Cnidarians mostly have two basic body forms: swimmingmedusae andsessilepolyps, both of which areradially symmetrical with mouths surrounded bytentacles that bear cnidocytes, which are specialized stinging cells used to capture prey. Both forms have a singleorifice and body cavity that are used fordigestion andrespiration. Many cnidarian species producecolonies that are single organisms composed of medusa-like or polyp-likezooids, or both (hence they aretrimorphic). Cnidarians' activities are coordinated by a decentralizednerve net andsimple receptors. Cnidarians also haverhopalia, which are involved in gravity sensing and sometimes chemoreception. Several free-swimming species ofCubozoa andScyphozoa possess balance-sensingstatocysts, and some havesimple eyes. Not all cnidariansreproduce sexually, but many species have complex life cycles ofasexual 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 withctenophores, also known as comb jellies, in the phylumCoelenterata, but increasing awareness of their differences caused them to be placed in separate phyla.[5] Most cnidarians are classified into four main groups: the almost whollysessileAnthozoa (sea anemones,corals,sea pens); swimmingScyphozoa (jellyfish);Cubozoa (box jellies); andHydrozoa (a diverse group that includes all the freshwater cnidarians as well as many marine forms, and which has both sessile members, such asHydra, and colonial swimmers (such as thePortuguese man o' war)).Staurozoa have recently been recognised as aclass in their own right rather than a sub-group of Scyphozoa, and the highly derived parasiticMyxozoa andPolypodiozoa were firmly recognized as cnidarians only in 2007.[6]
Most cnidarians prey onorganisms ranging in size fromplankton to animals several times larger than themselves, but many obtain much of their nutrition from symbioticdinoflagellates, and a few areparasites. Many are preyed on by other animals includingstarfish,sea slugs,fish,turtles, and even other cnidarians. Manyscleractinian corals—which form the structural foundation forcoral reefs—possess polyps that are filled with symbiotic photo-syntheticzooxanthellae. While reef-forming corals are almost entirely restricted to warm and shallow marine waters, other cnidarians can be found at great depths, inpolar regions, and in freshwater.
Cnidarians are a very ancient phylum, with fossils having been found in rocks formed about580 million years ago during theEdiacaranperiod, preceding theCambrian Explosion. Other fossils show that corals may have been present shortly before490 million years ago and diversified a few million years later.Molecular clock analysis ofmitochondrial genes suggests an even older age for thecrown group of cnidarians, estimated around741 million years ago, almost 200 million years before theCambrian period, as well as before any fossils.[7] Recentphylogenetic analyses supportmonophyly of cnidarians, as well as the position of cnidarians as thesister group ofbilaterians.[8]
Cnidarians form aphylum ofanimals that are more complex thansponges, about as complex asctenophores (comb jellies), and less complex thanbilaterians, 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-likebasement membranes;muscles;nervous systems; andsome havesensory organs. Cnidarians are distinguished from all other animals by havingcnidocytes that fireharpoon-like structures that are mainly used to capture prey. In some species, cnidocytes can also be used as anchors.[10] 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 themesoglea in cnidarians; more complexanimals have three main cell layers and no intermediate jelly-like layer. Hence, cnidarians and ctenophores have traditionally been labelleddiploblastic, along with sponges.[10][11] However, both cnidarians and ctenophores have a type ofmuscle that, in more complex animals, arises from themiddle cell layer.[12] As a result, some recent text books classify ctenophores astriploblastic,[11]: 182–195 and it has been suggested that cnidarians evolved from triploblastic ancestors.[12]
Most adult cnidarians appear as either free-swimmingmedusae orsessilepolyps, and manyhydrozoans species are known to alternate between the two forms.
Both areradially 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 withcnidocytes around their edges, and medusae generally have an inner ring of tentacles around the mouth. Some hydroids may consist of colonies ofzooids that serve different purposes, such as defence, reproduction and catching prey. Themesoglea 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 ofjet propulsion.[11]
In some colonial polyps, achitinousepidermis 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 withsediment particles.[11]
Cnidaria arediploblastic animals; in other words, they have two main cell layers, while more complex animals aretriploblasts having three main layers. The two main cell layers of cnidarians formepithelia that are mostly one cell thick, and are attached to a fibrousbasement membrane, which theysecrete. They also secrete the jelly-likemesoglea that separates the layers. The layer that faces outwards, known as theectoderm ("outside skin"), generally contains the following types of cells:[10]
Epitheliomuscular cells whose bodies form part of the epithelium but whose bases extend to formmuscle fibers in parallel rows.[11]: 103–104 The fibers of the outward-facing cell layer generally run at right angles to the fibers of the inward-facing one. InAnthozoa (anemones, corals, etc.) andScyphozoa (jellyfish), themesoglea also contains some muscle cells.[11]
Cnidocytes, the harpoon-like "nettle cells" that give thephylum Cnidaria its name. These appear between or sometimes on top of the muscle cells.[10]
Nerve cells.Sensory cells appear between or sometimes on top of the muscle cells,[10] and communicate viasynapses (gaps across which chemical signals flow) withmotor nerve cells, which lie mostly between the bases of the muscle cells.[11] Some form a simplenerve 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.[10]
In addition to epitheliomuscular, nerve and interstitial cells, the inward-facinggastroderm ("stomach skin") containsgland cells that secrete digestiveenzymes. In some species it also contains low concentrations of cnidocytes, which are used to subdue prey that is still struggling.[10][11]
The mesoglea contains small numbers ofamoeba-like cells,[11] and muscle cells in some species.[10] However, the number of middle-layer cells and types are much lower than in sponges.[11]
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 thepolyp andmedusa forms, or ofzooids within colonial organisms like those inHydrozoa.[17] InHydrozoans, colonial individuals arising from individual zooids will take on separate tasks.[18] For example, inObelia there are feeding individuals, thegastrozooids; the individuals capable of asexual reproduction only, the gonozooids, blastostyles and free-living or sexually reproducing individuals, themedusae.
These "nettle cells" function asharpoons, since theirpayloads remain connected to the bodies of the cells by threads. Three types ofcnidocytes are known:[10][11]
Firing sequence of the cnida in a hydra's nematocyst[11] Operculum (lid) "Finger" that turns inside out / / / Barbs Venom Victim's skin Victim's tissues
Nematocysts injectvenom into prey, and usually have barbs to keep them embedded in the victims. Most species have nematocysts.[10]
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 theorderCeriantharia,tube anemones.[11]
Ahydra's nematocyst, before firing. "trigger" cilium[11]
Acilium (fine hair) which projects above the surface and acts as a trigger. Spirocysts do not have cilia.
A tough capsule, thecnida, which houses the thread, its payload and a mixture of chemicals that may include venom oradhesives 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.
Anoperculum (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:[10]
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 ofChironex (the "sea wasp"), chemical changes in the cnida's contents may cause them to expand rapidly bypolymerization.
Chemical changes in the liquid in the cnida make it a much moreconcentrated solution, so thatosmotic pressure forces water in very rapidly to dilute it. This mechanism has been observed in nematocysts of the classHydrozoa, sometimes producing pressures as high as 140atmospheres, similar to that ofscuba air tanks, and fully extending the thread in as little as 2 milliseconds (0.002 second).[11]
Cnidocytes can only fire once, and about 25% of a hydra's nematocysts are lost from its tentacles when capturing abrine 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: nearbysensory 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.[10][11]
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.[11]
Hydras and somesea anemones can move slowly over rocks and sea or stream beds by various means: creeping like snails, crawling likeinchworms, or bysomersaulting. A few can swim clumsily by waggling their bases.[11]
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, 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.[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 have many of the same neurotransmitters as bilaterians, including chemicals such as glutamate, GABA, and glycine.[22] Serotonin, dopamine, noradrenaline, octopamine, histamine, and acetylcholine, on the other hand, are absent.[23]
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 and complex swimming colonies such assiphonophores andchondrophores sense tilt and acceleration by means ofstatocysts, 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 haveocelli ("simple eyes"), which can detect sources of light. However, the agilebox jellyfish are unique among Medusae because they possess four kinds of true eyes that haveretinas,corneas andlenses.[24] 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.[10][24]
Cnidarians feed in several ways:predation, absorbing dissolvedorganic chemicals,filtering food particles out of the water, obtainingnutrients fromsymbioticalgae within their cells, and parasitism. Most obtain the majority of their food from predation but some, including thecoralsHetroxenia andLeptogorgia, depend almost completely on theirendosymbionts and on absorbing dissolved nutrients.[10] Cnidaria give their symbiotic algaecarbon dioxide, some nutrients, and protection against predators.[11]
Predatory species use theircnidocytes to poison or entangle prey, and those with venomousnematocysts may start digestion by injecting digestiveenzymes. 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 ofcilia whose beating creates currents that flow towards the mouth, and some produce nets ofmucus to trap particles.[10] Their digestion is both intra and extracellular.
Once the food is in the digestive cavity,gland cells in thegastroderm 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.[11] Nutrients reach the outer cell layer bydiffusion or, for animals or zooids such as medusae which have thickmesogleas, are transported by mobile cells in the mesoglea.[10]
Indigestible remains of prey are expelled through the mouth. The main waste product of cells' internal processes isammonia, which is removed by the external and internal water currents.[11]
There are no respiratory organs, and both cell layers absorb oxygen from and expelcarbon 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. SomeAnthozoa 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 ahydrostatic skeleton, to control the water pressure in the cavity without expelling undigested food.[10]
Cnidaria that carryphotosyntheticsymbionts may have the opposite problem, an excess of oxygen, which may provetoxic. The animals produce large quantities ofantioxidants to neutralize the excess oxygen.[10]
All cnidarians canregenerate, allowing them to recover from injury and to reproduceasexually. 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.[10]
Cnidariansexual reproduction often involves a complex life cycle with bothpolyp andmedusa stages. For example, inScyphozoa (jellyfish) andCubozoa (box jellies), alarva 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 calledstrobilation. The juveniles swim off and slowly grow to maturity, while the polyp re-grows and may continue strobilating periodically. The adult medusae havegonads in thegastroderm, and these releaseova andsperm into the water in the breeding season.[10][11]
This phenomenon of succession of differently organized generations (one asexually reproducing, sessile polyp, followed by afree-swimming medusa or a sessile polyp that reproduces sexually)[25] is sometimes called "alternation of asexual and sexual phases" or "metagenesis", but should not be confused with thealternation 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) 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[26] andLaodicea undulata[27]) and Scyphozoa (Aurelia sp.1[28]).Anthozoa have no medusa stage at all and the polyps are responsible for sexual reproduction.[10]
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 thephase 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 — one famous example is theGreat Barrier Reef, where at least 110corals and a few non-cnidarianinvertebrates produce enough gametes to turn the water cloudy. These mass spawnings may producehybrids, 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.[10]
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 theanimal pole), while inbilaterians it forms at the other end (vegetal pole).[11] The larvae, calledplanulae, swim or crawl by means ofcilia.[10] 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.[11]
Anthozoan larvae either have largeyolks or are capable of feeding onplankton, and some already haveendosymbioticalgae 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 andmetamorphose into polyps. Instead, these species rely on their medusae to extend their ranges.[11]
All known cnidarians can reproduceasexually by various means, in addition to regenerating after being fragmented.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 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.[10][11]
Two classicalDNA repair pathways,nucleotide excision repair andbase excision repair, are present inhydra,[29] and these repair pathways facilitate unhindered reproduction. The identification of these pathways in hydra is based, in part, on the presence in the hydragenome of genes homologous to genes in other genetically well studied species that have been demonstrated to play key roles in these DNA repair pathways.[29]
Cnidarians were for a long time grouped withctenophores in the phylumCoelenterata, but increasing awareness of their differences caused them to be placed in separate phyla. Modern cnidarians are generally classified into four mainclasses:[10] sessileAnthozoa (sea anemones,corals,sea pens); swimmingScyphozoa (jellyfish) andCubozoa (box jellies); andHydrozoa, a diverse group that includes all the freshwater cnidarians as well as many marine forms, and has both sessile members such asHydra and colonial swimmers such as thePortuguese Man o' War.Staurozoa have recently been recognised as aclass in their own right rather than a sub-group of Scyphozoa, and the parasiticMyxozoa andPolypodiozoa are now recognized as highly derived cnidarians rather than more closely related to thebilaterians.[6][30]
Stauromedusae, smallsessile 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.[32]
TheMyxozoa, microscopicparasites, were first classified asprotozoans.[33] Research then found thatPolypodium hydriforme, a non-myxozoan parasitewithin the egg cells ofsturgeon, is closely related to the Myxozoa and suggested that bothPolypodium and the Myxozoa were intermediate between cnidarians andbilaterian animals.[34] 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.[6][30][35][36]
Some researchers classify the extinctconulariids as cnidarians, while others propose that they form a completely separatephylum.[37]
Many cnidarians are limited to shallow waters because they depend onendosymbioticalgae 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 have a worldwide range: some, such asHydra, live in freshwater;Obelia appears in the coastal waters of all the oceans; andLiriope can form large shoals near the surface in mid-ocean. Amonganthozoans, a fewscleractiniancorals,sea pens andsea fans live in deep, cold waters, and some sea anemones inhabit polar seabeds while others live nearhydrothermal vents over 10 km (33,000 ft) 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 and 28 °C (68 and 82 °F), highsalinity, and lowcarbon dioxide levels.Stauromedusae, although usually classified as jellyfish, are stalked,sessile animals that live in cool toArctic waters.[38] Cnidarians range in size from a mere handful of cells for the parasitic myxozoans[30] throughHydra's length of 5–20 mm (1⁄4–3⁄4 in),[39] to thelion's mane jellyfish, which may exceed 2 m (6 ft 7 in) in diameter and 75 m (246 ft) in length.[40]
Prey of cnidarians ranges from plankton to animals several times larger than themselves.[38][41] Some cnidarians areparasites, mainly on jellyfish but a few are major pests of fish.[38] Others obtain most of their nourishment from endosymbiotic algae or dissolved nutrients.[10] Predators of cnidarians include:sea slugs,flatworms andcomb jellies, which can incorporatenematocysts into their own bodies for self-defense (nematocysts used by cnidarian predators are referred to as kleptocnidae);[42][43][44]starfish, notably thecrown of thorns starfish, which can devastate corals;[38]butterfly fish andparrot fish, which eat corals;[45] and marineturtles, which eat jellyfish.[40] Some sea anemones and jellyfish have asymbiotic relationship with some fish; for exampleclownfish live among the tentacles of sea anemones, and each partner protects the other against predators.[38]
Coral reefs 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 effectiveprimary producers, in other words converters ofinorganic chemicals intoorganic 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.[46]Fringing reefs just below low-tide level also have a mutually beneficial relationship withmangrove forests at high-tide level andseagrass meadows in between: the reefs protect the mangroves and seagrass from strong currents and waves that would damage them orerode the sediments in which they are rooted, while the mangroves and seagrass protect the coral from large influxes ofsilt, fresh water andpollutants. 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.[47]
The earliest widely accepted animal fossils are rather modern-looking cnidarians, possibly from around580 million years ago, although fossils from theDoushantuo Formation can only be dated approximately.[48] The identification of some of these as embryos of animals has been contested, but other fossils from these rocks strongly resemble tubes and othermineralized structures made bycorals.[49] Their presence implies that the cnidarian andbilaterian lineages had already diverged.[50] Although the Ediacaran fossilCharnia used to be classified as ajellyfish orsea pen,[51] more recent study of growth patterns inCharnia and modern cnidarians has cast doubt on this hypothesis,[52][53] leaving the Canadian polypHaootia and the BritishAuroralumina as the only recognized cnidarian body fossils from the Ediacaran.Auroralumina is the earliest known animalpredator.[54] Few fossils of cnidarians without mineralizedskeletons are known from more recent rocks, except inLagerstätten that preserved soft-bodied animals.[55]
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 likeHydroconus andTuvaeconus, possessed external skeletons with features resembling bothscyphozoans andtetracorals. Their unique skeletal structures suggest a distinct lineage within early cnidarian evolution.[58]
It is difficult to reconstruct the early stages in theevolutionary "family tree" of animals using onlymorphology (their shapes and structures) because of the large differences between the major groups of animals. Hence, reconstructions now rely almost entirely onmolecular phylogenetics, which groups organisms based on theirbiochemistry, most commonly by analyzingDNA orRNA sequences.[59]
In 1866, it was proposed that Cnidaria and Ctenophora were more closely related to each other than to Bilateria and formed a group calledCoelenterata ("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",[a] named so because the earliest Bilateria were probably similar to theplanula larvae of Cnidaria.[60][61]
In 2005, Katja Seipel and Volker Schmid suggested that cnidarians and ctenophores are simplified descendants oftriploblastic animals, since ctenophores and the medusa stage of some cnidarians havestriated muscle, which in bilaterians arises from themesoderm. They did not commit themselves on whether bilaterians evolved from early cnidarians or from the hypothesized triploblastic ancestors of cnidarians.[12]
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 enigmaticPolypodium hydriforme and subphylumMyxozoa have been firmly placed within the Cnidaria and have been shown to be closely related to theMedusozoa. 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 classScyphozoa also includedStaurozoa andCubozoa, but significant morphological differences eventually lead to the split of the three.[62] The group containing them has since been named "Acraspeda". The relationships between these three andHydrozoa 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.[63][64][65] At the same time, other studies have recovered Acraspeda as being monophyletic.[66][67] The subphylumAnthozoa is argued to have either two or three classes, but the relationships between them is not disputed; thetube-dwelling anemones of the class Ceriantharia have consistently shown to be more closely related to theHexacorallia than to theOctocorallia.[66][68][67][63]
In molecular phylogenetics analyses from 2005 onwards, important groups of developmental genes show the same variety in cnidarians as inchordates.[69] In fact cnidarians, and especiallyanthozoans (sea anemones and corals), retain some genes that are present inbacteria,protists,plants andfungi but not in bilaterians.[70]
Jellyfish stings killed about 1,500 people in the 20th century,[71] and cubozoans are particularly dangerous. On the other hand, some large jellyfish are considered adelicacy inEast andSoutheast Asia.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.[72] However, despite their greatproductivity, reefs are vulnerable to over-fishing, because much of theorganic carbon they produce is exhaled ascarbon dioxide by organisms at the middle levels of thefood chain and never reaches the larger species that are of interest to fishermen.[46] 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 fromstorm drains; commercial fishing, including the use ofdynamite to stun fish and the capture of young fish foraquariums; and tourist damage caused by boat anchors and the cumulative effect of walking on the reefs.[72] Coral, mainly from thePacific Ocean has long been used injewellery, and demand rose sharply in the 1980s.[73]
Some largejellyfish species of theRhizostomeae order are commonly consumed inJapan,Korea and Southeast Asia.[74][75][76] 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.[76] The commercial value of jellyfish food products depends on the skill with which they are prepared, and "Jellyfish Masters" guard theirtrade secrets carefully. Jellyfish is very low incholesterol andsugars, but cheap preparation can introduce undesirable amounts ofheavy metals.[77]
The "sea wasp"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 byC. fleckeri cause only mild symptoms.[78] Seven otherbox jellies can cause a set of symptoms calledIrukandji syndrome,[79] which takes about 30 minutes to develop,[80] and from a few hours to two weeks to disappear.[81] Hospital treatment is usually required, and there have been a few deaths.[79]
^Subphyla Anthozoa, Endocnidozoa and Medusozoa based on"WoRMS - Cnidaria". World Register of Marine Species. Archived fromthe original on 2025-07-18. Retrieved2025-08-10.
^Park E, Hwang D, Lee J, et al. (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.Bibcode:2012MolPE..62..329P.doi:10.1016/j.ympev.2011.10.008.PMID22040765.
^Dunn, Casey W.; Wagner, Günter P. (16 September 2006). "The evolution of colony-level development in the Siphonophora (Cnidaria:Hydrozoa)".Development Genes and Evolution.216 (12):743–754.doi:10.1007/s00427-006-0101-8.PMID16983540.S2CID278540.
^Trumble, W.; Brown, L. (2002). "Cnida".Shorter Oxford English Dictionary. Oxford University Press.
^Štolc, A. (1899). "Actinomyxidies, nouveau groupe de Mesozoaires parent des Myxosporidies".Bull. Int. l'Acad. Sci. Bohème.12:1–12.
^Zrzavý, J.; Hypša, V. (April 2003). "Myxozoa,Polypodium, and the origin of the Bilateria: The phylogenetic position of "Endocnidozoa" in light of the rediscovery ofBuddenbrockia".Cladistics.19 (2):164–169.doi:10.1111/j.1096-0031.2003.tb00305.x.S2CID221583517.
^Miller, D.J.; Ball, E.E. & Technau, U. (October 2005). "Cnidarians and ancestral genetic complexity in the animal kingdom".Trends in Genetics.21 (10):536–539.doi:10.1016/j.tig.2005.08.002.PMID16098631.
^Technau, U.; Rudd, S. & Maxwell, P (December 2005). "Maintenance of ancestral complexity and non-metazoan genes in two basal cnidarians".Trends in Genetics.21 (12):633–639.doi:10.1016/j.tig.2005.09.007.PMID16226338.
^Omori, M.; Kitamura, M. (2004). "Taxonomic review of three Japanese species of edible jellyfish (Scyphozoa: Rhizostomeae)".Plankton Biol. Ecol.51 (1):36–51.
^Greenberg, M.I.; Hendrickson, R.G.; Silverberg, M.; et al. (2004). "Box Jellyfish Envenomation".Greenberg's Text-atlas of Emergency Medicine. Lippincott Williams & Wilkins. p. 875.ISBN978-0-7817-4586-4.
Arai, M.N. (1997).A Functional Biology of Scyphozoa. London: Chapman & Hall [p. 316].ISBN0-412-45110-7.
Ax, P. (1999).Das System der Metazoa I. Ein Lehrbuch der phylogenetischen Systematik. Gustav Fischer, Stuttgart-Jena: Gustav Fischer.ISBN3-437-30803-3.
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].ISBN0-632-04761-5.
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].ISBN0-521-77914-6.
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.ISBN3-8274-1482-2.
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.ISBN3-334-60474-8.
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. 8750.ISSN0097-3157
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.ISSN0737-4038
P. Schuchert:Phylogenetic analysis of the Cnidaria. in:Zeitschrift für zoologische Systematik und Evolutionsforschung. Paray, Hamburg-Berlin 31.1993, p. 161.ISSN0044-3808
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