Their shape is not hydrodynamic, which makes them slow
swimmers but this is little hindrance as they feed on plankton,
needing only to drift slowly through the water. It is more
important for them that their movements create a current where
the water (which contains their food) is being forced within
reach of their tentacles. They accomplish this by rhythmically
opening and closing their bell-like body.
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. 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.
The developmental stages of jellyfish.Most jellyfish pass through two different body forms during their life cycle. The first is the polyp stage; in this phase, 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 are facing 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 Spanish name 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 jelly, allowing the fertilization of the ova process to begin. Moon jellies, however, 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's 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.
Most jellyfish have tentacles or oral arms coated with thousands of microscopic nematocysts; generally, each of these 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 will swiftly unwind, launch into the target, and inject 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 actually 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.
A typical scyphomedusa jellyfish.A 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. They are able to digest with the help of 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 can easily diffuse in and out of their bodies. 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.
In cell biology, ectoplasm ("outer plasma") refers to the outer regions of jelly fish. The jelly like material called ectoplasma typically contains a smaller amount of protein granules and other organic compounds than inner cytoplasm, also referred to as endoplasm.
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. The formation of these blooms is a complex process
that depends on ocean currents, nutrients, temperature and
oxygen content. Jellyfish will sometimes mass breed during
blooms. Jellyfish population is reportedly raising major
ecological concerns for a possible jellyfish outbreak.
According to Claudia Mills of the University of Washington, the frequency of these blooms may be attributed to mankind's impact on marine life. 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 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 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, where "moon jellies have formed a kind of gelatinous net that stretches from end to end across the gulf," and the Adriatic Sea. Jellyfish are commonly spotted along coastal shores.
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, 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.
This Jellyfish Page is Copyright The Animal Web Guide © 2004 - 2009 Chuck Ayoub