Jellyfish
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[Photo] Sea nettle jellyfish, Chrysaora quinquecirrha at the Monterey Bay Aquarium. 2004. Source: Taken by user (Tom Murphy VII).

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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. 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 a jellyfish, but a colony of hydrozoan polyps. 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

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," . 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".

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