| Jellyfish Temporal range: 505–0 Ma Cambrian – Recent |
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|---|---|
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Chrysaora quinquecirrha |
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| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Cnidaria |
| Subphylum: | Medusozoa Petersen, 1979 |
| Classes | |
Jellyfish (also known as jellies or sea jellies or a stage of the life cycle of Medusozoa) are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish life stages among animals in the phylum. Jellyfish have multiple morphologies that represent cnidarian classes including the Scyphozoa (over 200 species), Staurozoa (about 50 species), Cubozoa (about 20 species), and Hydrozoa (about 1000–1500 species that make jellyfish and many more that do not).[1][2] .
Jellyfish are found in every ocean, from the surface to the deep sea. Some hydrozoan jellyfish, or hydromedusae, inhabit freshwater; freshwater jellyfish are less than an inch (2.5 cm) in diameter, are colorless and do not sting. Large, often colorful, jellyfish are common in coastal zones worldwide. Jellyfish have roamed the seas for at least 500 million years,[3] and possibly 700 million years or more, making them the oldest multi-organ animal.[4]
In its broadest sense, the term jellyfish also generally refers to members of the phylum Ctenophora. Although not closely related to cnidarian jellyfish, ctenophores are also free-swimming planktonic carnivores, are generally transparent or translucent, and exist in shallow to deep portions of all the world's oceans.
Alternative names for groups of jellyfish are scyphomedusae, stauromedusae, cubomedusae, and hydromedusae. These may relate to an entire order or class.[5]
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The word jellyfish (which has been in common usage for more than a century)[6] is used to denote several different kinds of cnidarians, all of which have a basic body structure that resembles an umbrella, including scyphozoans, staurozoans (stalked jellyfish), hydrozoans, and cubozoans (box jellyfish). Some textbooks and websites refer to scyphozoans as "true jellyfish".[7][8]
As jellyfish are not even vertebrates, let alone true fish, the usual word jellyfish is considered by some to be a misnomer, and American public aquariums have popularized use of the terms jellies or sea jellies instead.[9]
In its broadest usage, some scientists occasionally include members of the phylum Ctenophora (comb jellies) when they are referring to jellyfish.[10] 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.[11]
A group of jellyfish is sometimes called a bloom or a swarm.[12] "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.[13] Another collective name for a group of jellyfish is a smack.[14]
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.[15] Using "swarm" usually implies some kind of active ability to stay together, which a few species such as Aurelia, the moon jelly, demonstrate.[16]
Most jellyfish have a second stage to their life cycle, the planula larvae phase, following the initial egg and sperm phase. Although this is a short lived stage for jellyfish, it is an important phase when the fertilized eggs that had previously undergone embryonic development, hatch, and planulae emerge from the females mouth or brood pouch and are off on their own.[17]
Most jellyfish do not have specialized digestive, osmoregulatory, central nervous, respiratory, or circulatory systems. The manubrium is a stalk-like structure hanging down from the centre of the underside, with the mouth at its tip. 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.[18] 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. The Jellyfish body consist of over 95% water; most of their 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.
Most jellyfish do not have a brain or central nervous system, but employ a loose network of nerves, located in the epidermis, which is called a "nerve net". 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.
Another counter to the "brainless jellyfish" hypothesis is that some species explicitly adapt to tidal flux to control their location. 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 waiting for the tide to rise, ascending and allowing it to sweep them back into the bay. They monitor salinity to avoid fresh water from mountain snowmelt, again by diving until they find enough salt.[4]
Some jellyfish also 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 been revealed to be more advanced than their counterparts. The Box jellyfish has 24 eyes, two of which are capable of seeing color, and four parallel brains that act in competition, supposedly making it one of the only creatures to have a 360 degree view of its environment.[19] It is suggested that the two eyes that contain cornea and retina are attached to a central nervous system which enables the four brains to process images. It is unknown how this works, as the creature has a unique central nervous system.[20][4]
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.[4]
One of the smallest adult jellies is the Australian Irukandji, which is about the size of a fingernail.[4]
The lion's mane jellyfish, Cyanea capillata, were long-cited as the largest jellyfish, and arguably the longest animal in the world, with fine, thread-like tentacles up to 36.5 metres (120 ft) long (though most are nowhere near that large).[21][22][23] They have a 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 probably a much better candidate for "largest jellyfish", 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.[24][25] The large bell mass of the giant Nomura's jellyfish[26] can dwarf a diver and is nearly always much greater than the up-to-100 centimetres (39 in) bell diameter Lion's Mane.[27]
The rarely-encountered deep-sea jellyfish Stygiomedusa gigantea is another solid candidate for "largest jellyfish", with its 100 centimetres (39 in) wide, and thick, massive bell and four thick, "paddle-like" oral arms extending up to 6 metres (20 ft) in length,[28] very different than the typical fine, threadlike tentacles that rim the umbrella of more-typical-looking jellyfish, including the Lion's Mane.
Most jellyfish undergo two distinct life history stages (body forms) during their life cycle. The first is the polypoid stage. After fertilization and initial growth, a larval form, called the planula, develops. 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 surrounded by upward-facing tentacles like miniatures of the closely related anthozoan polyps (sea anemones and corals), also of the phylum Cnidaria. This polyp may be sessile, living on the bottom or on similar substrata such as floats or boat-bottoms, or it may be free-floating or attached to tiny bits of free-living plankton[29] or rarely, fish[30] or other invertebrates. Polyps may be solitary or colonial. Most are very small, measured in millimeters. They feed continuously.
Certain Box Jellyfish species sleep on the sea bed in shallow water.[31]
Jellyfish reproduce both sexually and asexually. Upon reaching adult size, jellyfish spawn daily if there is 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.[32]Jellyfish are usually either male or female (hermaphroditic specimens are occasionally found).
In most cases, adults release sperm and eggs into the surrounding water, where the (unprotected) eggs are fertilized and mature into new organisms. In a few species, the sperm swim into the female's mouth fertilizing the eggs within the female's 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.
After a growth interval, the polyp begins reproducing asexually by budding and, in the Scyphozoa, is called a segmenting polyp, or a scyphistoma. New scyphistomae may be produced by budding or form new, immature jellies called ephyrae. A few jellyfish species can produce new medusae by budding directly from the medusan stage. Budding sites vary by species; from the tentacle bulbs, the manubrium (above the mouth), or the gonads of hydromedusae. A few species of hydromedusae reproduce by fission (splitting in half).[29]
In the second stage, the tiny polyps asexually produce jellyfish, each of which is also known as a medusa. Tiny jellyfish (usually only a millimeter or two across) swim away from the polyp and then grow and feed in the plankton. Medusae have a radially symmetric, umbrella-shaped body called a bell, which is usually supplied with marginal tentacles - fringe-like protrusions from the bell's border that capture prey. A few species of jellyfish do not have the polyp portion of the life cycle, but go from jellyfish to the next generation of jellyfish through direct development of fertilized eggs.
Jellyfish lifespans typically range from a few hours (in the case of some very small hydromedusae) to several months. Life span and maximum size varies by species. One unusual species is reported to live as long as 30 years. Another species, Turritopsis dohrnii as T. nutricula, may be effectively immortal because of its ability to transform between medusa and polyp, thereby escaping death.[33] Most large coastal jellyfish live 2 to 6 months, during which they grow from a millimeter or two to many centimeters in diameter.
Jellies are carnivorous, feeding on plankton, crustaceans, fish eggs, small fish and other jellyfish, ingesting and voiding through the same convenient hole in the middle of the bell. Jellies hunt passively using their tentacles as drift nets. Should a fish brush against the often invisible extensions, the pressure activates the nematocysts, protecting the predator’s delicate tissue. Prey must be dead before ingestion, or it could puncture the jellyfish' stomach.[4]
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. 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.
Jellyfish blooms can grow to 100,000 individuals. 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. 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.[34]
Scientists have little quantitative data of historic or current jellyfish populations beyond "impressions" in the public memory.[15] Recent speculation about increases in jellyfish populations are based on no "before" data.
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.[15] 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."[35]
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 and the Caspian Sea, the Baltic Sea, the eastern Mediterranean coasts of Egypt and Israel[36], and the American Gulf coast. 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 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."[35]
Jellyfish populations may be expanding globally as a result of overfishing and the availability of excessive nutrients due to land runoff.[37][38] 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.[39][40] 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 eggs and larvae.[41][42]
In some countries, such as Japan, jellyfish are known as a delicacy. "Dried jellyfish" has become increasingly popular throughout the world. The jellyfish is dried to prevent from spoiling. Jellyfish can spoil within a matter of hours if it is not dried. Once jellyfish are 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.[43] Rhizostomes, especially Rhopilema esculentum in China (海蜇 hǎizhē, "sea stings") 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.[44]
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.[44][45] Desalted, ready-to-eat products are also available.[44]
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.[44]
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 Chemistry for 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.[46]
Jellyfish are also harvested for their collagen, which can be used for a variety of applications including the treatment of rheumatoid arthritis.
Jellyfish are displayed in many public aquaria. 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. As of 2009, jellyfish were becoming popular in home aquaria.[47][48][49][50]
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,[51] 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 poison flows through into the victim.[52] 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.[53] 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.[54][55] Jellyfish kill 20 to 40 people a year in the Philippines alone.[53] In 2006 the Spanish Red Cross treated 19,000 stung swimmers along the Costa Brava.[53]
An Australian box jellyfish called the sea wasp can kill a grown man in a matter of seconds or minutes. Because the harpoons are so shallow, however, Australians have learned that they can protect themselves while swimming in sea wasp waters simply by covering their exposed skin with pantyhose.[4]
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 need to wear barrier clothing, such as pantyhose, wet suits or full-body sting-proof suits. Deactivating the nematocysts (stinging cells) prevents further injection of venom.
Vinegar (3–10% aqueous acetic acid) is a common remedy to help with box jellyfish stings,[56][57] but not the stings of the Portuguese Man o' War (which is not a true jellyfish, but a colony).[56] For stings on or around the eyes, a towel dampened with vinegar is used to dab around the eyes, with care taken to avoid the eyeballs. Salt water is an alternative if vinegar is unavailable.[56][58] Fresh water is not used if the sting occurs in salt water, as changes in tonicity[59] 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.[60]
Clearing the area of jelly, tentacles, and wetness further reduces nematocyst firing.[60] Scraping the affected skin with a knife edge, safety razor, or credit card can remove remaining nematocysts.[61]
Beyond initial first aid, antihistamines such as diphenhydramine (Benadryl) can control skin irritation (pruritus).[61] For removal of venom in the skin, a paste of baking soda and water can be applied with a cloth covering on the sting. In some cases it is necessary to reapply paste every 15–20 minutes. Ice or fresh water should not be applied to the sting, as this may help the nematocysts to continue to release toxin.[62][63]
Jellyfish adversely affect humanity by interfering with public systems and harming swimmers.[53] The most obvious consequences are human injury or death and reduced coastal tourism. Jellies destroy fish nets, poison or crushing captured fish, and consume fish eggs and young fish.[64]
Jellyfish can clog cooling equipment, disabling power plants in several countries. Jellyfish caused a cascading blackout in the Philippines in 1999,[53] as well as damaging the Diablo Canyon Power Plant in California in 2010.[65] Clogging can stop desalination plants, as well as clogging ship engines[64] and infesting fishing nets.[66]
"Jellyfish Invasion," was an episode of the National Geographic Channel documentary series Explorer,[67][68][69] which includes research conducted by scientists in Australia, Hawaii and Japan.
The Disney Pixar animated film Finding Nemo illustrated the near fatal effects of swimming through a jellyfish bloom. The opening sequence of the animated film Ponyo depicts a massive jellyfish bloom off the coast of Japan. The Japanese science fiction film Dogora features jellyfish-like space creatures. The Japanese anime Kuragehime features a main character who is obsessed with jellyfish, and has jellyfish related plotlines. The "A Star is Born Again" episode of The Simpsons, depicts a Jellyfish Festival, celebrating the bloom of jellyfish that overruns the waters and beach.
In the Will Smith movie Seven Pounds, Will Smith's character had a pet jellyfish which played an important role at the end of the movie.
On an episode of Survivor Palau, the team winning the reward challenge got to swim in Jellyfish Lake, a lake full of moon and golden jellyfish, which are harmless to humans.
Taxonomic classification systematics within the Cnidaria, as with all organisms, are always in flux. 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. Presented here is one scheme, which includes all groups that produce medusae (jellyfish), derived from several expert sources:
| Jellyfish taxonomy (phylum Cnidaria: subphylum Medusozoa) | ||||
|---|---|---|---|---|
| Class | Subclass | Order | Suborder | Families |
| Hydrozoa [70][71] | Hydroidolina | Anthomedusae | Filifera | see [70] |
| Capitata | see [70] | |||
| Leptomedusae | Conica | see [70] | ||
| Proboscoida | see [70] | |||
| Siphonophorae | Physonectae | Agalmatidae, Apolemiidae, Erennidae, Forskaliidae, Physophoridae, Pyrostephidae, Rhodaliidae | ||
| Calycophorae | Abylidae, Clausophyidae, Diphyidae, Hippopodiidae, Prayidae, Sphaeronectidae | |||
| Cystonectae | Physaliidae, Rhizophysidae | |||
| Trachylina | Limnomedusae | Olindiidae, Monobrachiidae, Microhydrulidae, Armorhydridae | ||
| Trachymedusae | Geryoniidae, Halicreatidae, Petasidae, Ptychogastriidae, Rhopalonematidae | |||
| Narcomedusae | Cuninidae, Solmarisidae, Aeginidae, Tetraplatiidae | |||
| Actinulidae | Halammohydridae, Otohydridae | |||
| Staurozoa[72] | Eleutherocarpida | Lucernariidae, Kishinouyeidae, Lipkeidae, Kyopodiidae | ||
| Cleistocarpida | Depastridae, Thaumatoscyphidae, Craterolophinae | |||
| Cubozoa [73] | Carybdeidae, Alatinidae, Tamoyidae, Chirodropidae, Chiropsalmidae | |||
| Scyphozoa [73] | Coronatae | Atollidae, Atorellidae, Linuchidae, Nausithoidae, Paraphyllinidae, Periphyllidae | ||
| Semaeostomeae | Cyaneidae, Pelagiidae, Ulmaridae | |||
| Rhizostomeae | Cassiopeidae, Catostylidae, Cepheidae, Lychnorhizidae, Lobonematidae, Mastigiidae, Rhizostomatidae, Stomolophidae | |||
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