98-636: See text Semaeostomeae (literally "flag mouths") is an order of large jellyfish characterized by four long, frilly oral arms flanking their quadrate mouths. The umbrella is domed with scalloped margins, and the gastrovascular system consists of four unbranched pouches radiating outwards from the central stomach; no ring canal is present. They usually possess eight tentacles; four are per-radical and four are inter-radical. The order consists of five families. [REDACTED] The three traditional families, Pelagiidae, Cyaneidae, and Ulmaridae, are distinguishable by these characteristics: In addition, members of
196-404: A metachronal rhythm rather like that of a Mexican wave . From each balancer in the statocyst a ciliary groove runs out under the dome and then splits to connect with two adjacent comb rows, and in some species runs along the comb rows. This forms a mechanical system for transmitting the beat rhythm from the combs to the balancers, via water disturbances created by the cilia. The Lobata has
294-469: A phylum of marine invertebrates , commonly known as comb jellies , that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), and they are the largest animals to swim with the help of cilia. Depending on the species, adult ctenophores range from a few millimeters to 1.5 m (5 ft) in size. Only 186 living species are currently recognised. Their bodies consist of
392-493: A " nerve net " is located in the epidermis . Although traditionally thought not to have a central nervous system , nerve net concentration and ganglion -like structures could be considered to constitute one in most species. A jellyfish detects stimuli, and transmits impulses both throughout the nerve net and around a circular nerve ring, to other nerve cells. The rhopalial ganglia contain pacemaker neurones which control swimming rate and direction. In many species of jellyfish,
490-402: A "darting motion". Most Platyctenida have oval bodies that are flattened in the oral-aboral direction, with a pair of tentilla-bearing tentacles on the aboral surface. They cling to and creep on surfaces by everting the pharynx and using it as a muscular "foot". All but one of the known platyctenid species lack comb-rows. Platyctenids are usually cryptically colored, live on rocks, algae, or
588-513: A backbone, though shellfish , cuttlefish and starfish are not vertebrates either. In scientific literature, "jelly" and "jellyfish" have been used interchangeably. Many sources refer to only scyphozoans as "true jellyfish". A group of jellyfish is called a "smack" or a "smuck". The term jellyfish broadly corresponds to medusae, that is, a life-cycle stage in the Medusozoa . The American evolutionary biologist Paulyn Cartwright gives
686-400: A complex life cycle , and the medusa is normally the sexual phase, which produces planula larvae. These then disperse widely and enter a sedentary polyp phase which may include asexual budding before reaching sexual maturity. Jellyfish are found all over the world, from surface waters to the deep sea. Scyphozoans (the "true jellyfish") are exclusively marine , but some hydrozoans with
784-571: A domed head with vesicles (chambers) that contain adhesive; a stalk that anchors the cell in the lower layer of the epidermis or in the mesoglea; and a spiral thread that coils round the stalk and is attached to the head and to the root of the stalk. The function of the spiral thread is uncertain, but it may absorb stress when prey tries to escape, and thus prevent the colloblast from being torn apart. One species, Minictena luteola, which only measure 1.5mm in diameter, have five different types of colloblast cells. In addition to colloblasts, members of
882-436: A few are anchored to the seabed by stalks rather than being motile. They are made of an umbrella-shaped main body made of mesoglea , known as the bell , and a collection of trailing tentacles on the underside. Via pulsating contractions, the bell can provide propulsion for locomotion through open water. The tentacles are armed with stinging cells and may be used to capture prey or to defend against predators. Jellyfish have
980-421: A few ctenophore species lack them. Like cnidarians, ctenophores have two main layers of cells that sandwich a middle layer of jelly-like material, which is called the mesoglea in cnidarians and ctenophores; more complex animals have three main cell layers and no intermediate jelly-like layer. Hence ctenophores and cnidarians have traditionally been labelled diploblastic . Both ctenophores and cnidarians have
1078-529: A free-swimming medusa is Burgessomedusa from the mid-Cambrian Burgess Shale of Canada, which is likely either a stem group of box jellyfish (Cubozoa) or Acraspeda (the clade including Staurozoa, Cubozoa, and Scyphozoa). Other claimed records from the Cambrian of China and Utah in the United States are uncertain, and possibly represent ctenophores instead. The main feature of a true jellyfish
SECTION 10
#17328015276881176-462: A mass of jelly, with a layer two cells thick on the outside, and another lining the internal cavity. The phylum has a wide range of body forms, including the egg-shaped cydippids with a pair of retractable tentacles that capture prey, the flat, generally combless platyctenids , and the large-mouthed beroids , which prey on other ctenophores. Almost all ctenophores function as predators , taking prey ranging from microscopic larvae and rotifers to
1274-408: A medusa stage in their life cycle. The basic cycle is egg, planula larva, polyp, medusa, with the medusa being the sexual stage. The polyp stage is sometimes secondarily lost. The subphylum include the major taxa, Scyphozoa (large jellyfish), Cubozoa (box jellyfish) and Hydrozoa (small jellyfish), and excludes Anthozoa (corals and sea anemones). This suggests that the medusa form evolved after
1372-400: A mouth that can usually be closed by muscles; a pharynx ("throat"); a wider area in the center that acts as a stomach ; and a system of internal canals. These branch through the mesoglea to the most active parts of the animal: the mouth and pharynx; the roots of the tentacles, if present; all along the underside of each comb row; and four branches around the sensory complex at the far end from
1470-576: A nervous system, with the genes coding for the receptors for each of these neurotransmitters missing. Monofunctional catalase (CAT), one of the three major families of antioxidant enzymes that target hydrogen peroxide , an important signaling molecule for synaptic and neuronal activity, is also absent, most likely due to gene loss. They have been found to use L-glutamate as a neurotransmitter , and have an unusually high variety of ionotropic glutamate receptors and genes for glutamate synthesis and transport compared to other metazoans. The genomic content of
1568-566: A pair of lobes, which are muscular, cuplike extensions of the body that project beyond the mouth. Their inconspicuous tentacles originate from the corners of the mouth, running in convoluted grooves and spreading out over the inner surface of the lobes (rather than trailing far behind, as in the Cydippida). Between the lobes on either side of the mouth, many species of lobates have four auricles, gelatinous projections edged with cilia that produce water currents that help direct microscopic prey toward
1666-452: A relatively thick, jelly-like mesoglea sandwiched between two epithelia , layers of cells bound by inter-cell connections and by a fibrous basement membrane that they secrete . The epithelia of ctenophores have two layers of cells rather than one, and some of the cells in the upper layer have several cilia per cell. The outer layer of the epidermis (outer skin) consists of: sensory cells; cells that secrete mucus , which protects
1764-454: A ring around the mouth. The only known ctenophores with long nerves today is Euplokamis in the order Cydippida. Their nerve cells arise from the same progenitor cells as the colloblasts. In addition there is a less organized mesogleal nerve net consisting of single neurites. The largest single sensory feature is the aboral organ (at the opposite end from the mouth), which is underlined with its own nerve net. This organ's main component
1862-414: A similar appearance live in fresh water . Large, often colorful, jellyfish are common in coastal zones worldwide. The medusae of most species are fast-growing, and mature within a few months then die soon after breeding, but the polyp stage, attached to the seabed, may be much more long-lived. Jellyfish have been in existence for at least 500 million years, and possibly 700 million years or more, making them
1960-450: A solid surface by a basal disk, and resemble a polyp, the oral end of which has partially developed into a medusa with tentacle-bearing lobes and a central manubrium with four-sided mouth. Most jellyfish do not have specialized systems for osmoregulation , respiration and circulation , and do not have a central nervous system . Nematocysts, which deliver the sting, are located mostly on the tentacles; true jellyfish also have them around
2058-414: A supporting function. These normally beat so that the propulsion stroke is away from the mouth, although they can also reverse direction. Hence ctenophores usually swim in the direction in which the mouth is eating, unlike jellyfish . When trying to escape predators, one species can accelerate to six times its normal speed; some other species reverse direction as part of their escape behavior, by reversing
SECTION 20
#17328015276882156-499: A type of muscle that, in more complex animals, arises from the middle cell layer , and as a result some recent text books classify ctenophores as triploblastic , while others still regard them as diploblastic. The comb jellies have more than 80 different cell types , exceeding the numbers from other groups like placozoans, sponges, cnidarians, and some deep-branching bilaterians. Ranging from about 1 millimeter (0.04 in) to 1.5 meters (5 ft) in size, ctenophores are
2254-400: Is a statocyst , a balance sensor consisting of a statolith, a tiny grain of calcium carbonate, supported on four bundles of cilia , called "balancers", that sense its orientation. The statocyst is protected by a transparent dome made of long, immobile cilia. A ctenophore does not automatically try to keep the statolith resting equally on all the balancers. Instead, its response is determined by
2352-463: Is a common name, its mapping to biological groups is inexact. Some authorities have called the comb jellies and certain salps jellyfish, though other authorities state that neither of these are jellyfish, which they consider should be limited to certain groups within the medusozoa. The non-medusozoan clades called jellyfish by some but not all authorities (both agreeing and disagreeing citations are given in each case) are indicated with " ??? " on
2450-439: Is an intermediary to a better understanding of how visual systems evolved on Earth. Jellyfish exhibit immense variation in visual systems ranging from photoreceptive cell patches seen in simple photoreceptive systems to more derived complex eyes seen in box jellyfish. Major topics of jellyfish visual system research (with an emphasis on box jellyfish) include: the evolution of jellyfish vision from simple to complex visual systems),
2548-822: Is capable of multiple task-guided behaviors. Although they lack a true brain, cnidarian jellyfish have a "ring" nervous system that plays a significant role in motor and sensory activity. This net of nerves is responsible for muscle contraction and movement and culminates the emergence of photosensitive structures. Across Cnidaria , there is large variation in the systems that underlie photosensitivity. Photosensitive structures range from non-specialized groups of cells, to more "conventional" eyes similar to those of vertebrates . The general evolutionary steps to develop complex vision include (from more ancestral to more derived states): non-directional photoreception, directional photoreception, low-resolution vision, and high-resolution vision. Increased habitat and task complexity has favored
2646-404: Is circular rather than oval in cross-section, and the pharynx extends over the inner surfaces of the lobes. The Thalassocalycida , only discovered in 1978 and known from only one species, are medusa-like, with bodies that are shortened in the oral-aboral direction, and short comb-rows on the surface furthest from the mouth, originating from near the aboral pole. They capture prey by movements of
2744-405: Is not expected the populations will survive. The two limiting factors in saline lakes are availability of food and a varied diet, and high temperatures during hot summers. Because a parasitic isopod, Livoneca redmanii , was introduced at the same time, it is difficult to say how much of the ecological impact of invasive species is caused by the ctenophore alone. Ctenophores may be abundant during
2842-444: Is over they will not produce more gametes again until later. A population of Mertensia ovum in the central Baltic Sea have become paedogenetic , and consist solely of sexually mature larvae less than 1.6 mm. In Mnemiopsis leidyi , nitric oxide (NO) signaling is present both in adult tissues and differentially expressed in later embryonic stages suggesting the involvement of NO in developmental mechanisms. The mature form of
2940-660: Is partly parasitic . If food is plentiful, they can eat 10 times their own weight per day. While Beroe preys mainly on other ctenophores, other surface-water species prey on zooplankton (planktonic animals) ranging in size from the microscopic, including mollusc and fish larvae, to small adult crustaceans such as copepods , amphipods , and even krill . Members of the genus Haeckelia prey on jellyfish and incorporate their prey's nematocysts (stinging cells) into their own tentacles instead of colloblasts . Ctenophores have been compared to spiders in their wide range of techniques for capturing prey – some hang motionless in
3038-409: Is the umbrella-shaped bell. This is a hollow structure consisting of a mass of transparent jelly-like matter known as mesoglea , which forms the hydrostatic skeleton of the animal. The mesoglea is 95% or more composed of water, and also contains collagen and other fibrous proteins, as well as wandering amebocytes that can engulf debris and bacteria. The mesogloea is bordered by the epidermis on
Semaeostomeae - Misplaced Pages Continue
3136-403: The cydippid Pleurobrachia . Since the body of many species is almost radially symmetrical , the main axis is oral to aboral (from the mouth to the opposite end). However, since only two of the canals near the statocyst terminate in anal pores, ctenophores have no mirror-symmetry, although many have rotational symmetry. In other words, if the animal rotates in a half-circle it looks
3234-558: The green fluorescent protein used by some species for bioluminescence . This protein has been adapted as a fluorescent reporter for inserted genes and has had a large impact on fluorescence microscopy . The stinging cells used by jellyfish to subdue their prey can injure humans. Thousands of swimmers worldwide are stung every year, with effects ranging from mild discomfort to serious injury or even death. When conditions are favourable, jellyfish can form vast swarms, which may damage fishing gear by filling fishing nets, and sometimes clog
3332-400: The striated muscle . The wriggling motion is produced by smooth muscles , but of a highly specialized type. Coiling around prey is accomplished largely by the return of the tentilla to their inactive state, but the coils may be tightened by smooth muscle. There are eight rows of combs that run from near the mouth to the opposite end, and are spaced evenly round the body. The "combs" beat in
3430-701: The Greek suffix -φορος meaning "carrying". For a phylum with relatively few species, ctenophores have a wide range of body plans. Coastal species need to be tough enough to withstand waves and swirling sediment particles, while some oceanic species are so fragile that it is very difficult to capture them intact for study. In addition, oceanic species do not preserve well, and are known mainly from photographs and from observers' notes. Hence most attention has until recently concentrated on three coastal genera – Pleurobrachia , Beroe and Mnemiopsis . At least two textbooks base their descriptions of ctenophores on
3528-542: The Pelagiidae have no ring canal, and the marginal tentacles arise from umbrella margin. Three genera are in this family. This Scyphozoa -related article is a stub . You can help Misplaced Pages by expanding it . Jellyfish Jellyfish , also known as sea jellies , are the medusa -phase of certain gelatinous members of the subphylum Medusozoa , which is a major part of the phylum Cnidaria . Jellyfish are mainly free-swimming marine animals , although
3626-559: The adults of small crustaceans ; the exceptions are juveniles of two species, which live as parasites on the salps on which adults of their species feed. Despite their soft, gelatinous bodies, fossils thought to represent ctenophores appear in Lagerstätten dating as far back as the early Cambrian , about 525 million years ago. The position of the ctenophores in the "tree of life" has long been debated in molecular phylogenetics studies. Biologists proposed that ctenophores constitute
3724-676: The adults, lack both tentacles and tentacle sheaths. In some groups, such as the flat, bottom-dwelling platyctenids, the juveniles behave more like true larvae. They live among the plankton and thus occupy a different ecological niche from their parents, only attaining the adult form by a more radical ontogeny after dropping to the sea-floor. At least in some species, juvenile ctenophores appear capable of producing small quantities of eggs and sperm while they are well below adult size, and adults produce eggs and sperm for as long as they have sufficient food. If they run short of food, they first stop producing eggs and sperm, and then shrink in size. When
3822-431: The animal's buoyancy by pumping water into or out of the mesoglea. The outer surface bears usually eight comb rows, called swimming-plates, which are used for swimming. The rows are oriented to run from near the mouth (the "oral pole") to the opposite end (the "aboral pole"), and are spaced more or less evenly around the body, although spacing patterns vary by species and in most species the comb rows extend only part of
3920-415: The animal's "mood", in other words, the overall state of the nervous system. For example, if a ctenophore with trailing tentacles captures prey, it will often put some comb rows into reverse, spinning the mouth towards the prey. Research supports the hypothesis that the ciliated larvae in cnidarians and bilaterians share an ancient and common origin. The larvae's apical organ is involved in the formation of
4018-472: The animals. Among animal phyla, the Ctenophores are more complex than sponges , about as complex as cnidarians ( jellyfish , sea anemones , etc.), and less complex than bilaterians (which include almost all other animals). Unlike sponges, both ctenophores and cnidarians have: Ctenophores are distinguished from all other animals by having colloblasts , which are sticky and adhere to prey, although
Semaeostomeae - Misplaced Pages Continue
4116-415: The beating of their comb-rows. There are two known species, with worldwide distribution in warm, and warm-temperate waters: Cestum veneris (" Venus ' girdle") is among the largest ctenophores – up to 1.5 meters (4.9 ft) long, and can undulate slowly or quite rapidly. Velamen parallelum , which is typically less than 20 centimeters (0.66 ft) long, can move much faster in what has been described as
4214-443: The bell and possibly by using two short tentacles. The Cestida ("belt animals") are ribbon-shaped planktonic animals, with the mouth and aboral organ aligned in the middle of opposite edges of the ribbon. There is a pair of comb-rows along each aboral edge, and tentilla emerging from a groove all along the oral edge, which stream back across most of the wing-like body surface. Cestids can swim by undulating their bodies as well as by
4312-452: The bell pulsates, allowing box jellyfish to swim faster than true jellyfish. Hydrozoans are also similar, usually with just four tentacles at the edge of the bell, although many hydrozoans are colonial and may not have a free-living medusal stage. In some species, a non-detachable bud known as a gonophore is formed that contains a gonad but is missing many other medusal features such as tentacles and rhopalia. Stalked jellyfish are attached to
4410-597: The body surfaces of other invertebrates, and are often revealed by their long tentacles with many side branches, seen streaming off the back of the ctenophore into the current. Adults of most species can regenerate tissues that are damaged or removed, although only platyctenids reproduce by cloning , splitting off from the edges of their flat bodies fragments that develop into new individuals. Lab research on Mnemiopsis leidyi also show that when two individuals have parts of their bodies removed, they are able to fuse together, including their nervous and digestive systems, even when
4508-412: The body; and interstitial cells, which can transform into other types of cell. In specialized parts of the body, the outer layer also contains colloblasts , found along the surface of tentacles and used in capturing prey, or cells bearing multiple large cilia, for locomotion. The inner layer of the epidermis contains a nerve net , and myoepithelial cells that act as muscles . The internal cavity forms:
4606-441: The cilia on their comb rows for propulsion, although Leucothea has long and active auricles whose movements also contribute to propulsion. Members of the lobate genera Bathocyroe and Ocyropsis can escape from danger by clapping their lobes, so that the jet of expelled water drives them back very quickly. Unlike cydippids, the movements of lobates' combs are coordinated by nerves rather than by water disturbances created by
4704-452: The cilia, yet combs on the same row beat in the same Mexican wave style as the mechanically coordinated comb rows of cydippids and beroids. This may have enabled lobates to grow larger than cydippids and to have less egg-like shapes. An unusual species first described in 2000, Lobatolampea tetragona , has been classified as a lobate, although the lobes are "primitive" and the body is medusa -like when floating and disk-like when resting on
4802-471: The common moon jelly , Aurelia aurita , which migrates in response to changes in ambient light and solar position even though they lack proper eyes. Ctenophores Ctenophora ( / t ə ˈ n ɒ f ər ə / tə- NOF -ər-ə ; sg. : ctenophore / ˈ t ɛ n ə f ɔːr , ˈ t iː n ə -/ TEN -ə-for, TEE -nə- ; from Ancient Greek κτείς (kteis) 'comb' and φέρω (pherō) 'to carry') comprise
4900-409: The common coastal "sea gooseberry", Pleurobrachia , sometimes has an egg-shaped body with the mouth at the narrow end, although some individuals are more uniformly round. From opposite sides of the body extends a pair of long, slender tentacles, each housed in a sheath into which it can be withdrawn. Some species of cydippids have bodies that are flattened to various extents so that they are wider in
4998-448: The cooling systems of power and desalination plants which draw their water from the sea. The name jellyfish, in use since 1796, has traditionally been applied to medusae and all similar animals including the comb jellies ( ctenophores , another phylum). The term jellies or sea jellies is more recent, having been introduced by public aquaria in an effort to avoid use of the word "fish" with its modern connotation of an animal with
SECTION 50
#17328015276885096-479: The cydippid genus Pleurobrachia , are incapable of bioluminescence. When some species, including Bathyctena chuni , Euplokamis stationis and Eurhamphaea vexilligera , are disturbed, they produce secretions (ink) that luminesce at much the same wavelengths as their bodies. Juveniles will luminesce more brightly in relation to their body size than adults, whose luminescence is diffused over their bodies. Detailed statistical investigation has not suggested
5194-458: The deep-sea genus Bathocyroe is red, which hides the bioluminescence of copepods it has swallowed. The comb rows of most planktonic ctenophores produce a rainbow effect, which is not caused by bioluminescence but by the scattering of light as the combs move. Most species are also bioluminescent, but the light is usually blue or green and can only be seen in darkness. However some significant groups, including all known platyctenids and
5292-418: The distance from the aboral pole towards the mouth. The "combs" (also called "ctenes" or "comb plates") run across each row, and each consists of thousands of unusually long cilia, up to 2 millimeters (0.08 in). Unlike conventional cilia and flagella, which has a filament structure arranged in a 9 + 2 pattern, these cilia are arranged in a 9 + 3 pattern, where the extra compact filament is suspected to have
5390-451: The eight comb rows. In the genome of Mnemiopsis leidyi ten genes encode photoproteins. These genes are co-expressed with opsin genes in the developing photocytes of Mnemiopsis leidyi , raising the possibility that light production and light detection may be working together in these animals. Ctenophores are found in most marine environments: from polar waters at −2 °C to the tropics at 30 °C; near coasts and in mid-ocean; from
5488-421: The exterior, perhaps supplying good oxygenation to the gonads. Near the free edges of the septa, gastric filaments extend into the gastric cavity; these are armed with nematocysts and enzyme-producing cells and play a role in subduing and digesting the prey. In some scyphozoans, the gastric cavity is joined to radial canals which branch extensively and may join a marginal ring canal. Cilia in these canals circulate
5586-694: The eye morphology and molecular structures of box jellyfish (including comparisons to vertebrate eyes), and various uses of vision including task-guided behaviors and niche specialization. Experimental evidence for photosensitivity and photoreception in cnidarians antecedes the mid 1900s, and a rich body of research has since covered evolution of visual systems in jellyfish. Jellyfish visual systems range from simple photoreceptive cells to complex image-forming eyes. More ancestral visual systems incorporate extraocular vision (vision without eyes) that encompass numerous receptors dedicated to single-function behaviors. More derived visual systems comprise perception that
5684-400: The fluid in a regular direction. The box jellyfish is largely similar in structure. It has a squarish, box-like bell. A short pedalium or stalk hangs from each of the four lower corners. One or more long, slender tentacles are attached to each pedalium. The rim of the bell is folded inwards to form a shelf known as a velarium which restricts the bell's aperture and creates a powerful jet when
5782-965: The following phylogenetic tree by the presence of citations. Names of included jellyfish, in English where possible, are shown in boldface; the presence of a named and cited example indicates that at least that species within its group has been called a jellyfish. Anthozoa (corals) Polypodiozoa and Myxozoa (parasitic cnidarians) Staurozoa ( stalked jellyfish ) [REDACTED] Cubozoa ( box jellyfish ) [REDACTED] Discomedusae [REDACTED] Coronatae ( crown jellyfish ) [REDACTED] Aplanulata [REDACTED] Siphonophorae [REDACTED] Some Leptothecata e.g. crystal jelly [REDACTED] Filifera e.g. red paper lantern jellyfish [REDACTED] Limnomedusae , e.g. flower hat jelly [REDACTED] Narcomedusae , e.g. cosmic jellyfish [REDACTED] The subphylum Medusozoa includes all cnidarians with
5880-572: The following cladogram of the animal kingdom: Porifera Ctenophora (comb jellies) ??? [REDACTED] Cnidaria [REDACTED] (includes jellyfish and other jellies) Protostomia Ambulacraria Tunicata (includes salps ) ??? [REDACTED] Vertebrata Jellyfish are not a clade , as they include most of the Medusozoa, barring some of the Hydrozoa. The medusozoan groups included by authorities are indicated on
5978-497: The following general definition: Typically, medusozoan cnidarians have a pelagic , predatory jellyfish stage in their life cycle; staurozoans are the exceptions [as they are stalked]. The Merriam-Webster dictionary defines jellyfish as follows: A free-swimming marine coelenterate that is the sexually reproducing form of a hydrozoan or scyphozoan and has a nearly transparent saucer-shaped body and extensible marginal tentacles studded with stinging cells. Given that jellyfish
SECTION 60
#17328015276886076-504: The food supply improves, they grow back to normal size and then resume reproduction. These features make ctenophores capable of increasing their populations very quickly. Members of the Lobata and Cydippida also have a reproduction form called dissogeny; two sexually mature stages, first as larva and later as juveniles and adults. During their time as larva they are capable of releasing gametes periodically. After their first reproductive period
6174-420: The function of ctenophores' bioluminescence nor produced any correlation between its exact color and any aspect of the animals' environments, such as depth or whether they live in coastal or mid-ocean waters. In ctenophores, bioluminescence is caused by the activation of calcium-activated proteins named photoproteins in cells called photocytes , which are often confined to the meridional canals that underlie
6272-447: The genus Haeckelia , which feed mainly on jellyfish , incorporate their victims' stinging nematocytes into their own tentacles – some cnidaria-eating nudibranchs similarly incorporate nematocytes into their bodies for defense. The tentilla of Euplokamis differ significantly from those of other cydippids: they contain striated muscle , a cell type otherwise unknown in the phylum Ctenophora; and they are coiled when relaxed, while
6370-424: The genus Mnemiopsis , and it is thought that most of the hermaphroditic species are self-fertile. Development of the fertilized eggs is direct; there is no distinctive larval form. Juveniles of all groups are generally planktonic , and most species resemble miniature adult cydippids, gradually developing their adult body forms as they grow. In the genus Beroe , however, the juveniles have large mouths and, like
6468-420: The genus Ocyropsis and Bathocyroe fosteri in the genus Bathocyroe . The gonads are located in the parts of the internal canal network under the comb rows, and eggs and sperm are released via pores in the epidermis. Fertilization is generally external , but platyctenids use internal fertilization and keep the eggs in brood chambers until they hatch. Self-fertilization has occasionally been seen in species of
6566-510: The high-resolution visual systems common in derived cnidarians such as box jellyfish . Basal visual systems observed in various cnidarians exhibit photosensitivity representative of a single task or behavior. Extraocular photoreception (a form of non-directional photoreception), is the most basic form of light sensitivity and guides a variety of behaviors among cnidarians. It can function to regulate circadian rhythm (as seen in eyeless hydrozoans ) and other light-guided behaviors responsive to
6664-646: The intensity and spectrum of light. Extraocular photoreception can function additionally in positive phototaxis (in planula larvae of hydrozoans), as well as in avoiding harmful amounts of UV radiation via negative phototaxis . Directional photoreception (the ability to perceive direction of incoming light) allows for more complex phototactic responses to light, and likely evolved by means of membrane stacking. The resulting behavioral responses can range from guided spawning events timed by moonlight to shadow responses for potential predator avoidance. Light-guided behaviors are observed in numerous scyphozoans including
6762-486: The largest non-colonial animals that use cilia as their main method of locomotion. Most species have eight strips, called comb rows, that run the length of their bodies and bear comb-like bands of cilia, called "ctenes", stacked along the comb rows so that when the cilia beat, those of each comb touch the comb below. The name "ctenophora" means "comb-bearing", from the Greek κτείς (stem-form κτεν- ) meaning "comb" and
6860-416: The mouth "lips" in some species of Beroe , is a pair of narrow strips of adhesive epithelial cells on the stomach wall that "zip" the mouth shut when the animal is not feeding, by forming intercellular connections with the opposite adhesive strip. This tight closure streamlines the front of the animal when it is pursuing prey. The Ganeshida has a pair of small oral lobes and a pair of tentacles. The body
6958-470: The mouth and stomach. Jellyfish do not need a respiratory system because sufficient oxygen diffuses through the epidermis. They have limited control over their movement, but can navigate with the pulsations of the bell-like body; some species are active swimmers most of the time, while others largely drift. The rhopalia contain rudimentary sense organs which are able to detect light, water-borne vibrations, odour and orientation. A loose network of nerves called
7056-577: The mouth – two of these four branches terminate in anal pores. The inner surface of the cavity is lined with an epithelium , the gastrodermis . The mouth and pharynx have both cilia and well-developed muscles. In other parts of the canal system, the gastrodermis is different on the sides nearest to and furthest from the organ that it supplies. The nearer side is composed of tall nutritive cells that store nutrients in vacuoles (internal compartments), germ cells that produce eggs or sperm, and photocytes that produce bioluminescence . The side furthest from
7154-447: The mouth, which also functions as the anus, at its tip. There are often four oral arms connected to the manubrium, streaming away into the water below. The mouth opens into the gastrovascular cavity , where digestion takes place and nutrients are absorbed. This is subdivided by four thick septa into a central stomach and four gastric pockets. The four pairs of gonads are attached to the septa, and close to them four septal funnels open to
7252-411: The mouth. This combination of structures enables lobates to feed continuously on suspended planktonic prey. Lobates have eight comb-rows, originating at the aboral pole and usually not extending beyond the body to the lobes; in species with (four) auricles, the cilia edging the auricles are extensions of cilia in four of the comb rows. Most lobates are quite passive when moving through the water, using
7350-678: The nervous system genes is the smallest known of any animal, and could represent the minimum genetic requirements for a functional nervous system. The fact that portions of the nervous system feature directly fused neurons, without synapses, suggests that ctenophores might form a sister group to other metazoans, having developed a nervous system independently. If ctenophores are the sister group to all other metazoans, nervous systems may have either been lost in sponges and placozoans, or arisen more than once among metazoans. Cydippid ctenophores have bodies that are more or less rounded, sometimes nearly spherical and other times more cylindrical or egg-shaped;
7448-518: The nervous system. The aboral organ of comb jellies is not homologous with the apical organ in other animals, and the formation of their nervous system has therefore a different embryonic origin. Ctenophore nerve cells and nervous system have different biochemistry as compared to other animals. For instance, they lack the genes and enzymes required to manufacture neurotransmitters like serotonin , dopamine , nitric oxide , octopamine , noradrenaline , and others, otherwise seen in all other animals with
7546-420: The neurons are found to have synaptic connections , but the neurons in the nerve net are highly distinctive by being fused into a syncytium , rather than being connected by synapses. Some animals outside ctenophores also have fused nerve cells, but never to such a degree that they form a whole nerve net. Fossils shows that Cambrian species had a more complex nervous system, with long nerves which connected with
7644-430: The nutritive cells. The ciliary rosettes in the canals may help to transport nutrients to muscles in the mesoglea. The anal pores may eject unwanted small particles, but most unwanted matter is regurgitated via the mouth. Little is known about how ctenophores get rid of waste products produced by the cells. The ciliary rosettes in the gastrodermis may help to remove wastes from the mesoglea, and may also help to adjust
7742-506: The oldest multi-organ animal group. Jellyfish are eaten by humans in certain cultures. They are considered a delicacy in some Asian countries, where species in the Rhizostomeae order are pressed and salted to remove excess water. Australian researchers have described them as a "perfect food": sustainable and protein-rich but relatively low in food energy . They are also used in cell and molecular biology research, especially
7840-402: The open lagoon, where they feed, and back again. Box jellyfish have more advanced vision than the other groups. Each individual has 24 eyes, two of which are capable of seeing colour, and four parallel information processing areas that act in competition, supposedly making them one of the few kinds of animal to have a 360-degree view of its environment. The study of jellyfish eye evolution
7938-415: The organ is covered with ciliated cells that circulate water through the canals, punctuated by ciliary rosettes, pores that are surrounded by double whorls of cilia and connect to the mesoglea. When prey is swallowed, it is liquefied in the pharynx by enzymes and by muscular contractions of the pharynx. The resulting slurry is wafted through the canal system by the beating of the cilia , and digested by
8036-415: The outside and the gastrodermis on the inside. The edge of the bell is often divided into rounded lobes known as lappets , which allow the bell to flex. In the gaps or niches between the lappets are dangling rudimentary sense organs known as rhopalia , and the margin of the bell often bears tentacles. On the underside of the bell is the manubrium, a stalk-like structure hanging down from the centre, with
8134-435: The plane of the tentacles. The tentacles of cydippid ctenophores are typically fringed with tentilla ("little tentacles"), although a few genera have simple tentacles without these side branches. The tentacles and tentilla are densely covered with microscopic colloblasts that capture prey by sticking to it. Colloblasts are specialized mushroom -shaped cells in the outer layer of the epidermis, and have three main components:
8232-448: The polyps. Medusozoans have tetramerous symmetry, with parts in fours or multiples of four. The four major classes of medusozoan Cnidaria are: There are over 200 species of Scyphozoa, about 50 species of Staurozoa, about 50 species of Cubozoa, and the Hydrozoa includes about 1000–1500 species that produce medusae, but many more species that do not. Since jellyfish have no hard parts, fossils are rare. The oldest unambiguous fossil of
8330-526: The power stroke of the comb plate cilia. It is uncertain how ctenophores control their buoyancy, but experiments have shown that some species rely on osmotic pressure to adapt to the water of different densities. Their body fluids are normally as concentrated as seawater. If they enter less dense brackish water, the ciliary rosettes in the body cavity may pump this into the mesoglea to increase its bulk and decrease its density, to avoid sinking. Conversely, if they move from brackish to full-strength seawater,
8428-514: The presence of specific ctenophore genes that were markedly different from those of other species. Follow up analysis by Whelan et al. (2017) yielded further support for the 'Ctenophora sister' hypothesis; the issue remains a matter of taxonomic dispute. Schultz et al. (2023) found irreversible changes in synteny in the sister of the Ctenophora, the Myriazoa , consisting of the rest of
8526-404: The rhopalia include ocelli , light-sensitive organs able to tell light from dark. These are generally pigment spot ocelli, which have some of their cells pigmented. The rhopalia are suspended on stalks with heavy crystals at one end, acting like gyroscopes to orient the eyes skyward. Certain jellyfish look upward at the mangrove canopy while making a daily migration from mangrove swamps into
8624-472: The rosettes may pump water out of the mesoglea to reduce its volume and increase its density. Ctenophores have no brain or central nervous system , but instead have a subepidermal nerve net (rather like a cobweb) that forms a ring round the mouth and is densest near structures such as the comb rows, pharynx, tentacles (if present) and the sensory complex furthest from the mouth. The communication between nerve cells make use of two different methods; some of
8722-447: The same as when it started. The Ctenophore phylum has a wide range of body forms, including the flattened, deep-sea platyctenids , in which the adults of most species lack combs, and the coastal beroids , which lack tentacles and prey on other ctenophores by using huge mouths armed with groups of large, stiffened cilia that act as teeth. Like those of cnidarians , ( jellyfish , sea anemones , etc.), ctenophores' bodies consist of
8820-513: The same species is also able to revert back to the cydippid stage when triggered by environmental stressors. Most ctenophores that live near the surface are mostly colorless and almost transparent. However some deeper-living species are strongly pigmented, for example the species known as "Tortugas red" (see illustration here), which has not yet been formally described. Platyctenids generally live attached to other sea-bottom organisms, and often have similar colors to these host organisms. The gut of
8918-411: The sea-bed. The Beroida , also known as Nuda , have no feeding appendages, but their large pharynx , just inside the large mouth and filling most of the saclike body, bears "macrocilia" at the oral end. These fused bundles of several thousand large cilia are able to "bite" off pieces of prey that are too large to swallow whole – almost always other ctenophores. In front of the field of macrocilia, on
9016-453: The second-earliest branching animal lineage, with sponges being the sister-group to all other multicellular animals ( Porifera sister hypothesis ). Other biologists contend that ctenophores emerged earlier than sponges ( Ctenophora sister hypothesis ), which themselves appeared before the split between cnidarians and bilaterians . Pisani et al . reanalyzed the data and suggested that the computer algorithms used for analysis were misled by
9114-473: The summer months in some coastal locations, but in other places, they are uncommon and difficult to find. In bays where they occur in very high numbers, predation by ctenophores may control the populations of small zooplanktonic organisms such as copepods , which might otherwise wipe out the phytoplankton (planktonic plants), which are a vital part of marine food chains . Almost all ctenophores are predators – there are no vegetarians and only one genus that
9212-503: The surface waters to the ocean depths at more than 7000 meters. The best-understood are the genera Pleurobrachia , Beroe and Mnemiopsis , as these planktonic coastal forms are among the most likely to be collected near shore. No ctenophores have been found in fresh water. In 2013 Mnemiopsis was recorded in lake Birket Qarun, and in 2014 in lake El Rayan II, both near Faiyum in Egypt, where they were accidentally introduced by
9310-400: The tentilla of all other known ctenophores elongate when relaxed. Euplokamis ' tentilla have three types of movement that are used in capturing prey: they may flick out very quickly (in 40 to 60 milliseconds ); they can wriggle, which may lure prey by behaving like small planktonic worms; and they coil round prey. The unique flicking is an uncoiling movement powered by contraction of
9408-399: The transport of fish (mullet) fry. Though many species prefer brackish waters like estuaries and coastal lagoons in open connection with the sea, this was the first record from an inland environment. Both lakes are saline, with Birket Qarun being hypersaline, and shows that some ctenophores can establish themselves in saline limnic environments without connection to the ocean. In the long run it
9506-537: The two individuals are genetically different; a phenomenon that has so far only been found in comb jellies. The last common ancestor (LCA) of the ctenophores was hermaphroditic . Some are simultaneous hermaphrodites, which can produce both eggs and sperm at the same time, while others are sequential hermaphrodites, in which the eggs and sperm mature at different times. There is no metamorphosis . At least three species are known to have evolved separate sexes ( dioecy ); Ocyropsis crystallina and Ocyropsis maculata in
9604-542: The water using their tentacles as "webs", some are ambush predators like Salticid jumping spiders , and some dangle a sticky droplet at the end of a fine thread, as bolas spiders do. This variety explains the wide range of body forms in a phylum with rather few species. The two-tentacled "cydippid" Lampea feeds exclusively on salps , close relatives of sea-squirts that form large chain-like floating colonies, and juveniles of Lampea attach themselves like parasites to salps that are too large for them to swallow. Members of
#687312