A typically bell-shaped, marine coelenterate with a ring of marginal tentacles containing stinging cells and a central mouth on undersurface of bell; body displays a 4-part radial symmetry; endodermal gastric tentacles present in gut; represents the medusa phase of the coelenterate life-cycle, the polyp phase being reduced or absent. (Phylum: Cnidaria. Class: Scyphozoa.) The box jellyfish or sea wasp (Chironex fleckeri) is one of the most deadly stinging animals in the sea to which more than 60 deaths in Australia's northern waters have been attributed during the 20th-c. An antivenom has been developed but must be administered within minutes of an attack (Class: Cubozoa).
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Sea nettle, Chrysaora quinquecirrha |
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Stauromedusae |
Jellyfish are marine invertebrates belonging to the Scyphozoan class, and in turn the phylum Cnidaria. The body of an adult jellyfish is composed of a bell-shaped, jellylike substance enclosing its internal structure, from which the creature's tentacles suspend. Each tentacle is covered with stinging cells (cnidocytes) that can sting or kill other animals: most jellyfish use them to secure prey or as a defense mechanism. To compensate for its lack of basic sensory organs and a brain, the jellyfish exploits its nervous system and rhopalia to perceive stimuli, such as light or odor, and orchestrate expedient responses.
Most jellyfish are passive drifters that feed on small fish and zooplankton that become caught in their tentacles. Jellyfish have an incomplete digestive system, meaning that the same orifice is used for both food intake and waste expulsion.
Since jellyfish do not biologically qualify as actual "fish", the term "jellyfish" is considered a misnomer by some, who instead employ the names "jellies" or "sea jellies". The name "jellyfish" is also often used to denote either Hydrozoa or the box jellyfish, Cubozoa.
Life cycle and reproduction
Most jellyfish pass through two different body forms during their life cycle. in this phase, the jellyfish takes the form of either a sessile stalk which catches passing food, or a similar free-floating configuration.
In the second stage, the jellyfish is known as a medusa. Medusae have a radially symmetric, umbrella-shaped body called a bell.
Jellyfish are dioecious (that is, they are either male or female). Many jellyfish can bud off new medusae directly from the medusan stage.
Defense and feeding mechanisms
Most jellyfish have tentacles or oral arms coated with thousands of microscopic nematocysts;
Although most jellyfish are not perniciously dangerous to humans, a few are highly toxic, such as Cyanea capillata. The recently discovered Carukia barnesi is also suspected of causing two deaths in Australia. Contrary to popular belief, the menacingly infamous Portuguese Man o' War (Physalia) is not actually a jellyfish, but a colony of hydrozoan polyps.
Body systems
The jellyfish can detect the touch of other animals using a nervous system called a "nerve net", which is found in its epidermis. Impulses to the nerve cells are sent from nerve rings that have collected information from the environment of the jellyfish through the rhopalial lappet, which is located around the animal's body. Jellyfish also have ocelli that cannot form images, but are sensitive to light; the jellyfish can use these to determine up from down, basing its judgement on sunlight shining on the surface of the water.
Jellyfish do not have a specialized digestive system, osmoregulatory system, central nervous system, respiratory system, or circulatory system. Jellyfish have limited control over their movement and mostly free-float, but can use a hydrostatic skeleton that controls the water pouch in their body to actuate vertical movement.
Blooms and grouping
A group of jellyfish is often called a "smack." Many species of jellyfish are also capable of congregating into large swarms or "blooms" consisting of hundreds or even thousands of individuals. Jellyfish will sometimes mass breed during blooms, thereby causing large booms in jellyfish population and reportedly raising major ecological concern for a possible jellyfish outbreak. 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 nutrient in the water, ascribed to agricultural runoff, has also been cited as an antecedent to the recent proliferation of jellyfish numbers: scientist Monty Graham says, "ecosystems in which there are high levels of nutrient ... 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.
By sampling sea life in a heavily fished region off the coast of Namibia, researchers have found that jellyfish have actually overtaken fish in terms of the biomass they contribute to this ocean region. The findings represent a careful quantitative analysis of what's been called a "jellyfish explosion" after intense fishing in the area in the last few decades.
Areas seriously affected by jellyfish blooms include the northern Gulf of Mexico, where "moon jellies have formed a kind of gelatinous net that stretches from end to end across the gulf," and the Adriatic Sea.
Jellyfish in captivity
Jellyfish are commonly displayed in aquariums across the United States and in other countries;
Cuisine
Sliced and marinated jellyfish bells (often known as sesame jellyfish or jellyfish salad) is a common appetizer in Chinese cuisine. Korean version of the dish, haepari naengchae (cold jellyfish salad), is a summertime delicacy in the country, and is usually served with sweet and sour seasoning with mustard.
Packages or slabs of jellyfish bells (Chinese: 海蜇皮;
Treatment of stings
When stung by a jellyfish, first aid may be in order. Though most jellyfish stings are not deadly, other stings, such as those perpetrated by the box jellyfish (Chironex fleckeri) may be fatal. Serious stings may cause anaphylaxis and eventual paralysis, and hence people stung by jellyfish must get out of the water to avoid drowning.
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.
Vinegar (3 to 10 % acetic acid in water) should be applied for box jellyfish stings. After large pieces of the jellyfish are removed, shaving cream may be applied to the area and a knife edge, safety razor, or credit card may be used to take away any remaining nematocysts.
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