jellyfish

 

common name for the free-swimming stage (see polyp and medusa), of certain invertebrate animals of the phylum Cnidaria (the coelenterates). The body of a jellyfish is shaped like a bell or umbrella, with a clear, jellylike material filling most of the space between the upper and lower surfaces. A mouth is located in the center of the undersurface and tentacles dangle from the bell margin. Many jellyfish are colored, with pink or orange internal structures visible through the colorless or delicately tinted bell, and all are exquisitely designed; they are among the most beautiful of animal types. Typically, jellyfish catch their prey with the aid of stinging cells located in the tentacles; many jellyfish can cause irritating or even dangerous stings to humans. Food is carried by the tentacles to the mouth, then is moved into the stomach and is distributed to the body through radial canals. Jellyfish move up and down by contracting and relaxing the bell, using muscles that circle the bell margin; they are carried horizontally by waves and currents. Jellyfish of the class Hydrozoa are small, ranging from ¹/₈ in. (0.32 cm) to several inches in diameter, and usually have four tentacles. They have several (often four) unbranched radial canals and simple sense organs. In this group the polyp, or attached stage, is often larger and more conspicuous than the medusa. Jellyfish of the class Scyphozoa, sometimes called true jellyfish, are larger and often have numerous tentacles; they have branched radial canals and complex sense organs. In this group the medusa is the prominent form and the polyp is reduced to a small larval stage. Scyphozoan jellyfish are commonly ³/₄ in. to 16 in. (2–40 cm) in diameter; Aurelia, the flattened jellyfish common along North American coasts, may be as much as 1 ft (30 cm) across. One species of Cyanea found in cold northern seas may reach 6 ft (1.8 m) across and have tentacles over 100 ft (30 m) long. Tiny Craspedacusta, a hydrozoan jellyfish less than 1 in. (2.5 cm) long, occurs in freshwater lakes and ponds, but all other jellyfish are marine, living in ocean depths as well as along the coasts. The hydrozoan Physalia, or Portuguese man-of-war, is actually a large colony of modified individuals, some medusalike and some polyplike; a large gas-filled sac acts as a float for the colony. The tentacles of such a colony may extend 60 ft (18 m) into the water and can cause severe injuries to swimmers. Physalia is usually bright blue, sometimes with tints of pink and orange. The purple sail, Velella, a floating colony 1 to 3 in. (2.5–7.5 cm) across, may be blue or purple. Jellyfish are classified in the phylum Cnidaria, classes Hydrozoa and Scyphozoa.

Jellyfish
Sea nettle, Chrysaora quinquecirrha
Scientific classification
Kingdom: Animalia Phylum: Cnidaria Class: Scyphozoa Goette, 1887
Orders
Stauromedusae Coronatae Semaeostomeae Rhizostomae

Jellyfish are marine invertebrates belonging to the Class Scyphozoa within the Phylum Cnidaria. They can be found in every ocean in the world. The use of the term "jellyfish" is actually a misnomer since scyphozoans are not fish, which are vertebrates. The term is also (incorrectly) commonly-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 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, and the bell shape consists of a layer of epidermis, gastrodermis, and a thick layer called mesoglea that produces most of the jelly and separates the epidermis from the gastrodermis.

Defense

Most jellyfish have tentacles or oral arms coated with thousands of microscopic nematocysts; generally, each nematocyst has a "trigger" (cnidocil) paired with a capsule containing a coiled stinging filament, as well as barbs on the exterior. Upon contact, the filament rapidly unwinds, launches into the target, and injects toxins. It can then pull the victim into its mouth, if appropriate.

Although most jellyfish are not perniciously dangerous to humans, a few are highly toxic, such as Cyanea capillata. Contrary to popular belief, the menacingly infamous Portuguese Man o' War (Physalia) is not a jellyfish, but a colony of hydrozoan polyps, and the box jellies notorious along the coast of Australia are cubozoans, not true scyphozoan jellyfish. Regardless of the actual toxicity of the stings, many victims find them very painful, and some individuals may have severe allergic reactions, anaphylactic shock, similar to bee sting allergic reactions^[1]

Body systems

A jellyfish detects the touch of other animals using a nervous system called a "nerve net", found in its epidermis. Touch stimuli are collected by nerve rings, through the rhopalial lappet located around the animal's body, to the nerve cells. 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. They digest using the gastrodermis that lines the gastrovascular cavity, where nutrients from their food are absorbed. They do not need a respiratory system since their skin is thin enough that oxygen diffuses into their bodies. They 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.

The outer side of jellyfish is lined with a jelly-like material called ectoplasm ("outer plasma"). It typically contains a smaller amount of protein granules and other organic compounds than inner cytoplasm, also referred to as endoplasm.

Ecology, behavior and life history

Behavior

Many species of jellyfish are also capable of congregating into large swarms or "blooms" consisting of hundreds or even thousands of individuals. The formation of these blooms is a complex process that depends on ocean currents, nutrients, temperature and oxygen content. Jellyfish sometimes mass breed during blooms. Jellyfish population is reportedly raising major ecological concerns for a possible jellyfish outbreak.

The frequency of these blooms may be attributed to humankind's impact on marine life, according to Claudia Mills of the University of Washington. She says that the breeding jellyfish may merely be taking the place of already 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 recent proliferation of jellyfish numbers. Scientist Monty Graham 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 the biomass they contribute to this ocean region. 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 seriously affected by jellyfish blooms include the northern Gulf of Mexico, Graham states, "Moon jellies have formed a kind of gelatinous net that stretches from end to end across the gulf," .^[2] Jellyfish are commonly spotted along coastal shores.

Life history

Most jellyfish pass through two different body forms during their life cycle. The first is the polyp stage, when the jellyfish takes the form of either a sessile stalk which catches passing food, or a similar free-floating configuration. The polyp's mouth and tentacles face upwards.

In the second stage, the jellyfish is known as a medusa. Medusae have a radially symmetric, umbrella-shaped body called a bell. The medusa's tentacles hang from the border of the bell. (Medusa is also the Hebrew, Spanish and Italian word for jellyfish.)

Jellyfish are dioecious; that is, they are either male or female. In most cases, to reproduce, a male releases his sperm into the surrounding water. The sperm then swims into the mouth of the female, allowing the fertilization of the ova to begin. However, moon jellies use a different process: their eggs become lodged in pits on the oral arms, which form a temporary brood chamber to accommodate fertilization.

After fertilization and initial growth, a larval form, called the planula, develops from the egg. The planula larva is small and covered with cilia. It settles onto a firm surface and develops into a polyp. The polyp is cup-shaped with tentacles surrounding a single orifice, perhaps resembling a tiny sea anemone. Once the polyp begins reproducing asexually by budding, it is called a segmenting polyp, or a scyphistoma. New scyphistomae may be produced by budding or new, immature jellies called ephyra may be formed. Many jellyfish can bud off new medusae directly from the medusan stage.

Most jellyfish have a lifespan of two and a half months; few live longer than six months.

Etymology and taxonomic history

Since jellyfish are not fish, some people consider the term "jellyfish" a misnomer, and instead use the term "jellies" or "sea jellies". The word "jellyfish" is also often used to denote either Hydrozoa or the box jellyfish, Cubozoa. The class name Scyphozoa comes from the Greek word skyphos, denoting a kind of drinking cup and alluding to the cup shape of the organism.

A group of jellyfish is often called a "smuck".^[3]

Importance to humans

Culinary uses

Jellyfish is an important food in the Chinese community and many Asian countries.^[1] Only jellyfish belonging to the Order Rhizostomeae are harvested for food. The rhizostomes (Rhopilema esculentum, Chinese name: 海蜇) are favoured because they are typically larger and have more rigid bodies than other scyphozoan orders. Traditional processing methods involve a multi-phase processing procedure using a mixture of table salt and alum, and then desalting.^[1] Processing makes the jellyfish drier and more acidic, producing a "crunchy and crispy texture."^[1] Nutritionally, jellyfish prepared this way are roughly 95% water and 4-5% protein, making it a relatively low calorie food.^[1]

In biotechnology

In 1961, Green Fluorescent Protein was discovered in the jellyfish Aequorea victoria by scientists studying bioluminescence. This protein has since become one of the most useful tools in biology.^[2]

In captivity

Jellyfish are commonly displayed in aquariums in many countries; among the more known are the Monterey Bay Aquarium, Long Beach Aquarium of the Pacific, Vancouver Aquarium, Seattle Aquarium, National Aquarium in Baltimore and Maui Ocean Center. Often the tank's background is blue with the animals illuminated by side lighting to produce a high contrast effect. In natural conditions, many of the jellies are so transparent that they can be almost impossible to see.

Holding jellies in captivity also presents other problems: for one, they are not adapted to closed spaces or areas with walls, which aquariums by definition have. They also depend on the natural currents of the ocean to transport them from place to place. To compensate for this, professional exhibits feature precise water flow patterns, 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 in order. Though most jellyfish stings are not deadly, some stings, such as those perpetrated by the box jellyfish (Chironex fleckeri) may be fatal. Serious stings may cause anaphylaxis and eventual death, and hence people stung by jellyfish must get out of the water to avoid drowning. In these serious cases, advanced professional care must be sought. This care may include administration of an antivenom and other supportive care such as required to treat the symptoms of anaphylactic shock. The most serious threat that humans face from jellyfish is the sting of the Irukandji, which has the most potent and deadly venom of any known jellyfish species.

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% acetic acid in water) should be applied for box jellyfish stings.^[3][4] However, vinegar is not recommended for Portuguese Man o' War stings.^[3] In the case of stings on or around the eyes, vinegar may be placed on a towel and dabbed around the eyes, but not in them. Salt water may also be used in case vinegar is not readily available.^[3][5] 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.^[6]

Once deactivated, the stinging cells must be removed. This can be accomplished by picking off tentacles left on the body.^[6] First aid providers should be careful to use gloves or another readily available barrier device to prevent personal injury, and to follow standard universal precautions. 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.^[7]

Beyond initial first aid, antihistamines such as diphenhydramine (Benadryl) may be used to control skin irritation (pruritus).^[7] 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.

For at least some jellyfish, it seems to be safe to touch their tentacles with the palm of the hand.

In entertainment

Anthropomorphized jellyfish are characters in several popular video games, such as Pokémon's Tentacool, and in TV and film animation, such as SpongeBob SquarePants and Shark Tale. Like many other aquatic life forms, both real and anthropomorphized jellyfish have been represented in countless other aquatic-themed entertainment as well.

Gallery

Sea nettle Jellyfish (Chrysaora quinquecirrha) at the Monterey Bay Aquarium.	Blubber jelly (Catostylus mosaicus) at the Monterey Bay Aquarium.	Mastigias papua at the Monterey Bay Aquarium.	Australian spotted jellyfish (Phyllorhiza punctata).
Mediterranean Jellyfish (Cotylorhiza tuberculata) at the Monterey Bay Aquarium.	Chrysaora quinquecirrha	Narcomedusoid	Flower hat jelly at the Monterey Bay Aquarium.
Catostylus species of stingless jellyfish from Palau, Micronesia.

See also

• Turritopsis nutricula an immortal jellyfish
• Sea nettle
• Irukandji jellyfish
• Moon jelly
• Phacellophora camtschatica
• Cubozoa (the box jellyfish)
• Physconect siphonophore
• Cassiopeas
• Physalia physalis, Portuguese Man O' War (not a true jellyfish)
• Cotylorrhiza tuberculata
• Rhizostoma pulmo (also known as the Rhizostoma octopus or white jellyfish)
• Pelagia noctiluca (jellyfish mainly found in British water and Mediterranean)
• Craspedacusta sowerbyi, freshwater "jellyfish" (not a true jellyfish)
• Lion's mane jellyfish, with the longest known tentacles (over 100 feet)
• Ocean sunfish Predator of jellyfish

References

Bibliography

External links

• Sea Science: Jellyfish
• Cotylorhiza tuberculata
• Treatment of Coelenterate and Jellyfish Envenomations
• British Marine Life Study Society - Jellyfish Page
• Jellyfish - Curious creatures of the sea
• Sting treatment
• Jellyfish Grown with Twelve Heads

cdo:Tá

Donate to Wikimedia