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Cnidaria
Cnidaria
(/naɪˈdɛəriə/[4]) is a phylum containing over 10,000[5] species of animals found exclusively in aquatic (freshwater and marine) environments: they are predominantly marine species. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick. They have two basic body forms: swimming medusae and sessile polyps, both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes. Both forms have a single orifice and body cavity that are used for digestion and respiration. Many cnidarian species produce colonies that are single organisms composed of medusa-like or polyp-like zooids, or both (hence they are trimorphic). Cnidarians' activities are coordinated by a decentralized nerve net and simple receptors. Several free-swimming species of Cubozoa
Cubozoa
and Scyphozoa
Scyphozoa
possess balance-sensing statocysts, and some have simple eyes. Not all cnidarians reproduce sexually, with many species having complex life cycles of asexual polyp stages and sexual medusae. Some, however, omit either the polyp or the medusa stage. Cnidarians
Cnidarians
were formerly grouped with ctenophores in the phylum Coelenterata, but increasing awareness of their differences caused them to be placed in separate phyla.[when?] Cnidarians
Cnidarians
are classified into four main groups: the almost wholly sessile Anthozoa
Anthozoa
(sea anemones, corals, sea pens); swimming Scyphozoa
Scyphozoa
(jellyfish); 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, and colonial swimmers, such as the Portuguese Man o' War. Staurozoa have recently been recognised as a class in their own right rather than a sub-group of Scyphozoa, and the parasitic Myxozoa
Myxozoa
and Polypodiozoa
Polypodiozoa
were only firmly recognized as cnidarians in 2007.[6] Most cnidarians prey on organisms ranging in size from plankton to animals several times larger than themselves, but many obtain much of their nutrition from dinoflagellates, and a few are parasites. Many are preyed on by other animals including starfish, sea slugs, fish, turtles, and even other cnidarians. Many scleractinian corals—which form the structural foundation for coral reefs—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 regions, and in freshwater. Recent phylogenetic analyses support monophyly of cnidarians, as well as the position of cnidarians as the sister group of bilaterians.[7] Fossil cnidarians have been found in rocks formed about 580 million years ago, and other fossils show that corals may have been present shortly before 490 million years ago and diversified a few million years later. However, molecular clock analysis of mitochondrial genes suggests a much older age for the crown group of cnidarians, estimated around 741 million years ago, almost 200 million years before the Cambrian
Cambrian
period as well as any fossils.[8]

Contents

1 Distinguishing features 2 Description

2.1 Basic body forms 2.2 Skeletons 2.3 Main cell layers 2.4 Polymorphism 2.5 Cnidocytes 2.6 Locomotion 2.7 Nervous system
Nervous system
and senses 2.8 Feeding and excretion 2.9 Respiration 2.10 Regeneration

3 Reproduction

3.1 Sexual 3.2 Asexual

4 Classification 5 Ecology 6 Evolutionary history

6.1 Fossil record 6.2 Family tree

7 Interaction with humans 8 Notes 9 Further reading

9.1 Books 9.2 Journal articles

10 External links

Distinguishing features[edit] Further information: Sponge, Ctenophore, and Bilateria Cnidarians
Cnidarians
form an animal phylum that are more complex than sponges, about as complex as ctenophores (comb jellies), and less complex than bilaterians, which include almost all other animals. However, both cnidarians and ctenophores are more complex than sponges as they have: cells bound by inter-cell connections and carpet-like basement membranes; muscles; nervous systems; and some have sensory organs. Cnidarians
Cnidarians
are distinguished from all other animals by having cnidocytes that fire like harpoons and are used mainly to capture prey. In some species, cnidocytes can also be used as anchors.[9] 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 animals have three main cell layers and no intermediate jelly-like layer. Hence, cnidarians and ctenophores have traditionally been labelled diploblastic, along with sponges.[9][10] However, both cnidarians and ctenophores have a type of muscle that, in more complex animals, arises from the middle cell layer.[11] As a result, some recent text books classify ctenophores as triploblastic,[12] and it has been suggested that cnidarians evolved from triploblastic ancestors.[11]

  Sponges[13][14] Cnidarians[9][10] Ctenophores[9][12] Bilateria[9]

Cnidocytes No Yes No

Colloblasts No Yes No

Digestive and circulatory organs No Yes

Number of main cell layers Two, with jelly-like layer between them Two[9] or Three[11][12] Three

Cells in each layer bound together cell-adhesion molecules, but no basement membranes except Homoscleromorpha.[15] inter-cell connections; basement membranes

Sensory organs No Yes

Number of cells in middle "jelly" layer Many Few (Not applicable)

Cells in outer layers can move inwards and change functions Yes No (Not applicable)

Nervous system No Yes, simple Simple to complex

Muscles None Mostly epitheliomuscular Mostly myoepithelial Mostly myocytes

Description[edit] Basic body forms[edit]

Aboral end Oral end Mouth Oral end Aboral end      Exoderm      Gastroderm (Endoderm)      Mesoglea      Digestive cavity

Medusa
Medusa
(left) and polyp (right)[10]

Oral end of actinodiscus polyp, with close-up of the mouth

Most adult cnidarians appear as either swimming medusae or sessile polyps, and many hydrozoan species are known to alternate between the two forms. Both are 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 cnidocytes around their edges, and medusae generally have an inner ring of tentacles around the mouth. Some hydroids may consist of colonies of zooids that serve different purposes, such as defense, 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.[10] Skeletons[edit] In medusae the only supporting structure is the mesoglea. Hydra and most sea anemones 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
Tubularia
use columns of water-filled cells for support. Sea pens stiffen the mesoglea with calcium carbonate spicules and tough fibrous proteins, rather like sponges.[10] In some colonial polyps, a chitinous periderm gives support and some protection to the connecting sections and to the lower parts of individual polyps. Stony corals secrete massive calcium carbonate exoskeletons. 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.[10] Main cell layers[edit] Cnidaria
Cnidaria
are diploblastic animals; in other words, they have two main cell layers, while more complex animals are triploblasts 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:[9]

Epitheliomuscular cells whose bodies form part of the epithelium but whose bases extend to form muscle fibers in parallel rows.[16] The fibers of the outward-facing cell layer generally run at right angles to the fibers of the inward-facing one. In Anthozoa
Anthozoa
(anemones, corals, etc.) and Scyphozoa
Scyphozoa
(jellyfish), the mesoglea also contains some muscle cells.[10] Cnidocytes, the harpoon-like "nettle cells" that give the phylum Cnidaria
Cnidaria
its name. These appear between or sometimes on top of the muscle cells.[9] Nerve
Nerve
cells. Sensory cells appear between or sometimes on top of the muscle cells,[9] and communicate via synapses (gaps across which chemical signals flow) with motor nerve cells, which lie mostly between the bases of the muscle cells.[10] 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.[9]

In addition to epitheliomuscular, nerve and interstitial cells, the inward-facing gastroderm ("stomach skin") contains gland cells that secrete digestive enzymes. In some species it also contains low concentrations of cnidocytes, which are used to subdue prey that is still struggling.[9][10] The mesoglea contains small numbers of amoeba-like cells,[10] and muscle cells in some species.[9] However, the number of middle-layer cells and types are much lower than in sponges.[10] Polymorphism[edit] 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 and medusa forms, or of zooids within colonial organisms like those in Hydrozoa.[17] In Hydrozoans, colonial individuals arising from individuals zooids will take on separate tasks.[18] For example, in Obelia
Obelia
there are feeding individuals, the gastrozooids; the individuals capable of asexual reproduction only, the gonozooids, blastostyles and free-living or sexually reproducing individuals, the medusae. Cnidocytes[edit] These "nettle cells" function as harpoons, since their payloads remain connected to the bodies of the cells by threads. Three types of cnidocytes are known:[9][10]

Firing sequence of the cnida in a hydra's nematocyst[10]      Operculum (lid)      "Finger" that turns inside out / / / Barbs      Venom      Victim's skin      Victim's tissues

Nematocysts inject venom into prey, and usually have barbs to keep them embedded in the victims. Most species have nematocysts.[9] Spirocysts do not penetrate the victim or inject venom, but entangle it by means of small sticky hairs on the thread. Ptychocysts are not used for prey capture — instead the threads of discharged ptychocysts are used for building protective tubes in which their owners live. Ptychocysts are found only in the order Ceriantharia, tube anemones.[10]

The main components of a cnidocyte are:[9][10]

A hydra's nematocyst, before firing.      "trigger" cilium[10]

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 adhesives or both. ("cnida" is derived from the Greek word κνίδη, which means "nettle"[19]) 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 (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:[9]

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
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 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 atmospheres, similar to that of scuba air tanks, and fully extending the thread in as little as 2 milliseconds (0.002 second).[10]

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 hours. To minimise wasteful firing, two types of stimulus are generally required to trigger cnidocytes: nearby 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.[9][10] Locomotion[edit]

Play media

A swimming sea nettle known as the purple-striped jelly (Chrysaora colorata)

Medusae
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.[10] Hydras and some sea anemones can move slowly over rocks and sea or stream beds by various means: creeping like snails, crawling like inchworms, or by somersaulting. A few can swim clumsily by waggling their bases.[10] Nervous system
Nervous system
and senses[edit] Cnidarians
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.[20] These sensory structures, usually called rhopalia, can generate signals in response to various types of stimuli such as light, pressure, and much more. Medusa
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
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.[21] 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
Cnidarians
have many of the same neurotransmitters as many animals, including chemicals such as glutamate, GABA, and acetylcholine.[22] 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.[20][21] Medusae
Medusae
and complex swimming colonies such as siphonophores and chondrophores sense tilt and acceleration by means of statocysts, 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
Jellyfish
are unique among Medusae
Medusae
because they possess four kinds of true eyes that have retinas, corneas and lenses.[23] Although the eyes probably do not form images, Cubozoa
Cubozoa
can clearly distinguish the direction from which light is coming as well as negotiate around solid-colored objects.[9][23] Feeding and excretion[edit] Cnidarians
Cnidarians
feed in several ways: predation, absorbing dissolved organic chemicals, 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 corals Hetroxenia and Leptogorgia, depend almost completely on their endosymbionts and on absorbing dissolved nutrients.[9] Cnidaria
Cnidaria
give their symbiotic algae carbon dioxide, some nutrients, a place in the sun and protection against predators.[10] Predatory species use their cnidocytes to poison or entangle prey, and those with venomous nematocysts may start digestion by injecting digestive enzymes. The "smell" of fluids from wounded prey makes the tentacles fold inwards and wipe the prey off into the mouth. In medusae the tentacles round 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. Medusae
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.[9] Their digestion is both intra and extracellular. Once the food is in the digestive cavity, gland cells in the 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.[10] Nutrients reach the outer cell layer by diffusion or, for animals or zooids such as medusae which have thick mesogleas, are transported by mobile cells in the mesoglea.[9] 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.[10] Respiration[edit] 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
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.[9] Cnidaria
Cnidaria
that carry photosynthetic symbionts may have the opposite problem, an excess of oxygen, which may prove toxic. The animals produce large quantities of antioxidants to neutralize the excess oxygen.[9] Regeneration[edit] All cnidarians can regenerate, allowing them to recover from injury and to reproduce asexually. Medusae
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.[9] Reproduction[edit]

 1   2   3   4   5   6   7   8   9   10   11   12   13   14 

Life cycle of a jellyfish:[9][10] 1–3 Larva
Larva
searches for site 4–8 Polyp
Polyp
grows 9–11 Polyp
Polyp
strobilates 12–14 Medusa
Medusa
grows

Sexual[edit] Cnidarian
Cnidarian
sexual reproduction often involves a complex life cycle with both polyp and medusa stages. For example, in Scyphozoa
Scyphozoa
(jellyfish) and Cubozoa
Cubozoa
(box jellies) a 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 adults have gonads in the gastroderm, and these release ova and sperm into the water in the breeding season.[9][10] 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)[24] 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
Hydrozoa
have a variety of life cycles. Some have no polyp stages and some (e.g. 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
Hydrozoa
(Turritopsis dohrnii[25] and Laodicea undulata[26]) and Scyphozoa
Scyphozoa
(Aurelia sp.1[27]). Anthozoa
Anthozoa
have no medusa stage at all and the polyps are responsible for sexual reproduction.[9] 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 phase of the moon. Many species of Cnidaria
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 — one famous example is the Great Barrier Reef, where at least 110 corals and a few non-cnidarian invertebrates produce enough gametes to turn the water cloudy. These mass spawnings may produce 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.[9] 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 bilaterians it forms at the other end (vegetal pole).[10] The larvae, called planulae, swim or crawl by means of cilia.[9] 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.[10] Anthozoan larvae either have large yolks 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.[10] Asexual[edit] All known cnidaria can reproduce asexually by various means, in addition to regenerating after being fragmented. Hydrozoan
Hydrozoan
polyps only bud, while the medusae of some hydrozoans can divide down the middle. Scyphozoan polyps can both bud and split down the middle. In addition to both of these methods, Anthozoa
Anthozoa
can split horizontally just above the base. Asexual reproduction
Asexual reproduction
makes the daughter cnidarian a clone of the adult.[9][10] Classification[edit] Cnidarians
Cnidarians
were for a long time grouped with Ctenophores 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 classes:[9] sessile Anthozoa
Anthozoa
(sea anemones, corals, sea pens); swimming Scyphozoa
Scyphozoa
(jellyfish) and Cubozoa
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 and colonial swimmers such as the Portuguese Man o' War. Staurozoa have recently been recognised as a class in their own right rather than a sub-group of Scyphozoa, and the parasitic Myxozoa
Myxozoa
and Polypodiozoa
Polypodiozoa
are now recognized as highly derived cnidarians rather than more closely related to the bilaterians.[6][28]

Hydrozoa Scyphozoa Cubozoa Anthozoa

Number of species[5] 3,600 228 42 6,100

Examples Hydra, siphonophores Jellyfish Box jellies Sea anemones, corals, sea pens

Cells found in mesoglea No Yes Yes Yes

Nematocysts in exodermis No Yes Yes Yes

Medusa
Medusa
phase in life cycle In some species Yes Yes No

Number of medusae produced per polyp Many Many One (not applicable)

Stauromedusae, small 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.[29] The Myxozoa, microscopic parasites, were first classified as protozoans.[30] Research then suggested that Polypodium hydriforme, a parasite within the egg cells of sturgeon, is closely related to the Myxozoa
Myxozoa
and that both Polypodium and the Myxozoa
Myxozoa
were intermediate between cnidarians and bilaterian animals.[31] More recent research demonstrated 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
Hydrozoa
and Scyphozoa
Scyphozoa
than to Anthozoa.[32][6][28][33] Some researchers classify the extinct conulariids as cnidarians, while others propose that they form a completely separate phylum.[34] Ecology[edit] 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. Hydrozoans
Hydrozoans
have a worldwide range: some, such as Hydra, live in freshwater; Obelia appears in the coastal waters of all the oceans; and Liriope can form large shoals near the surface in mid-ocean. Among anthozoans, a few scleractinian corals, sea pens and sea fans live in deep, cold waters, and some sea anemones inhabit polar seabeds while others live near hydrothermal vents over 10 km (6.2 mi) below sea-level. Reef-building corals are limited to tropical seas between 30°N and 30°S with a maximum depth of 46 m (151 ft), temperatures between 20 °C (68 °F) and 28 °C (82 °F) high salinity and low carbon dioxide levels. Stauromedusae, although usually classified as jellyfish, are stalked, sessile animals that live in cool to Arctic
Arctic
waters.[35] Cnidarians
Cnidarians
range in size from a mere handful of cells for myxozoan[28] through Hydra's 5–20 mm (0.20–0.79 in) long,[36] to the Lion's mane jellyfish, which may exceed 2 m (6.6 ft) in diameter and 75 m (246 ft) in length.[37] Prey of cnidarians ranges from plankton to animals several times larger than themselves.[35][38] Some cnidarians are parasites, mainly on jellyfish but a few are major pests of fish.[35] Others obtain most of their nourishment from endosymbiotic algae or dissolved nutrients.[9] Predators of cnidarians include: sea slugs, which can incorporate nematocysts into their own bodies for self-defense;[39] starfish, notably the crown of thorns starfish, which can devastate corals;[35] butterfly fish and parrot fish, which eat corals;[40] and marine turtles, which eat jellyfish.[37] Some sea anemones and jellyfish have a symbiotic relationship with some fish; for example clown fish live among the tentacles of sea anemones, and each partner protects the other against predators.[35] Coral
Coral
reefs form some of the world's most productive ecosystems. Common coral reef cnidarians include both Anthozoans (hard corals, octocorals, anemones) and Hydrozoans
Hydrozoans
(fire corals, lace corals). The endosymbiotic algae of many cnidarian species are very effective primary producers, in other words converters of inorganic chemicals into 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.[41] Fringing reefs just below low-tide level also have a mutually beneficial relationship with mangrove forests at high-tide level and seagrass meadows in between: the reefs protect the mangroves and seagrass from strong currents and waves that would damage them or erode the sediments in which they are rooted, while the mangroves and seagrass protect the coral from large influxes of silt, fresh water and 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.[42] Evolutionary history[edit]

The fossil coral Cladocora
Cladocora
from Pliocene
Pliocene
rocks in Cyprus

Fossil record[edit] The earliest widely accepted animal fossils are rather modern-looking cnidarians, possibly from around 580 million years ago, although fossils from the Doushantuo Formation can only be dated approximately.[43] The identification of some of these as embryos of animals has been contested, but other fossils from these rocks strongly resemble tubes and other mineralized structures made by corals.[44] Their presence implies that the cnidarian and bilaterian lineages had already diverged.[45] Although the Ediacaran
Ediacaran
fossil Charnia
Charnia
used to be classified as a jellyfish or sea pen,[46] more recent study of growth patterns in Charnia
Charnia
and modern cnidarians has cast doubt on this hypothesis,[47][48] leaving only the Canadian polyp, Haootia, as the only bona-fide cnidarian body fossil from the Ediacaran. Few fossils of cnidarians without mineralized skeletons are known from more recent rocks, except in lagerstätten that preserved soft-bodied animals.[49] A few mineralized fossils that resemble corals have been found in rocks from the Cambrian
Cambrian
period, and corals diversified in the Early Ordovician.[49] These corals, which were wiped out in the Permian- Triassic
Triassic
extinction about 251 million years ago,[49] did not dominate reef construction since sponges and algae also played a major part.[50] During the Mesozoic
Mesozoic
era rudist bivalves were the main reef-builders, but they were wiped out in the Cretaceous–Paleogene extinction event 65 million years ago,[51] and since then the main reef-builders have been scleractinian corals.[49] Family tree[edit] Further information: Phylogeny

Metazoa

Glass sponges

Demosponges

Calcareous sponges

Eumetazoa

Ctenophora
Ctenophora
(comb jellies)

Planulozoa

Cnidaria

Anthozoa (sea anemones and corals)

Myxozoa

Medusozoa

Hydrozoa (Hydra, siphonophores, etc.)

Cubozoa (box jellies)

Staurozoa

"Scyphozoa" (jellyfish, excluding Staurozoa)

Placozoa

Bilateria

Family tree of Cnidaria
Cnidaria
and the origins of animals[2][52][53][54][6] It is difficult to reconstruct the early stages in the evolutionary "family tree" of animals using only morphology (their shapes and structures), because the large differences between Porifera
Porifera
(sponges), Cnidaria
Cnidaria
plus Ctenophora
Ctenophora
(comb jellies), Placozoa
Placozoa
and Bilateria
Bilateria
(all the more complex animals) make comparisons difficult. Hence reconstructions now rely largely or entirely on molecular phylogenetics, which groups organisms according to similarities and differences in their biochemistry, usually in their DNA
DNA
or RNA.[55] It is now generally thought that the Calcarea
Calcarea
(sponges with calcium carbonate spicules) are more closely related to Cnidaria, Ctenophora (comb jellies) and Bilateria
Bilateria
(all the more complex animals) than they are to the other groups of sponges.[52][56][57] In 1866 it was proposed that Cnidaria
Cnidaria
and Ctenophora
Ctenophora
were more closely related to each other than to Bilateria
Bilateria
and formed a group called Coelenterata ("hollow guts"), because Cnidaria
Cnidaria
and Ctenophora
Ctenophora
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
Ctenophora
and Bilateria were more closely related to each other, since they shared features that Cnidaria
Cnidaria
lack, for example muscles in the middle layer (mesoglea in Ctenophora, mesoderm in Bilateria). However more recent analyses indicate that these similarities are rather vague, and the current view, based on molecular phylogenetics, is that Cnidaria
Cnidaria
and Bilateria are more closely related to each other than either is to Ctenophora. This grouping of Cnidaria
Cnidaria
and Bilateria
Bilateria
has been labelled "Planulozoa" because it suggests that the earliest Bilateria
Bilateria
were similar to the planula larvae of Cnidaria.[2][53] Within the Cnidaria, the Anthozoa
Anthozoa
(sea anemones and corals) are regarded as the sister-group of the rest, which suggests that the earliest cnidarians were sessile polyps with no medusa stage. However, it is unclear how the other groups acquired the medusa stage, since Hydrozoa
Hydrozoa
form medusae by budding from the side of the polyp while the other Medusozoa
Medusozoa
do so by splitting them off from the tip of the polyp. The traditional grouping of Scyphozoa
Scyphozoa
included the Staurozoa, but morphology and molecular phylogenetics indicate that Staurozoa are more closely related to Cubozoa
Cubozoa
(box jellies) than to other "Scyphozoa". Similarities in the double body walls of Staurozoa and the extinct Conulariida
Conulariida
suggest that they are closely related. The position of Anthozoa
Anthozoa
nearest the beginning of the cnidarian family tree also implies that Anthozoa
Anthozoa
are the cnidarians most closely related to Bilateria, and this is supported by the fact that Anthozoa and Bilateria
Bilateria
share some genes that determine the main axes of the body.[2][58] However, 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.[11] In molecular phylogenetics analyses from 2005 onwards, important groups of developmental genes show the same variety in cnidarians as in chordates.[59] In fact cnidarians, and especially anthozoans (sea anemones and corals), retain some genes that are present in bacteria, protists, plants and fungi but not in bilaterians.[60] The mitochondrial genome in the medusozoan cnidarians, unlike those in other animals, is linear with fragmented genes.[61] The reason for this difference is unknown. Interaction with humans[edit]

The dangerous "sea wasp" Chironex
Chironex
fleckeri

Jellyfish
Jellyfish
stings killed about 1,500 people in the 20th century,[62] and cubozoans are particularly dangerous. On the other hand, some large jellyfish are considered a delicacy in East and Southeast Asia. Coral
Coral
reefs 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. 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.[63] However, despite their great 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.[41] 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 drains; commercial fishing, including the use of dynamite to stun fish and the capture of young fish for aquariums; and tourist damage caused by boat anchors and the cumulative effect of walking on the reefs.[63] Coral, mainly from the Pacific Ocean
Ocean
has long been used in jewellery, and demand rose sharply in the 1980s.[64]

The dangerous Carukia barnesi, one of the known species of box jellyfish which can cause Irukandji syndrome.

Some large jellyfish species of the Rhizostomae
Rhizostomae
order are commonly consumed in Japan, Korea
Korea
and Southeast Asia.[65][66][67] 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.[67] The commercial value of jellyfish food products depends on the skill with which they are prepared, and " Jellyfish
Jellyfish
Masters" guard their trade secrets carefully. Jellyfish
Jellyfish
is very low in cholesterol and sugars, but cheap preparation can introduce undesirable amounts of heavy metals.[68] The "sea wasp" Chironex
Chironex
fleckeri has been described as the world's most venomous jellyfish and is held responsible for 67 deaths, although it is difficult to identify the animal as it is almost transparent. Most stingings by C. fleckeri cause only mild symptoms.[69] Seven other box jellies can cause a set of symptoms called Irukandji syndrome,[70] which takes about 30 minutes to develop,[71] and from a few hours to two weeks to disappear.[72] Hospital treatment is usually required, and there have been a few deaths.[70]

A number of Myxozoans are commercially important pathogens in salmonid aquaculture. Notes[edit]

^ Classes in Medusozoa
Medusozoa
based on "ITIS Report – Taxon: Subphylum Medusozoa". Universal Taxonomic Services. Retrieved 2018-03-18.  ^ a b c d Collins, A.G. (May 2002). " Phylogeny
Phylogeny
of Medusozoa
Medusozoa
and the Evolution
Evolution
of Cnidarian
Cnidarian
Life Cycles" (PDF). Journal of Evolutionary Biology. 15 (3): 418–432. doi:10.1046/j.1420-9101.2002.00403.x. Retrieved 2008-11-27.  ^ Subphyla Anthozoa
Anthozoa
and Medusozoa
Medusozoa
based on "The Taxonomicon – Taxon: Phylum
Phylum
Cnidaria". Universal Taxonomic Services. Archived from the original on 2007-09-29. Retrieved 2007-07-10.  ^ Dictionary.com Unabridged. Random House, Inc. Cnidaria. Retrieved May 15, 2013. ^ a b Zhang, Z.-Q. (2011). " Animal
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biodiversity: An introduction to higher-level classification and taxonomic richness" (PDF). Zootaxa. 3148: 7–12.  ^ a b c d E. Jímenez-Guri; et al. (July 2007). "Buddenbrockia is a cnidarian worm". Science. 317 (116): 116–118. doi:10.1126/science.1142024. PMID 17615357.  ^ Zapata F, Goetz FE, Smith SA, Howison M, Siebert S, Church SH, et al. (2015). "Phylogenomic Analyses Support Traditional Relationships within Cnidaria". PLOS ONE. 10 (10): e0139068. doi:10.1371/journal.pone.0139068. PMC 4605497 . PMID 26465609.  ^ Park E, Hwang D, Lee J, Song J, Seo T, Won Y (January 2012). "Estimation of divergence times in cnidarian evolution based on mitochondrial protein-coding genes and the fossil record". Molecular Phylogenetics & Evolution. 62 (1): 329–45. doi:10.1016/j.ympev.2011.10.008. PMID 22040765.  ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae af Hinde, R.T. (1998). "The Cnidaria
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Further reading[edit] Books[edit]

Arai, M.N. (1997). A Functional Biology of Scyphozoa. London: Chapman & Hall [p. 316]. ISBN 0-412-45110-7. Ax, P. (1999). Das System der Metazoa
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. 54]. ISBN 0-632-04761-5. Brusca, R.C., G.J. Brusca (2003). Invertebrates. Sunderland, Mass.: Sinauer Associates. 2nd edition [chapter 8, p. 219]. ISBN 0-87893-097-3. 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. 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.

Journal articles[edit]

D. Bridge, B. Schierwater, C. W. Cunningham, R. DeSalle R, L. W. Buss: Mitochondrial DNA
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. 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. 679. ISSN 0737-4038 D. G. Fautin: Reproduction of Cnidaria. in: Canadian Journal of Zoology. Ottawa Ont. 80.2002, p. 1735. (PDF, online) ISSN 0008-4301 G. O. Mackie: What's new in cnidarian biology? in: Canadian Journal of Zoology. Ottawa Ont. 80.2002, p. 1649. (PDF, online) ISSN 0008-4301 P. Schuchert: Phylogenetic
Phylogenetic
analysis of the Cnidaria. in: Zeitschrift für zoologische Systematik und Evolutionsforschung. Paray, Hamburg-Berlin 31.1993, p. 161. ISSN 0044-3808 G. Kass-Simon, A. A. Scappaticci Jr.: The behavioral and developmental physiology of nematocysts. in: Canadian Journal of Zoology. Ottawa Ont. 80.2002, p. 1772. (PDF, online) ISSN 0044-3808 J. Zrzavý (2001). "The interrelationships of metazoan parasites: a review of phylum- and higher-level hypotheses from recent morphological and molecular phylogenetic analyses" (PDF). Folia Parasitologica. 48 (2): 81–103. doi:10.14411/fp.2001.013. PMID 11437135. Archived from the original (PDF) on 2007-10-25. Retrieved 2009-01-26. 

External links[edit]

Wikispecies
Wikispecies
has information related to Cnidaria

The Wikibook Dichotomous Key has a page on the topic of: Cnidaria

Wikimedia Commons has media related to Cnidaria.

Look up Cnidaria
Cnidaria
in Wiktionary, the free dictionary.

YouTube: Nematocysts Firing YouTube:My Anemone Eat Meat Defensive and feeding behaviour of sea anemone Cnidaria
Cnidaria
- Guide to the Marine Zooplankton of south eastern Australia, Tasmanian Aquaculture & Fisheries Institute A Cnidaria
Cnidaria
homepage maintained by University of California, Irvine Cnidaria
Cnidaria
page at Tree of Life Fossil Gallery: Cnidarians The Hydrozoa
Hydrozoa
Directory Hexacorallians of the World

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Eukaryota

Domain Archaea Bacteria Eukaryota (Supergroup Plant Hacrobia Heterokont Alveolata Rhizaria Excavata Amoebozoa Opisthokonta

Animal Fungi)

Diaphoretickes

Archaeplastida

Glaucophyta Rhodophyta

Viridiplantae Plantae s.s.

Chlorophyta Streptophyta

Cryptista

Corbihelia Cryptophyta

A+H

Ancoracysta twista Haptista

Centroheliozoa Haptophyta

SAR

Halvaria

Alveolata

Ciliates Miozoa

Acavomonadia Colponemidia Myzozoa

Stramenopiles (heterokonts)

Bicosoecea Developea Hyphochytrea Ochrophyta Peronosporomycota Pirsoniomycota Placidozoa Platysulcea Sagenista

Rhizaria

Filosa Phytomyxea Retaria

Ectoreta Marimyxia

Vampyrellidea

Incertae sedis

Kamera lens

Excavata

Ancyromonadida Malawimonadea Metamonada
Metamonada
(Anaeromonada, Trichozoa)

Discoba

Jakobea Tsukubea

Discicristata

Euglenozoa Percolozoa

Podiata

Amorphea

Amoebozoa

Conosa
Conosa
(Archamoebae, Semiconosia) Lobosa
Lobosa
(Cutosea, Discosea, Tubulinea)

Obazoa

Apusomonadida Breviatea

Opisthokonta

Holomycota

Cristidiscoidea Opisthosporidia

Aphelida Cryptomycota Microsporidia

True fungi

Holozoa

Choanoflagellates Filasterea Metazoa
Metazoa
or Animals Ichthyosporea Pluriformea

Syssomonas Corallochytrea

CRuMs

Diphyllatea Mantamonadida Rigifilida Discocelida? Micronucleariida?

Incertae sedis

Parakaryon myojinensis †Acritarcha †Charnia †Gakarusia †Galaxiopsis †Grypania †Leptoteichos

Major kingdoms are underlined. See also: protist. Sources and alternative views: Wikispecies.

v t e

Extant Animal
Animal
phyla

Domain Archaea Bacteria Eukaryota (Supergroup Plant Hacrobia Heterokont Alveolata Rhizaria Excavata Amoebozoa Opisthokonta

Animal Fungi)

A n i m a l i a

Porifera
Porifera
(sponges)

Diploblasts (Eumetazoa)

Ctenophora
Ctenophora
(comb jellies)

ParaHoxozoa

Placozoa
Placozoa
(Trichoplax)

Planulozoa

Cnidaria
Cnidaria
(jellyfish and relatives)

Bilateria (Triploblasts)

(see below↓)

Bilateria

Xenacoelomorpha

Xenoturbellida (Xenoturbella) Acoelomorpha

acoels nemertodermatids

N e p h r o z o a

Deuterostomia

Chordata

lancelets tunicates craniates / vertebrates

Ambulacraria

Echinodermata (starfish and relatives) Hemichordata

acorn worms pterobranchs

P r o t o s t o m i a

Ecdysozoa

Scalidophora

Kinorhyncha
Kinorhyncha
(mud dragons) Priapulida
Priapulida
(penis worms)

N+L+P

Nematoida

Nematoda (roundworms) Nematomorpha
Nematomorpha
(horsehair worms)

L+P

Loricifera

Panarthropoda

Arthropoda (arthropods) Tardigrada (waterbears) Onychophora
Onychophora
(velvet worms)

S p i r a l i a

Gnathifera¹

Chaetognatha
Chaetognatha
(arrow worms) Gnathostomulida (jaw worms) Micrognathozoa (Limnognathia) Syndermata

Rotifera Acanthocephala

Platytrochozoa

R+M

Mesozoa

Orthonectida Dicyemida
Dicyemida
or Rhombozoa

Rouphozoa¹

Platyhelminthes (flatworms) Gastrotricha (hairybacks)

Lophotrochozoa

Cycliophora (Symbion) Mollusca
Mollusca
(molluscs)

A+N

Annelida (ringed worms) Nemertea
Nemertea
(ribbon worms)

Lophophorata

Bryozoa

Entoprocta
Entoprocta
or Kamptozoa Ectoprocta (moss animals)

Brachiozoa

Brachiopoda (lamp shells) Phoronida (horseshoe worms)

Major groups within phyla

Sponges

Calcareous Hexactinellid Demosponge Homoscleromorpha

Cnidarians

Anthozoa
Anthozoa
inc. corals Medusozoa
Medusozoa
inc. jellyfish Myxozoa

Vertebrates

Jawless fish Cartilaginous fish Bony fish Amphibians Reptiles/Birds Mammals

Echinoderms

Sea lilies Asterozoa
Asterozoa
inc. starfish Echinozoa

Nematodes

Chromadorea Enoplea Secernentea

Arthropods

Chelicerates/Arachnids Myriapods Crustaceans Hexapods/Insects

Platyhelminths

Turbellaria Trematoda Monogenea Cestoda

Bryozoans

Phylactolaemata Stenolaemata Gymnolaemata

Annelids

Polychaetes Clitellata Echiura

Molluscs

Gastropods Cephalopods Bivalves Chitons Tusk shells

Phyla with ≥5000 extant species bolded See also Diploblasts Monoblastozoa (nomen dubium)

¹Platyzoa

v t e

Extant Cnidaria
Cnidaria
classes

Anthozoa

Anthozoa
Anthozoa
(corals, anemones)

Medusozoa

Cubozoa
Cubozoa
(box jellyfish) Hydrozoa
Hydrozoa
(hydrozoans) Scyphozoa
Scyphozoa
(true jellyfish) Staurozoa (stalked jellyfish) Polypodiozoa
Polypodiozoa
(Polypodium)

Myxozoa

Malacosporea Myxosporea

Taxon identifiers

Wd: Q25441 ADW: Cnidaria EoL: 1745 EPPO: 1CNIDP Fauna Europaea: 10979 Fossilworks: 4524 ITIS: 48738 NCBI: 6073 WoRMS: 1267

Authority control

LCCN: sh98001754 GND: 4171498-2 BNF: cb119653720 (d