi=10.1186/1471-2148-13-5 |doi-access=free |pmid=23302374 |bibcode=2013BMCEE..13....5K |pmc=3598815 }}</ref><ref>{{cite journal |author1=Kayal, Ehsan |display-authors=etal |title=Evolution of Linear Mitochondrial Genomes in Medusozoan Cnidarians |journal=Oxford Academic |date=22 November 2011 |volume=4 |issue=1 |pages=1–12 |doi=10.1093/gbe/evr123|pmid=22113796 |pmc=3267393 }}</ref><ref>{{cite journal |last1=Kang Ling |first1=Min |title=Revisiting mitogenome evolution in Medusozoa with eight new mitochondrial genomes |journal=iScience |date=17 November 2023 |volume=26 |issue=11 |article-number=108252 |doi=10.1016/j.isci.2023.108252 |pmid=37965150 |bibcode=2023iSci...26j8252L |pmc=10641506 }}</ref> At the same time, other studies have recovered Acraspeda as being monophyletic.<ref name="Kayal18">{{cite journal |author1=Kayal, Ehsan |display-authors=etal |title=Phylogenomics provides a robust topology of the major cnidarian lineages and insights on the origins of key organismal traits |journal=  BMC Evolutionary Biology|date=13 April 2018 |volume=18 |issue=1 |article-number=68 |doi=10.1186/s12862-018-1142-0 |doi-access=free |bibcode=2018BMCEE..18...68K |pmc=5932825 }}</ref><ref name="Novosolov2022">{{cite journal |author1=Novosolov, Maria |display-authors=etal |title=The Phylogenetic Position of the Enigmatic, Polypodium hydriforme (Cnidaria, Polypodiozoa): Insights from Mitochondrial Genomes |journal=Oxford Academic |date=22 July 2022 |volume=14 |issue=8 |article-number=evac112 |doi=10.1093/gbe/evac112 |pmid=35867352 |pmc=9380995 |url=https://academic.oup.com/gbe/article/14/8/evac112/6648524#369282096|hdl=1808/33631 |hdl-access=free }}</ref> The subphylum [[Anthozoa]] is argued to have either two or three classes, but the relationships between them is not disputed; the [[tube-dwelling anemone]]s of the class Ceriantharia have consistently shown to be more closely related to the [[Hexacorallia]] than to the [[Octocorallia]].<ref name="Kayal18" /><ref>{{cite journal |author1=McFadden, Catherine |display-authors=etal |title=Phylogenomics, Origin, and Diversification of Anthozoans (Phylum Cnidaria) |journal=Systematic Biology |date=28 January 2021 |volume=70 |issue=4 |pages=635–647 |doi=10.1093/sysbio/syaa103 |pmid=33507310 |url=https://academic.oup.com/sysbio/article/70/4/635/6122449|url-access=subscription }}</ref><ref name="Novosolov2022" /><ref name="Kayal2013" />

{{clade
|label1='''Cnidaria'''
|1={{clade
   |label1=[[Anthozoa]]
   |1={{clade
      |label1=
      |1={{clade
         |label1=
         |1=[[Ceriantharia]] [[File:Cerianthus membranaceus Spallanzani, 1784.jpg|60px]]
         |2=[[Hexacorallia]] [[File:Brain coral.jpg|60px]]
         }}
      |2=[[Octocorallia]] [[File:Virgularia sp. (Purple sea pen).jpg|55px]]
      }}
   |2={{clade
      |label1=Endocnidozoa
      |1={{clade
         |label1=[[Myxozoa]]
         |1={{clade
            |label1=
            |1=[[Malacosporea]] [[File:T. bryosalmonae parasites in rainbow trout kidney. Tissue section stained with haematoxylin and eosin.jpg|60px]]
            |2=[[Myxosporea]] [[File:Fdl17-9-grey.jpg|55px]]
            }}
         |2=[[Polypodiozoa]] [[File:Polypodium hydriforme C.jpg|60px]]
         }}
      |label2=[[Medusozoa]]
      |2={{clade
         |label1=
         |state1=dashed
         |1=[[Hydrozoa]] [[File:Porpita_porpita.jpg|60px]]
         |2=[[Staurozoa]] [[File:Haliclystus octoradiatus - Becherqualle an Seegras.jpg|60px]]
         |3={{clade
            |label1=
            |1=[[Cubozoa]] [[File:Cubozoa.jpg|60px]]
            |2=[[Scyphozoa]] [[File:Jellyfish (4159680946).jpg|60px]]
            }}
         }}
      }}
   }}
}}

In molecular phylogenetics analyses from 2005 onwards, important groups of developmental genes show the same variety in cnidarians as in [[chordate]]s.<ref>{{cite journal|author1=Miller, D.J. |author2=Ball, E.E. |author3=Technau, U. |name-list-style=amp | title=Cnidarians and ancestral genetic complexity in the animal kingdom| journal=Trends in Genetics| volume=21| issue=10|date=October 2005| pages=536–539| doi=10.1016/j.tig.2005.08.002| pmid=16098631}}</ref> In fact cnidarians, and especially [[anthozoa]]ns (sea anemones and corals), retain some genes that are present in [[bacteria]], [[protist]]s, [[plant]]s and [[Fungus|fungi]] but not in bilaterians.<ref>{{cite journal|author1=Technau, U. |author2=Rudd, S. |author3=Maxwell, P |name-list-style=amp | title=Maintenance of ancestral complexity and non-metazoan genes in two basal cnidarians| journal=Trends in Genetics| volume=21| issue=12|date=December 2005| pages=633–639| doi=10.1016/j.tig.2005.09.007| pmid=16226338}}</ref>

==Interaction with humans==
[[File:Cubozoas.JPG|thumb|The dangerous ''[[Carukia barnesi]]'', one of the known species of [[box jellyfish]] which can cause [[Irukandji syndrome]].]]

Jellyfish stings killed about 1,500 people in the 20th century,<ref>{{cite book|author1=Williamson, J.A. |author2=Fenner, P.J. |author3=Burnett, J.W. |author4=Rifkin, J. |name-list-style=amp | title=Venomous and Poisonous Marine Animals: A Medical and Biological Handbook| publisher=UNSW Press| year=1996| isbn=978-0-86840-279-6| pages=65–68| url=https://books.google.com/books?id=YsZ3GryFIzEC&q=mollusc+venom+fatal&pg=PA75| access-date=2008-10-03}}</ref> and cubozoans are particularly dangerous. On the other hand, some large jellyfish are considered a [[delicacy]] in [[East Asia|East]] and [[Southeast Asia]]. [[Coral reef]]s have long been economically important as providers of fishing grounds, protectors of shore buildings against currents and tides, and more recently as centers of tourism. However, they are vulnerable to over-fishing, mining for construction materials, [[pollution]], and damage caused by tourism.

{{annotated image| float=left| caption=The dangerous "sea wasp" ''[[Chironex fleckeri]]''
| image=Avispa marina.jpg| width=130| image-width=150| height=160| image-top=-90
| annotations=
}}

Beaches protected from tides and storms by coral reefs are often the best places for housing in tropical countries. Reefs are an important food source for low-technology fishing, both on the reefs themselves and in the adjacent seas.<ref name="Clark">{{cite book| title=Coastal Seas: The Conservation Challenge| author=Clark, J.R.| publisher=Blackwell| year=1998| isbn=978-0-632-04955-4| pages=[https://archive.org/details/coastalseasconse0000clar/page/8 8]–9| url=https://archive.org/details/coastalseasconse0000clar| url-access=registration| quote=Coral Reef productivity.| access-date=2008-11-28}}</ref> However, despite their great [[primary production|productivity]], reefs are vulnerable to over-fishing, because much of the [[organic carbon]] they produce is exhaled as [[carbon dioxide]] by organisms at the middle levels of the [[food chain]] and never reaches the larger species that are of interest to fishermen.<ref name="Barnes" /> Tourism centered on reefs provides much of the income of some tropical islands, attracting photographers, divers and sports fishermen. However, human activities damage reefs in several ways: mining for construction materials; [[pollution]], including large influxes of fresh water from [[storm drain]]s; commercial fishing, including the use of [[dynamite]] to stun fish and the capture of young fish for [[aquarium]]s; and tourist damage caused by boat anchors and the cumulative effect of walking on the reefs.<ref name="Clark" /> Coral, mainly from the [[Pacific Ocean]] has long been used in [[jewellery]], and demand rose sharply in the 1980s.<ref>{{cite book| title=Marine Minerals in Exclusive Economic Zones| author=Cronan, D.S.| year=1991| publisher=Springer| pages=63–65| isbn=978-0-412-29270-5| url=https://books.google.com/books?id=4g4nhd8USO8C&q=coral+jewellery&pg=PA63| access-date=2008-11-28}}</ref>

Some large [[jellyfish]] species of the [[Rhizostomeae]] order are commonly consumed in [[Japan]], [[Korea]] and Southeast Asia.<ref>{{Cite journal|author1=Kitamura, M. |author2=Omori, M.| title = Synopsis of edible jellyfishes collected from Southeast Asia, with notes on jellyfish fisheries| journal = Plankton and Benthos Research| volume = 5| issue = 3| pages = 106–118| year = 2010| issn = 1880-8247| doi=10.3800/pbr.5.106| doi-access = free|bibcode=2010PBenR...5..106K }}</ref><ref>{{Cite journal|author1=Omori, M. |author2=Kitamura, M.| title = Taxonomic review of three Japanese species of edible jellyfish (Scyphozoa: Rhizostomeae)| journal = Plankton Biol. Ecol.| volume = 51| issue = 1| pages = 36–51| year = 2004}}</ref><ref name=Omori2001/> In parts of the range, fishing industry is restricted to daylight hours and calm conditions in two short seasons, from March to May and August to November.<ref name=Omori2001>{{cite journal | doi = 10.1023/A:1011879821323 | last1 = Omori | first1 = M. | last2 = Nakano | first2 = E. | date = May 2001 | title = Jellyfish fisheries in southeast Asia | journal = Hydrobiologia | volume = 451 | issue = 1–3 | pages = 19–26 | bibcode = 2001HyBio.451...19O | s2cid = 6518460 }}</ref> The commercial value of jellyfish food products depends on the skill with which they are prepared, and "Jellyfish Masters" guard their [[trade secret]]s carefully. Jellyfish is very low in [[cholesterol]] and [[sugar]]s, but cheap preparation can introduce undesirable amounts of [[Heavy metal (chemistry)|heavy metal]]s.<ref>{{Cite journal|author1=Y-H. Peggy Hsieh |author2=Fui-Ming Leong |author3=Jack Rudloe | title = Jellyfish as food| journal = Hydrobiologia| volume = 451| issue = 1–3| pages = 11–17| date = May 2001| doi = 10.1023/A:1011875720415|bibcode=2001HyBio.451...11P |s2cid=20719121 }}</ref>

The "sea wasp" ''[[Chironex fleckeri]]'' has been described as the world's most venomous jellyfish and is held responsible for 67&nbsp;deaths, although it is difficult to identify the animal as it is almost transparent. Most stingings by ''C. fleckeri'' cause only mild symptoms.<ref>{{cite book| title=Greenberg's Text-atlas of Emergency Medicine
|author1=Greenberg, M.I. |author2=Hendrickson, R.G. |author3=Silverberg, M. |author4=Campbell, C. |author5=Morocco, A. |display-authors=3|name-list-style=amp | publisher=Lippincott Williams & Wilkins| year=2004|isbn=978-0-7817-4586-4| page=875| chapter=Box Jellyfish Envenomation}}</ref> Seven other [[Cubozoa|box jellies]] can cause a set of symptoms called [[Irukandji syndrome]],<ref name="Little">{{cite journal|author1=Little, M. |author2=Pereira, P. |author3=Carrette, T. |author4=Seymour, J. |name-list-style=amp | title = Jellyfish Responsible for Irukandji Syndrome| journal = QJM| volume=99| issue = 6| pages = 425–427| date = June 2006| doi = 10.1093/qjmed/hcl057| pmid = 16687419| doi-access = free}}</ref> which takes about 30&nbsp;minutes to develop,<ref>{{cite journal| author = Barnes, J.| title = Cause and effect in Irukandji stingings| journal = Medical Journal of Australia| volume = 1| issue = 24| pages = 897–904| year = 1964| pmid = 14172390| doi=10.5694/j.1326-5377.1964.tb114424.x}}</ref> and from a few hours to two&nbsp;weeks to disappear.<ref>{{cite journal|vauthors=Grady J, Burnett J | title = Irukandji-like syndrome in South Florida divers| journal = Annals of Emergency Medicine| volume = 42| issue = 6| pages = 763–6| date = December 2003| pmid = 14634600| doi = 10.1016/S0196-0644(03)00513-4}}</ref> Hospital treatment is usually required, and there have been a few deaths.<ref name="Little" />

A number of the parasitic [[myxozoa]]ns are commercially important pathogens in [[salmonid]] aquaculture.<ref>{{Cite journal |last1=Braden |first1=Laura M. |last2=Rasmussen |first2=Karina J. |last3=Purcell |first3=Sara L. |last4=Ellis |first4=Lauren |last5=Mahony |first5=Amelia |last6=Cho |first6=Steven |last7=Whyte |first7=Shona K. |last8=Jones |first8=Simon R. M. |last9=Fast |first9=Mark D. |display-authors=3|date=December 19, 2017 |editor-last=Appleton |editor-first=Judith A. |title=Acquired Protective Immunity in Atlantic Salmon Salmo salar against the Myxozoan Kudoa thyrsites Involves Induction of MHIIβ + CD83 + Antigen-Presenting Cells |journal=Infection and Immunity |language=en |volume=86 |issue=1 |pages=e00556–17 |doi=10.1128/IAI.00556-17 |issn=0019-9567 |pmc=5736826 |pmid=28993459}}</ref> A [[Scyphozoa]] species – ''[[Pelagia noctiluca]]'' – and a [[Hydrozoa]] – ''[[Muggiaea atlantica]]'' – have caused repeated mass mortality in [[salmon farm]]s over the years around [[Ireland]].<ref name="IrishTimes">{{cite web | last=O'Sullivan | first=Kevin | title=Stinger jellyfish swarms wipe out farmed salmon in west of Ireland | website=The [[Irish Times]] | date=2017-10-06 | url=http://www.irishtimes.com/news/environment/stinger-jellyfish-swarms-wipe-out-farmed-salmon-in-west-of-ireland-1.3247245 | access-date=2022-04-23}}</ref> A loss valued at £1 million struck in November 2007, 20,000 died off [[Clare Island]] in 2013 and four fish farms collectively lost tens of thousands of salmon in September 2017.<ref name="IrishTimes" />
{{clear}}

==Notes==
{{notelist}}

==References==
{{reflist|colwidth=30em}}

==Further reading==

===Books===
*Arai, M.N. (1997). ''A Functional Biology of Scyphozoa.'' London: Chapman & Hall [p.&nbsp;316]. {{ISBN|0-412-45110-7}}.
*Ax, P. (1999). ''Das System der Metazoa I. Ein Lehrbuch der phylogenetischen Systematik.'' Gustav Fischer, Stuttgart-Jena: Gustav Fischer. {{ISBN|3-437-30803-3}}.
*Barnes, R.S.K., P. Calow, P. J. W. Olive, D. W. Golding & J. I. Spicer (2001). ''The invertebrates—a synthesis.'' Oxford: Blackwell. 3rd edition [chapter 3.4.2, p.&nbsp;54]. {{ISBN|0-632-04761-5}}.
*Brusca, R.C., G.J. Brusca (2003). ''Invertebrates.'' Sunderland, Mass.: Sinauer Associates. 2nd edition [chapter 8, p.&nbsp;219]. {{ISBN|0-87893-097-3}}.
*[[Andrew Dalby|Dalby, A.]] (2003). ''Food in the Ancient World: from A to Z.'' London: Routledge.
*Moore, J.(2001). ''An Introduction to the Invertebrates.'' Cambridge: Cambridge University Press [chapter 4, p.&nbsp;30]. {{ISBN|0-521-77914-6}}.
*Schäfer, W. (1997). Cnidaria, Nesseltiere. In Rieger, W. (ed.) ''Spezielle Zoologie. Teil 1. Einzeller und Wirbellose Tiere.'' Stuttgart-Jena: Gustav Fischer. Spektrum Akademischer Verl., Heidelberg, 2004. {{ISBN|3-8274-1482-2}}.
*Werner, B. 4. Stamm Cnidaria. In: V. Gruner (ed.) ''Lehrbuch der speziellen Zoologie.'' Begr. von Kaestner. 2 Bde. Stuttgart-Jena: Gustav Fischer, Stuttgart-Jena. 1954, 1980, 1984, Spektrum Akad. Verl., Heidelberg-Berlin, 1993. 5th edition. {{ISBN|3-334-60474-8}}.
<!-- The following are now cited inline ****
*Anderson, D.T. (2001). ''Invertebrate Zoology.'' Oxford: Oxford University Press. 2nd edition [chapter 3, p.31]. ISBN 0-19-551368-1.
*Ruppert, E.E., R.S. Fox & R.P. Barnes (2004). ''Invertebrate Zoology—a Functional Evolutionary Approach.'' Belmont: Brooks-Cole [chapter 7, p.111]. ISBN 0-03-025982-7.
****** -->

===Journal articles===
* D. Bridge, B. Schierwater, C. W. Cunningham, R. DeSalle R, L. W. Buss: ''Mitochondrial DNA structure and the molecular phylogeny of recent cnidaria classes.'' in: ''Proceedings of the Academy of Natural Sciences of Philadelphia.'' Philadelphia USA 89.1992, p.&nbsp;8750. {{ISSN|0097-3157}}
* D. Bridge, C. W. Cunningham, R. DeSalle, L. W. Buss: ''Class-level relationships in the phylum Cnidaria—Molecular and morphological evidence.'' in: ''Molecular biology and evolution.'' Oxford University Press, Oxford 12.1995, p.&nbsp;679. {{ISSN|0737-4038}}
* D. G. Fautin: ''[http://www.nrcresearchpress.com/doi/abs/10.1139/z02-133 Reproduction of Cnidaria]{{closed access}}.'' in: ''Canadian Journal of Zoology.'' Ottawa Ont. 80.2002, p.&nbsp;1735. ([[PDF]], online) {{ISSN|0008-4301}}
* G. O. Mackie: ''[http://www.nrcresearchpress.com/doi/abs/10.1139/z02-138 What's new in cnidarian biology?] {{Webarchive|url=https://web.archive.org/web/20190218062823/http://www.nrcresearchpress.com/doi/abs/10.1139/z02-138 |date=2019-02-18 }}{{closed access}}'' in: ''Canadian Journal of Zoology.'' Ottawa Ont. 80.2002, p.&nbsp;1649. (PDF, online) {{ISSN|0008-4301}}
* P. Schuchert: ''Phylogenetic analysis of the Cnidaria.'' in: ''Zeitschrift für zoologische Systematik und Evolutionsforschung.'' Paray, Hamburg-Berlin 31.1993, p.&nbsp;161. {{ISSN|0044-3808}}
* G. Kass-Simon, A. A. Scappaticci Jr.: ''[http://www.nrcresearchpress.com/doi/abs/10.1139/z02-135 The behavioral and developmental physiology of nematocysts.]{{closed access}}'' in: ''Canadian Journal of Zoology.'' Ottawa Ont. 80.2002, p.&nbsp;1772. (PDF, online) {{ISSN|0044-3808}}
* {{Cite journal
| author=J. Zrzavý| year=2001| journal=Folia Parasitologica| volume=48| issue=2| pages=81–103
| title=The interrelationships of metazoan parasites: a review of phylum- and higher-level hypotheses from recent morphological and molecular phylogenetic analyses
| pmid=11437135| doi=10.14411/fp.2001.013
| doi-access=free}}
<!-- The following are now cited inline ****
*{{cite journal | last1 = Marques | first1 = A. C. | last2 = Collins | first2 = A. G. | year = 2004 | title = Cladistic analysis of Medusozoa and cnidarian evolution | journal = Invertebrate Biology | volume = 123 | issue = 1| pages = 23–42 | doi=10.1111/j.1744-7410.2004.tb00139.x|doi-access=free}}
******** -->

==External links==
{{Wikispecies|Cnidaria}}
{{Wikibooks|Dichotomous Key|Cnidaria}}
{{Commons category|Cnidaria}}
{{Wiktionary|Cnidaria}}
*[https://www.youtube.com/watch?v=6zJiBc_N1Zk YouTube: Nematocysts Firing]
*[https://www.youtube.com/watch?v=WmvjGc2bojk YouTube:My Anemone Eat Meat] Defensive and feeding behaviour of sea anemone
* [https://web.archive.org/web/20120119003639/http://www.tafi.org.au/zooplankton/imagekey/cnidaria/index.html Cnidaria - Guide to the Marine Zooplankton of south eastern Australia], [https://web.archive.org/web/20080930064242/http://www.tafi.org.au/ Tasmanian Aquaculture & Fisheries Institute]
* [http://tolweb.org/tree?group=Cnidaria&contgroup=Animals Cnidaria page at ''Tree of Life'']
* [http://www.paleoportal.org/index.php?globalnav=fossil_gallery&sectionnav=search&taxon_id=55&state_id=&period_id=&assemblage_id=&last_section=search&p=0 Fossil Gallery: Cnidarians] {{Webarchive|url=https://web.archive.org/web/20071024144150/http://www.paleoportal.org/index.php?globalnav=fossil_gallery&sectionnav=search&taxon_id=55&state_id=&period_id=&assemblage_id=&last_section=search&p=0 |date=2007-10-24 }}
* [https://web.archive.org/web/20180908223624/http://hercules.kgs.ku.edu/Hexacoral/Anemone2/ Hexacorallians of the World]

{{Animalia}}
{{Cnidaria}}
{{Life on Earth}}

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[[Category:Cnidarians| ]]
[[Category:Articles containing video clips]]
[[Category:Animal phyla]]
[[Category:Freshwater animals]]
[[Category:Marine animals]]
[[Category:Ediacaran first appearances]]
[[Category:Taxa named by Berthold Hatschek]]