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38-400: 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 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

76-399: 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,

114-718: A "perfect food": sustainable and protein-rich but relatively low in food energy . They are also used in cell and molecular biology research, especially 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

152-505: 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

190-415: 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 a complex life cycle , and the medusa is normally the sexual phase, which produces planula larvae. These then disperse widely and enter

228-578: 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 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

266-528: 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

304-407: 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

342-452: 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 the mouth and stomach. Jellyfish do not need a respiratory system because sufficient oxygen diffuses through

380-457: A middle undifferentiated layer called mesoglea, and radial symmetry. Some examples are corals , which are typically colonial; hydrae , jellyfish , sea anemones , and Aurelia , which are solitary; Pennatula ; Portuguese man o' war ; Gorgonia ; and Physalia . Coelenterata lack a specialized circulatory system, relying instead on diffusion across the tissue layers. All coelenterates are aquatic , mostly marine , animals. The body form

418-441: 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 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

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456-608: A subphylum of phylum Chordata . Subphylum is: Where convenient, subphyla in turn can be divided into infraphyla ; in turn such an infraphylum also would be superordinate to any classes or superclasses in the hierarchy . Not all fauna phyla are divided into subphyla. Those that are include: Examples of infraphyla include the Mycetozoa , the Gnathostomata and the Agnatha . Coelenterata Coelenterata

494-485: Is radially symmetrical , diploblastic and does not have a coelom . The body has a single opening, the hypostome , surrounded by sensory tentacles equipped with either nematocysts or colloblasts to capture mostly planktonic prey. These tentacles are surrounded by a spacious cavity called the gastrovascular cavity , or coelenteron. Digestion is both intracellular and extracellular. Respiration and excretion are accomplished by simple diffusion. A network of nerves

532-459: 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

570-494: Is a term encompassing the animal phyla Cnidaria ( coral animals, true jellies, sea anemones, sea pens , and their relatives) and Ctenophora (comb jellies). The name comes from Ancient Greek κοῖλος ( koîlos )  'hollow' and ἔντερον ( énteron )  'intestine', referring to the hollow body cavity common to these two phyla. They have very simple tissue organization, with only two layers of cells ( ectoderm and endoderm ), along with

608-437: 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),

646-819: 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

684-615: Is spread throughout the body. Many Cnidaria exhibit polymorphism , wherein different types of individuals are present in a colony for different functions. These individuals are called zooids . These animals generally reproduce asexually by budding, though sexual reproduction does occur in some groups. The scientific validity of the term coelenterate is currently rejected, as the Cnidaria and Ctenophora have less in common than previously assumed. Coelentera may only be monophyletic if both Placozoa and Bilateria are included. In particular,

722-407: 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

760-450: The common moon jelly , Aurelia aurita , which migrates in response to changes in ambient light and solar position even though they lack proper eyes. Subphylum The taxonomic rank of " subdivision " in fungi and plant taxonomy is equivalent to "subphylum" in zoological taxonomy. Some plant taxonomists have also used the rank of subphylum, for instance monocotyledons as a subphylum of phylum Angiospermae and vertebrates as

798-445: 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

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836-413: 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 a solid surface by a basal disk, and resemble a polyp, the oral end of which has partially developed into

874-448: 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 a " nerve net " is located in the epidermis . Although traditionally thought not to have

912-421: 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

950-693: 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

988-541: 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 the bell pulsates, allowing box jellyfish to swim faster than true jellyfish. Hydrozoans are also similar, usually with just four tentacles at

1026-956: 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

1064-566: 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

1102-495: 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

1140-509: 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

1178-642: 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

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1216-446: 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

1254-399: 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

1292-414: 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

1330-447: The phylogenetic position of Ctenophora is controversial; it was first considered a sub-group of coelenterata but Hyman regarded it as a separate phylum. Most researchers think that Coelenterata is not monophyletic , and therefore any group containing Cnidaria and Ctenophora but excluding other phyla would be paraphyletic . Previously, some genomic studies have found support for monophyletic coelenterates. Despite this uncertainty,

1368-444: 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

1406-404: 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

1444-455: The term coelenterate is still used in informal settings to refer to the Cnidaria and Ctenophora. Complicating the issue is the 1997 work of Lynn Margulis (revising an earlier model by Thomas Cavalier-Smith ) that placed the Cnidaria and Ctenophora alone in the branch Radiata within Eumetazoa . (The latter refers to all the animals except the sponges and Placozoa ). Neither grouping

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