Jellyfish

Feb 14, 2010 Features / Columnists, Interesting Creatures in Guyana

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Jellyfish (also jellies or sea jellies) are free-swimming members of the phylum Cnidaria. Jellyfish have several different morphologies that represent several different classes.
They can be found in every ocean, from the surface to the deep sea. Some are also found in fresh water; freshwater species are less than an inch (25 mm) in diameter, are colourless and do not sting. Many of the best-known jellyfish are large, often colourful, jellyfish that are common in coastal zones worldwide.
Since jellyfish are not actually fish, some people consider the term “jellyfish” a misnomer, and American public aquariums have popularized use of the terms jellies or sea jellies instead. Others find the word jellyfish, which has been in common usage for more than a century, to be equally useful and picturesque, and prefer it over jellies.
A group of jellyfish is sometimes called a bloom or a swarm. “Bloom” is usually used for a large group of jellyfish that gather in a small area, but may also have a time component, referring to seasonal increases, or numbers beyond what was expected.
Jellyfish are “bloomy” by nature of their life cycles, being produced by their benthic polyps usually in the spring when sunshine and plankton increase, so they appear rather suddenly and often in large numbers, even when an ecosystem is in balance.
Most jellyfish have a second part of their life cycle, which is called the polyp phase. When single polyps, arising from a single fertilized egg, develop into a multiple-polyp cluster, connected to each other by strands of tissue called stolons, they are said to be “colonial.” A few polyps never proliferate and are referred to as “solitary” species.
Jellyfish do not have 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, but can use their hydrostatic skeleton to accomplish movement through contraction-pulsations of the bell-like body; some species actively swim most of the time, while others are passive much of the time.
Jellyfish are composed of more than 90 percent water; most of their umbrella mass is a gelatinous material – the jelly – called mesoglea which is surrounded by two layers of epithelial cells which form the umbrella (top surface) and subumbrella (bottom surface) of the bell, or body.
Jellyfish do not have a brain or central nervous system, but rather have a loose network of nerves, located in the epidermis, which is called a “nerve net.” A jellyfish detects various stimuli including the touch of other animals via this nerve net, which then transmits impulses both throughout the nerve net and around a circular nerve ring, through the rhopalial lappet, located at the rim of the jellyfish body, to other nerve cells.
Some jellyfish are harvested for food; about 12 of the approximately 85 species are harvested and sold on international markets.
They are displayed in aquariums in many countries. Often the tank’s background is blue and the animals are illuminated by side light, increasing the contrast between the animal and the background.
In natural conditions, many jellyfish are so transparent that they are nearly invisible.
Jellyfish are not adapted to closed spaces. They depend on currents to transport them from place to place.
All jellyfish sting their prey using nematocysts, also called cnidocysts, stinging structures located in specialized cells called cnidocytes.
Contact with a jellyfish tentacle can trigger millions of nematocysts to pierce the skin and inject venom, yet the sting of only some jellyfish species causes an adverse reaction in humans.
Touching, or being touched by a jellyfish can be very uncomfortable, sometimes requiring medical assistance; sting effects range from no effect to extreme pain to death. Because of the wide variation in response to jellyfish stings, it is wisest not to contact jellyfish with bare skin. Even beached and dying jellyfish can still sting, when touched.
Some species stings are often uncomfortable, though not generally deadly, but some species of the class Cubozoa, or the Box jellyfish, such as the famous and especially toxic Irukandji, can be deadly. Stings may cause anaphylaxis, which may result in death. Hence, victims should immediately get out of the water. Medical care may include administration of anti-venom.
(Source: Wikipedia – The Free Online Encyclopedia)

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