Cnidaria Scyphozoa Aurelia

Jellyfish or jellies are softbodied, free-swimming aquatic animals with a gelatinous umbrella-shaped bell and trailing tentacles. The bell can pulsate to acquire propulsion and locomotion. The tentacles may be utilized to capture prey or defend against predators by emitting toxins in a painful sting. Jellyfish species are classified in the subphylum Medusozoa which makes up a major part of the phylum Cnidaria, although not all Medusozoa species are considered to be jellyfish.

Jellyfish are found in every ocean, from the surface to the deep sea. Scyphozoans are exclusively marine, but some hydrozoans live in freshwater. Large, often colorful, jellyfish are common in coastal zones worldwide. Jellyfish have roamed the seas for at least 500 million years, and possibly 700 million years or more, making them the oldest multi-organ animal.

Terminology
The popular English name jellyfish has been in use since 1796. It has traditionally also been applied to other animals sharing a superficial resemblance, for example ctenophores (members from another phylum of common, gelatinous and generally transparent or translucent, free-swimming planktonic carnivores now known as comb jellies) were included as "jellyfishes". Even some scientists include the phylum ctenophora when they are referring to jellyfish. Other scientists prefer to use the more all-encompassing term gelatinous zooplankton, when referring to these, together with other soft-bodied animals in the water column.

As jellyfish are not true fish, which are vertebrates, the word jellyfish is considered by some to be a misnomer. Public aquaria often use the terms jellies or sea jellies instead. The term "jellies" may have become more popular than "jellyfish". In scientific literature, "jelly" and "jellyfish" are often used interchangeably. Some sources may use the term "jelly" to refer to organisms in this taxon, as "jellyfish" may be considered inappropriate.

Many textbooks and sources refer to only scyphozoans as "true jellyfish".

A group of jellyfish is sometimes called a bloom or a swarm. "Bloom" is usually used for a large group of jellyfish that gather in a small area, but may also have a time component, referring to seasonal increases, or numbers beyond what was expected. Other collective names for a group of jellyfish are "fluther" and "smack," though neither term is commonly used by scientists who study jellyfish. Jellyfish are "bloomy" by nature of their life cycles, being produced by their benthic polyps usually in the spring when sunshine and plankton increase, so they appear rather suddenly and often in large numbers, even when an ecosystem is in balance. Using "swarm" usually implies some kind of active ability to stay together, which a few species such as Aurelia, the moon jelly, demonstrate.

Medusa jellyfish may be classified as scyphomedusae ("true" jellyfish), stauromedusae (stalked jellyfish), cubomedusae (box jellyfish), or hydromedusae, according to which clade their species belongs.

The term medusa was coined by Linnaeus in 1752, alluding to the tentacled head of Medusa in Greek mythology. This term refers exclusively to the non-polyp life-stage which occurs in many cnidarians, which is typified by a large pulsating gelatinous bell with long trailing tentacles. All medusa-producing species belong to the sub-phylum Medusozoa.

In biology, a medusa (plural: medusae) is a form of cnidarian in which the body is shaped like an umbrella, in contrast with polyps. Medusae vary from bell-shaped to the shape of a thin disk, scarcely convex above and only slightly concave below. The upper or aboral surface is called the exumbrella and the lower surface is called the subumbrella; the mouth is located on the lower surface, which may be partially closed by a membrane extending inward from the margin (called the velum). The digestive cavity consists of the gastrovascular cavity and radiating canals which extend toward the margin; these canals may be simple or branching, and vary in number from few to many. The margin of the disk bears sensory organs and tentacles.

German biologist Ernst Haeckel popularized medusae through his vivid illustrations, particularly in Kunstformen der Natur.

Anatomy
Most jellyfish do not have specialized digestive, osmoregulatory, central nervous, respiratory, or circulatorysystems. The manubrium is a stalk-like structure hanging down from the centre of the underside, often surrounded by oral arms, which connects with the mouth/anus at the base of the bell. This opens into the gastrovascular cavity, where digestion takes place and nutrients are absorbed. It is joined to the radial canals which extend to the margin of the bell, where tentacles are attached. Nematocysts, which deliver the sting, are located mostly on the tentacles; Scyphozoans also have them around the mouth and stomach. Jellyfish do not need a respiratory system since their skin is thin enough that the body is oxygenated by diffusion. They have limited control over movement, but can use their hydrostatic skeleton to navigate through contraction-pulsations of the bell-like body; some species actively swim most of the time, while others are mostly passive. Depending on the species, the body contains between 95 and 98% water. Most of the umbrella mass is a gelatinous material — the jelly — called mesoglea which is surrounded by two layers of protective skin. The top layer is called the epidermis, and the inner layer is referred to as gastrodermis, which lines the gut.

Nervous system
Jellyfish employ a loose network of nerves, located in the epidermis, which is called a "nerve net". 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 various stimuli including the touch of other animals via this nerve net, which then transmits impulses both throughout the nerve net and around a circular nerve ring, through the rhopalial lappet, located at the rim of the jellyfish body, to other nerve cells.

Vision
Some jellyfish have ocelli: light-sensitive organs that do not form images but which can detect light and are used to determine up from down, responding to sunlight shining on the water's surface. These are generally pigment spot ocelli, which have some cells (not all) pigmented.

Certain species of jellyfish, such as the box jellyfish, have more advanced vision than their counterparts. The box jellyfish has 24 eyes, two of which are capable of seeing color, and four parallel information processing areas or rhopalia that act in competition, supposedly making it one of the few creatures to have a 360-degree view of its environment.

The eyes are suspended on stalks with heavy crystals on one end, acting like a gyroscope to orient the eyes skyward. They look upward to navigate from roots in mangrove swamps to the open lagoon and back, watching for the mangrove canopy, where they feed.

Size
Jellyfish range from about one millimeter in bell height and diameter to nearly 2 metres (6.6 ft) in bell height and diameter; the tentacles and mouth parts usually extend beyond this bell dimension.

The smallest jellyfish are the peculiar creeping jellyfish in the genera Staurocladia and Eleutheria, which have bell disks from 0.5 mm to a few millimeters in diameter, with short tentacles that extend out beyond this, which these jellyfish use to move across the surface of seaweed or the bottoms of rocky pools. Many of these tiny creeping jellyfish cannot be seen in the field without a hand lens or microscope; they can reproduce asexually by splitting in half (called fission). Other very small jellyfish, which have bells about one millimeter, are the hydromedusae of many species that have just been released from their parent polyps; some of these live only a few minutes before shedding their gametes in the plankton and then dying, while others will grow in the plankton for weeks or months. The hydromedusae Cladonema radiatum and Cladonema californicum are also very small, living for months, yet never growing beyond a few mm in bell height and diameter. Another small species of jellyfish is the Australian Irukandji, which is about the size of a fingernail.

The lion's mane jellyfish, Cyanea capillata, was long-cited as the largest jellyfish, and arguably the longest animal in the world, with fine, thread-like tentacles that may extend up to 36.5 metres (120 ft) long (though most are nowhere near that large). They have a moderately painful, but rarely fatal, sting.

The increasingly common giant Nomura's jellyfish, Nemopilema nomurai, found in some, but not all years in the waters of Japan, Korea and China in summer and autumn is another candidate for "largest jellyfish", in terms of diameter and weight, since the largest Nomura's jellyfish in late autumn can reach 200 centimetres (79 in) in bell (body) diameter and about 200 kilograms (440 lb) in weight, with average specimens frequently reaching 90 centimetres (35 in) in bell diameter and about 150 kilograms (330 lb) in weight. The large bell mass of the giant Nomura's jellyfish can dwarf a diver and is nearly always much greater than the up-to-100 centimetres (39 in) bell diameter Lion's Mane.

The rarely encountered deep-sea jellyfish Stygiomedusa gigantea is another candidate for "largest jellyfish", with its thick, massive bell up to 100 centimetres (39 in) wide, and four thick, "strap-like" oral arms extending up to 6 metres (20 ft) in length, very different from the typical fine, threadlike tentacles that rim the umbrella of more-typical-looking jellyfish, including the Lion's Mane.

Taxonomy
Jellyfish belong to Medusozoa, the clade of cnidarians which excludes Anthozoa (e.g., corals and anemones). This suggests that the medusa form evolved after the polyps.

The phylogenetics of this group are complex and evolving. The Medusozoa and Octocorallia are proposed as sister groups according to research published in 2012. That research also proposes coronate Scyphozoa and Cubozoa as a sister clade to Hydrozoa and discomedusan Scyphozoa, which are themselves sister groups. The hydroidolinans are a sister group to Limnomedusae, also called Trachylina. Semaeostomae is paraphyletic with Rhizostomeae. The class Storozoa was the earliest group of Medusozoa to diverge and the Limnomedusae were the earliest Hydrozoa to diverge.

The four major classes of medusozoan Cnidaria are: Some other animals are frequently associated with or mistaken for medusa jellyfish. There are over 200 species of Scyphozoa, about 50 species of Staurozoa, about 20 species of Cubozoa, and the Hydrozoa includes about 1000–1500 species that produce medusae (and many more hydrozoan species that do not produce medusae).
 * Scyphozoa are sometimes called true jellyfish, though they are no more truly jellyfish than the others listed here. They have tetra-radial symmetry. Most have tentacles around the outer margin of the bowl-shaped bell, and long, oral arms around the mouth in the center of the subumbrella.
 * Cubozoa (box jellyfish) have a (rounded) box-shaped bell, and their velarium assists them to swim more quickly. Box jellyfish may be related more closely to scyphozoan jellyfish than either are to the Hydrozoa.
 * Hydrozoa medusae also have tetra-radial symmetry, nearly always have a velum attached just inside the bell margin, do not have oral arms, but a much smaller central manubrium with terminal mouth opening, and are distinguished by the absence of cells in the mesoglea. The majority of hydrozoan species maintain the polyp form for their entire life cycle and do not form medusa at all (such as Hydra, which is hence not considered a jellyfish).
 * Staurozoa (stalked jellyfish) are characterized by a medusa form that is generally sessile, oriented upside down and with a stalk emerging from the apex of the "calyx" (bell), which attaches to the substrate. Some Staurozoa (all? – it is not known yet) also have a polyp form that alternates with the medusoid portion of the life cycle. Until recently, Staurozoa were classified within the Scyphozoa.
 * Siphonophorae are an order of hydrozoa which generally live as colonies (for example, free-swimming chains of repeated units, some units similar to polyps or to medusa). They are not considered medusa jellyfish. A well-known example is the Portuguese man o' war.
 * Ctenophores (comb jellies) are a separate phylum from Cnidaria. Their method of propulsion is coordinated movement of thousands of cilia used as paddles, rather than a pulsating bell, although a few species of ctenophores swim by flapping large lobes.
 * Salps are transparent, gelatinous marine organisms that form pelagic colonies like siphonophores. Salps are chordates, and as such are actually more closely related to humans than they are to cnidarians and comb jellies.

Many scientists who work on relationships between these groups are reluctant to assign ranks, although there is general agreement on the different groups, regardless of their absolute rank. Here is one scheme, which includes all groups that produce jellyfish, derived from several expert sources:

Phases
Jellyfish development occurs in multiple phases. Sperm fertilize eggs which develop into larval planulae, become polyps, bud into ephyrae and then transform into adult medusae. In some species, specimens may skip some phases.

The planula is a small larva covered with cilia. It settles onto a firm surface and develops into a polyp.

The polyp is generally a small stalk with a mouth that is ringed by upward-facing tentacles. The polyps resemble the closely related Cnidaria anthozoan (sea anemones and corals) polyps. The jellyfish polyp may be sessile, living on the bottom or on another substrate such as floats or boat hulls, or it may be free-floating or attached to tiny bits of free-living plankton or rarely, fish or other invertebrates. Polyps may be solitary or colonial. Polyp colonies form by strobilation, in which multiple polyps share a common stomach cavity. Most polyps are only millimeters in size. They feed continuously. The polyp stage may last for years.

The next stage is the ephyra, which is a free-swimming precursor of the final adult stage.

The ephyra then develops into a medusa. The medusa is the life stage that is typically identified as a jellyfish.

Reproduction
Jellyfish reproduce both sexually and asexually. Upon reaching adult size, jellyfish spawn daily given enough food. In most species, spawning is controlled by light, so the entire population spawns at about the same time of day, often at either dusk or dawn. Jellyfish are usually either male or female (with occasional hermaphrodites). In most cases, adults release sperm and eggs into the surrounding water, where the (unprotected) eggs are fertilized and mature into new organisms.

After a growth interval, the polyp begins reproducing asexually by budding and, in the Scyphozoa, is called a segmenting polyp, or a scyphistoma. Budding produces more scyphistomae and also ephyrae. Budding sites vary by species; from the tentacle bulbs, the manubrium (above the mouth), or the gonads of hydromedusae. Polyps asexually produce free-swimming ephyra, which then become a medusa. New specimens (usually only a millimeter or two across) swim away from the polyp and then grow. Some polyps can asexually produce a creeping frustule larval form, which then develops into another polyp.

A few species can produce new medusae by budding directly from the medusan stage. Some hydromedusae reproduce by fission (splitting in half). A few omit the planula, polyp and ephyra phases and produce new medusae directly from eggs.

In a few species, the sperm swim into the female's mouth, fertilizing the eggs within her body, where they remain during early development stages. In moon jellies, the eggs lodge in pits on the oral arms, which form a temporary brood chamber for the developing planula larvae.

Lifespan
Jellyfish lifespans typically range from a few hours (in the case of some very small hydromedusae) to several months; there are some indications that deep sea species may live on the order of years. Life span varies by species. Most large coastal jellyfish live 2 to 6 months, during which they grow from a millimeter or two to many centimeters in diameter.

Aquarium jellyfish that are carefully tended, fed daily even when food might be seasonally rare in the wild, and sometimes treated with antibiotics if they develop infections, may live several years, though this would be very unusual in the wild.

An unusual species, Turritopsis dohrnii, formerly classified as T. nutricula, might be effectively immortal because of its ability under certain circumstances to transform from medusa back to the polyp stage, thereby escaping the death that typically awaits medusae post-reproduction if they have not otherwise been eaten by some other ocean organism. So far this reversal has been observed only in the laboratory. At least one professor at the Seto Marine Biological Laboratory at Kyoto University in Japan has concluded that there are three species of jellyfish that are immortal, and says their immortality may hold the key to immortality for human beings, as he says that genetically they are not that much different from humans.

Movement
Jellyfish have been proven to be the most energy efficient swimmers of all animals. They move through the water by radially expanding and contracting their bell-shaped bodies to push water behind them. They pause between the contraction and expansion to create two vortex rings. Muscles are used for the contraction of the body, which sheds the first vortex and pushes the animal forward, but the mesoglea is so elastic that the expansion is powered exclusively by relaxing the bell, which releases the energy stored from the contraction. By doing so, the second vortex ring rolls under it and begins to spin faster. This sucks in water which refills the bell and is pushed up against the centre of the body, giving it a secondary and "free" boost forward. The mechanism, called passive energy recapture, only works at low speeds and relatively small body sizes, allowing the animal to travel 30 percent farther on each swimming cycle. Jellyfish achieved a 48 percent lower cost of transport (the amount of food and oxygen consumed, versus energy spent in movement) than other animals in similar studies.

Diet
Medusae are carnivorous, feeding on plankton, crustaceans, fish eggs, small fish and other jellyfish, ingesting and voiding through the same hole in the middle of the bell. Jellies hunt passively using their tentacles as drift nets. Their swimming technique also helps them to capture prey; when their body expands it displaces more water which brings more potential prey within the reach of their tentacles.

Predation
Other species of jellyfish are among the most common and important jellyfish predators, some of which specialize in jellies. Other predators include tuna, shark, swordfish, sea turtles, and at least one species of Pacific salmon. In general however, there are few predators preying on jellyfish and they can be considered top predators in the food chain. Not only do they eat fish eggs and juvenile fish, but they also compete for food resources, leading to jellyfish having a difficult-to-reverse dominant position in the ecosystem.

Sea birds sometimes pick symbiotic crustaceans from the jellyfish bells near the sea's surface, inevitably feeding also on the jellyfish hosts of these amphipods or young crabs and shrimp.

Blooms
Jellyfish bloom formation is a complex process that depends on ocean currents, nutrients, sunshine, temperature, season, prey availability, reduced predation and oxygen concentrations. Ocean currents tend to congregate jellyfish into large swarms or "blooms", consisting of hundreds or thousands of individuals. Blooms can also result from unusually high populations in some years. A recent study tracking swimming jellyfish revealed that these medusae can detect marine currents and swim against the current to congregate in blooms. Jellyfish are better able to survive in nutrient-rich, oxygen-poor water than competitors, and thus can feast on plankton without competition. Jellyfish may also benefit from saltier waters, as saltier waters contain more iodine, which is necessary for polyps to turn into jellyfish. Rising sea temperatures caused by climate change may also contribute to jellyfish blooms, because many species of jellyfish are relatively better able to survive in warmer waters.

One hypothesis is that the global increase in jellyfish bloom frequency may stem from human impact. In some locations jellyfish may be filling ecological niches formerly occupied by now overfished creatures, but this hypothesis lacks supporting data. Youngbluth states that "jellyfish feed on the same kinds of prey as adult and young fish, so if fish are removed from the equation, jellyfish are likely to move in."

Some jellyfish populations that have shown clear increases in the past few decades are invasive species, newly arrived from other habitats: examples include the Black Sea, Caspian Sea, Baltic Sea, central and eastern Mediterranean, Hawaii, and tropical and subtropical parts of the West Atlantic (including the Caribbean, Gulf of Mexico and Brazil). Invasive populations can expand rapidly because they often face no predators in the new habitat.

Increased nutrients, ascribed to agricultural runoff, have been cited as contributing to jellyfish proliferation. Graham states, "ecosystems in which there are high levels of nutrients ... provide nourishment for the small organisms on which jellyfish feed. In waters where there is eutrophication, low oxygen levels often result, favoring jellyfish as they thrive in less oxygen-rich water than fish can tolerate. The fact that jellyfish are increasing is a symptom of something happening in the ecosystem."

Population
Jellyfish populations may be expanding globally as a result of overfishing of their natural predators and the availability of excessive nutrients due to land runoff. When marine ecosystems become disturbed jellyfish can proliferate. For example, jellyfish reproduce rapidly and have fast growth rates; they predate many species, while few species predate them; and they feed via touch rather than visually, so they can feed effectively at night and in turbid waters. It may become difficult for fish stocks to reestablish themselves in marine ecosystems once they have become dominated by jellyfish, because jellyfish feed on plankton, which includes fish eggsand larvae.

Habitats
Most jellyfish are marine animals, although a few hydromedusae inhabit freshwater. The best known freshwater example is the cosmopolitan hydrozoan jellyfish, Craspedacusta sowerbii. It is less than an inch (2.5 cm) in diameter, colorless and it does not sting.

Some jellyfish populations have become restricted to coastal saltwater lakes, such as Jellyfish Lake in Palau.

Although what first comes to mind as the common domain of jellyfish is living well up off the ocean floor in the plankton, a few species of jellyfish are closely associated with the bottom for much of their lives (that is, they can be considered benthic). The upside-down jellyfish in the genus Cassiopea typically lie on the bottom of shallow lagoons where they sometimes pulsate gently with their umbrella top facing down. The tiny creeping jellyfish Staurocladia and Eleutheria (see section on Size, above) cannot swim and "walk" around on seaweed fronds or rocky bottoms on their tentacles. Most hydromedusae and scyphomedusae that live in coastal habitats find themselves on the bottom periodically, where they may stop swimming for a while, and certain box jellyfish species also rest on the sea bed in shallow water. Even some deep-sea species of hydromedusae and scyphomedusae are usually collected on or near the bottom. All of the stauromedusae are found attached to either seaweed or rocky or other firm material on the bottom.

Some species explicitly adapt to tidal flux. In Roscoe Bay, jellyfish ride the current at ebb tide until they hit a gravel bar, and then descend below the current. They remain in still waters until the tide rises, ascending and allowing it to sweep them back into the bay. They also actively avoid fresh water from mountain snowmelt, diving until they find enough salt.

Parasites
Jellyfish function as hosts for a wide variety of organisms. Endoparasitic helminths are transmitted from intermediate host jellyfish to definitive host fish via predation. Some digenean trematodes, especially species of the family Lepocreadiidae, are known to use jellyfish as their second intermediate hosts and/or paratenic hosts. Medusivorous fish become infected by trematodes through predation of infected jellyfish and act as definitive hosts.

Fisheries
Fisheries have begun harvesting the American cannonball jellyfish, Stomolophus meleagris, along the southern Atlantic coast of the United States and in the Gulf of Mexico for export to Asia.

Jellyfish are also harvested for their collagen, which can be used for a variety of applications including the treatment of rheumatoid arthritis.

Products
In some countries, such as China, Japan, and Korea, jellyfish are known as a delicacy. "Dried jellyfish" has become increasingly popular throughout the world. The jellyfish is dried to prevent spoiling; if not dried they can spoil within a matter of hours. Once dried, they can be stored for weeks at a time. Only scyphozoan jellyfish belonging to the order Rhizostomeae are harvested for food; about 12 of the approximately 85 species. Most of the harvest takes place in southeast Asia. Rhizostomes, especially Rhopilema esculentum in China (海蜇 hǎizhé, "sea stingers") and Stomolophus meleagris (cannonball jellyfish) in the United States, are favored because of their larger and more rigid bodies and because their toxins are harmless to humans.

Traditional processing methods, carried out by a Jellyfish Master, involve a 20- to 40-day multi-phase procedure in which after removing the gonads and mucous membranes, the umbrella and oral arms are treated with a mixture of table salt and alum, and compressed. Processing reduces liquefaction, odor, the growth of spoilage organisms, and makes the jellyfish drier and more acidic, producing a "crunchy and crispy texture." Jellyfish prepared this way retain 7–10% of their original weight, and the processed product contains approximately 94% water and 6% protein. Freshly processed jellyfish has a white, creamy color and turns yellow or brown during prolonged storage.

In China, processed jellyfish are desalted by soaking in water overnight and eaten cooked or raw. The dish is often served shredded with a dressing of oil, soy sauce, vinegar and sugar, or as a salad with vegetables. In Japan, cured jellyfish are rinsed, cut into strips and served with vinegar as an appetizer. Desalted, ready-to-eat products are also available.

In Israel, a start-up company called Cine'al has developed a super-absorbent substance made from jellyfish known as hydromash which they claim can be used to make diapers, tampons, and paper towels. Hydrosmash was inspired by research from the University of Tel Aviv, which found that jellyfish were made up of a material that could "absorb high volume of liquids and hold them without disintegrating or dissolving."

Biotechnology
In 1961, Osamu Shimomura extracted green fluorescent protein (GFP) and another bioluminescent protein, called aequorin, from the large and abundant hydromedusa Aequorea victoria, while studying photoproteins that cause bioluminescence in this species. Three decades later, Douglas Prasher sequenced and cloned the gene for GFP. Martin Chalfie figured out how to use GFP as a fluorescent marker of genes inserted into other cells or organisms. Roger Tsien later chemically manipulated GFP to produce other fluorescent colors to use as markers. In 2008, Shimomura, Chalfie and Tsien won the Nobel Prize in Chemistryfor their work with GFP.

Man-made GFP became commonly used as a fluorescent tag to show which cells or tissues express specific genes. The genetic engineering technique fuses the gene of interest to the GFP gene. The fused DNA is then put into a cell, to generate either a cell line or (via IVF techniques) an entire animal bearing the gene. In the cell or animal, the artificial gene turns on in the same tissues and the same time as the normal gene, making GFP instead of the normal protein. Illuminating the animal or cell reveals what tissues express that protein—or at what stage of development. The fluorescence shows where the gene is expressed.

Aquaria
Jellyfish are displayed in many public aquariums. Often the tank's background is blue and the animals are illuminated by side light, increasing the contrast between the animal and the background. In natural conditions, many jellies are so transparent that they are nearly invisible.

Jellyfish are not adapted to closed spaces. They depend on currents to transport them from place to place. Professional exhibits feature precise water flows, typically in circular tanks to avoid trapping specimens in corners. The Monterey Bay Aquarium uses a modified version of the kreisel (German for "spinning top") for this purpose. The outflow is spread out over a large surface area and the inflow enters as a sheet of water in front of the outflow, so the jellyfish do not get sucked into it. As of 2009, jellyfish were becoming popular in home aquariums. These home aquariums generate this special water flow pattern using an air-lift pump and require special food for the jellyfish, which can be shipped from suppliers to their final destination.

Toxicity
Jellyfish sting their prey using nematocysts, also called cnidocysts, stinging structures located in specialized cells called cnidocytes, which are characteristic of all Cnidaria. Contact with a jellyfish tentacle can trigger millions of nematocysts to pierce the skin and inject venom, yet only some species' venom cause an adverse reaction in humans. When a nematocyst is triggered by contact by predator or prey, pressure builds up rapidly inside it up to 2,000 pounds per square inch (14,000 kPa) until it bursts. A lance inside the nematocyst pierces the victim's skin, and venom flows through into the victim. Touching or being touched by a jellyfish can be very uncomfortable, sometimes requiring medical assistance; sting effects range from no effect to extreme pain to death. Even beached and dying jellyfish can still sting when touched.

Scyphozoan jellyfish stings range from a twinge to tingling to agony. Most jellyfish stings are not deadly, but stings of some species of the class Cubozoa and the Box jellyfish, such as the famous and especially toxic Irukandji jellyfish, can be deadly. Stings may cause anaphylaxis, which can be fatal. Medical care may include administration of an antivenom.

In 2010, at a New Hampshire beach, pieces of a single dead lion's mane jellyfish stung between 125 and 150 people. Jellyfish kill 20 to 40 people a year in the Philippines alone. In 2006 the Spanish Red Cross treated 19,000 stung swimmers along the Costa Brava.

The sea wasp, a box jellyfish found in Australian waters, and more recently, Florida, can kill an adult human within a few minutes. A thin skin covering such as pantyhose was found to be sufficient protection. The pantyhose were formerly thought to work because of the length of the nematocysts, but it is now known to be related to the way the stinger cells work. The stinging cells on a box jellyfish's tentacles are not triggered by pressure. Instead, they are triggered by the chemicals found on skin. Pantyhose hinders the detection of the chemicals preventing the nematocysts from firing.

Treatment of stings
The three goals of first aid for uncomplicated stings are to prevent injury to rescuers, deactivate the nematocysts, and remove tentacles attached to the patient. Rescuers usually wear barrier clothing, such as pantyhose, wet suits or full-body sting-proof suits while removing jellies or tentacles from injured. Deactivating the nematocysts (stinging cells) prevents further injection of venom.

Vinegar (3–10% aqueous acetic acid) may be used as a common remedy to help with box jellyfish stings, but not the stings of the Portuguese man o' war (which is not a true jellyfish, but a siphonophore). For stings on or around the eyes, a towel dampened with vinegar may be used to dab around the eyes, with care taken to avoid the eyeballs. Salt water may be used as an alternative if vinegar is unavailable; and may be preferred over vinegar. Fresh water is not usually used if the sting occurs in salt water, as changes in tonicity can release additional venom. Rubbing wounds, or using alcohol, spirits, ammonia, or urine may have strongly negative effects as these can encourage the release of venom.

Clearing the area of jelly, tentacles, and wetness further reduces nematocyst firing. Scraping the affected skin with a knife edge, safety razor, or credit card may remove remaining nematocysts.

Beyond initial first aid, antihistamines such as diphenhydramine (Benadryl) may control skin irritation (pruritus). Ice or fresh water is not usually applied to stings, since they may cause nematocysts to continue to release toxin. Immunobased antivenins have been available since the 1970s; administration requires medical personnel and refrigeration and are used in extreme cases as with regard to the box jellyfish, Chironex fleckeri.

Hazards
Jellyfish adversely affect humanity by interfering with public systems and harming swimmers. The most obvious consequences are human injury or death and reduced coastal tourism. Jellies destroy fish nets, poison or crush captured fish, and consume fish eggs and young fish. The most venomous jellyfish is the box jellyfish which produces enough poison to kill 60 humans and is the reason for 1 death per year.

Jellyfish can clog cooling equipment, disabling power plants in several countries. Jellyfish caused a cascading blackout in the Philippines in 1999, as well as damaging the Diablo Canyon Power Plant in California in 2008. Clogging can stop desalination plants, as well as clogging ship engines and infesting fishing nets.