AskDefine | Define jellyfish

Dictionary Definition



1 large siphonophore having a bladderlike float and stinging tentacles [syn: Portuguese man-of-war, man-of-war]
2 any of numerous usually marine and free-swimming coelenterates that constitute the sexually reproductive forms of hydrozoans and scyphozoans [syn: medusa, medusan] [also: jellyfishes (pl)]

User Contributed Dictionary




  1. Almost transparent aquatic being; any one of the acalephs, especially one of the larger species, having a jellylike appearance.


aquatic being

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Extensive Definition

Jellyfish are marine invertebrates belonging to the class Scyphozoa of the phylum Cnidaria. They can be found in every ocean in the world and even in some fresh water. The term "jellyfish" is a common name, and therefore does not imply any systematic relationship to vertebrate fish — jellyfish are cnidarians. The term "jellyfish" is also applied to some close relatives of true scyphozoans, such as the Hydrozoa and the Cubozoa.

Anatomy and morphology

The body of an adult jellyfish consists of a bell shape producing jelly and enclosing its internal structure, from which tentacles are suspended. Each tentacle is covered with cells called cnidocytes, that can sting or kill other animals. Most jellyfish use these cells to secure prey or for defense. Others, such as the Rhizostomae, do not have tentacles at all.
Jellyfish lack basic sensory organs and a brain, but their nervous systems and rhopalia allow them to perceive stimuli, such as light and odor, and respond quickly. They feed on small fish and zooplankton that become caught in their tentacles. Most jellyfish are passive drifters and slow swimmers, as their shape is not hydrodynamic. Instead, they move so as to create a current forcing the prey within reach of their tentacles. They do this by rhythmically opening and closing their bell-like body. Their digestive system is incomplete: the same orifice is used to take in food and expel waste. The body of an adult is made up of 94–98% water. The bell consists of a layer of epidermis, gastrodermis, and a thick, intervening layer called mesoglea that produces most of the jelly.

Body systems

A jellyfish detects the touch of other animals using a nervous system called a "nerve net", located in its epidermis. Touch stimuli are conducted by nerve rings, through the rhopalial lappet, located around the animal's body, to the nerve cells. Jellyfish also have ocelli: light-sensitive organs that do not form images but are used to determine up from down, responding to sunlight shining on the water's surface.
Jellyfish don't have a specialized digestive, osmoregulatory, central nervous, respiratory, or circulatory systems. They digest using the gastrodermal lining of the gastrovascular cavity, where nutrients are absorbed. They 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 and mostly free-float, but can use the hydrostatic skeleton of the water pouch to accomplish vertical movement through pulsations of the disc-like body.
The outer side of a jellyfish is lined with a jelly-like material called ectoplasm (ecto meaning outer and plasm meaning living matter). The ectoplasm typically contains a smaller amount of protein granules and other organic compounds than inner cytoplasm, also referred to as endoplasm (endo meaning inner).

Jellyfish blooms

Many species of jellyfish are capable of congregating into large swarms or "blooms", consisting of hundreds of individuals. The formation of these blooms is a complex process that depends on ocean currents, nutrients, temperature and ambient oxygen concentrations. Jellyfish sometimes mass breed during blooms. During such times of rapid population expansion, some people will raise ecological concerns about the potential noxious effects of a jellyfish "outbreak".
According to Claudia Mills of the University of Washington, the frequency of jellyfish blooms may be attributed to humans' impact on marine systems. She says that the breeding jellyfish may merely be filling ecological niches formerly occupied by overfished creatures. Jellyfish researcher Marsh Youngbluth further clarifies that "jellyfish feed on the same kinds of prey as adult and young fishes, so if fish are removed from the equation, jellyfish are likely to move in." Increased nutrients in the water, ascribed to agricultural runoff, have also been cited as an antecedent to the proliferation of jellyfish. Monty Graham, of the Dauphin Island Sea Lab in Alabama, says that "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."
By sampling sea life in a heavily fished region off the coast of Namibia, researchers found that jellyfish have overtaken fish in terms of biomass. The findings represent a careful, quantitative analysis of what has been called a "jellyfish explosion" following intense fishing in the area in the last few decades. The findings were reported by Andrew Brierley of the University of St. Andrews and his colleagues in the July 12, 2006 issue of the journal Current Biology.
Areas which have been seriously affected by jellyfish blooms include the northern Gulf of Mexico. In that case, Graham states, "Moon jellies have formed a kind of gelatinous net that stretches from end to end across the gulf."

Importance to humans

Culinary uses

Jellyfish are an important source of food to the Chinese community and in many Asian countries. Only jellyfish belonging to the order Rhizostomeae are harvested for food. Rhizostomes, especially Rhopilema esculentum in China (Chinese name: hǎizhē) and Stomolophus meleagris (cannonball jellyfish) in the United States, are favoured because they are typically larger and have more rigid bodies than other scyphozoans. Furthermore, their toxins are innocuous to humans. Desalted, ready-to-eat products are also available. Jellyfish are also harvested for their collagen, which can be used for a variety of scientific applications including the treatment of rheumatoid arthritis.

In captivity

Jellyfish are commonly displayed in aquaria in many countries. Often the tank's background is blue and the animals are illuminated by side light to produce a high contrast effect. In natural conditions, many jellies are so transparent that they are almost impossible to see.
Holding jellyfish in captivity presents other problems. For one, they are not adapted to closed spaces. They depend on currents to transport them from place to place. To compensate for this, professional exhibits feature precise water flows, typically in circular tanks to prevent specimens from becoming trapped in corners. The Monterey Bay Aquarium uses a modified version of the kreisel (German for "spinning top") for this purpose.

Toxicity to humans

When stung by a jellyfish, first aid may be needed immediately. The stings of true Scyphozoan jellyfish are not generally deadly, though species of the completely separate class Cubozoa (box jellyfish) such as the famous and especially toxic Irukandji can be fatal. However, even nonfatal jellyfish stings are known to be extremely painful. Serious stings may cause anaphylaxis and may result in death. Hence, people stung by jellyfish must get out of the water to avoid drowning. In serious cases, advanced professional care must be sought. This care may include administration of an antivenin and other supportive care such as required to treat the symptoms of anaphylactic shock.
There are three goals of first aid for uncomplicated jellyfish stings: prevent injury to rescuers, inactivate the nematocysts, and remove any tentacles stuck on the patient. To prevent injury to rescuers, barrier clothing should be worn. This protection may include anything from panty hose to wet suits to full-body sting-proof suits. Inactivating the nematocysts, or stinging cells, prevents further injection of venom into the patient. Vinegar (3 to 10% aqueous acetic acid) should be applied for box jellyfish stings. Vinegar, however, is not recommended for Portuguese Man o' War stings. Fresh water should not be used if the sting occurred in salt water, as a change in pH can cause the release of additional venom. Rubbing the wound, or using alcohol, spirits, ammonia, or urine will encourage the release of venom and should be avoided. Though often not available, a shower or bath as hot as can be tolerated can neutralize stings. However, if hypothermia is suspected this method may cause other serious complications.
Once deactivated, the stinging cells must be removed. This can be accomplished by picking off tentacles left on the body.
Beyond initial first aid, antihistamines such as diphenhydramine (Benadryl) may be used to control skin irritation (pruritus). To remove the venom in the skin, apply a paste of baking soda and water and apply a cloth covering on the sting. If possible, reapply paste every 15-20 minutes. Ice can be applied to stop the spread of venom until either of these is available.

See also

commons Jellyfish
jellyfish in Arabic: قنديل البحر
jellyfish in Bengali: জেলিফিশ
jellyfish in Catalan: Medusa
jellyfish in Czech: Medúzovci
jellyfish in German: Quallen
jellyfish in Spanish: Scyphozoa
jellyfish in Esperanto: Meduzo
jellyfish in Finnish: Meduusat
jellyfish in French: Scyphozoa
jellyfish in Galician: Scyphozoa
jellyfish in Korean: 해파리
jellyfish in Croatian: Meduze
jellyfish in Ido: Meduzo
jellyfish in Indonesian: Ubur-ubur
jellyfish in Italian: Scyphozoa
jellyfish in Hebrew: מדוזה
jellyfish in Lithuanian: Scifomedūzos
jellyfish in Malay (macrolanguage): Ampai-ampai
jellyfish in Min Dong Chinese: Tá
jellyfish in Dutch: Kwallen
jellyfish in Japanese: クラゲ
jellyfish in Norwegian: Manet
jellyfish in Norwegian Nynorsk: Manet
jellyfish in Polish: Krążkopławy
jellyfish in Portuguese: Medusa (desambiguação)
jellyfish in Quechua: Kachu k'arachiq
jellyfish in Russian: Медуза (биология)
jellyfish in Simple English: Jellyfish
jellyfish in Slovak: Medúzovce
jellyfish in Serbian: Сцифомедузе
jellyfish in Swedish: Maneter
jellyfish in Thai: แมงกะพรุน
jellyfish in Turkish: Medüz
jellyfish in Ukrainian: Клас Сцифоїдні
jellyfish in Chinese: 钵水母纲

Synonyms, Antonyms and Related Words

Chilopoda, Chordata, Echiuroidea, Ectoprocta, Entoprocta, Milquetoast, Monoplacophora, Nemertinea, Phoronidea, baby, big baby, chicken, chicken liver, coward, crybaby, doormat, dull tool, fence-sitter, fraid-cat, fraidy-cat, funk, funker, gutless wonder, invertebrate, lightweight, lily liver, meek soul, milksop, mollycoddle, mouse, mugwump, namby-pamby, nebbish, nonentity, pansy, pantywaist, pushover, sad sack, scaredy-cat, shilly-shally, sissy, softling, softy, sop, waverer, weak sister, weakling, white feather, white liver, wobbler
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