The name jellyfish is actually a misnomer: these animals aren't fish at all. They are part of a group of animals called coelenterates, which also includes corals and sea anemones.
There are two stages in the jellyfish life cycle. First is the polyp stage, when the animal looks like a small stalk on the ocean floor. In the second stage, the animal assumes the familiar umbrella shape, called a medusa, which swims away from the stalk to grow by feeding on plankton. Larger jellyfish may feed on shrimp or small fishes. Prey is captured in the tentacles, where it is stunned or killed by stinging cells called nematocysts. Even a dead or dying jellyfish can sting when touched.
Their behavior is simple: most swim slowly or are carried by ocean currents. Jellyfish appear transparent because less than 5% of their body mass is organic matter, the rest is water.
In recent years, scientists have noticed a huge rise in jellyfish populations in tropical areas, and suspect this increase may be caused by global climate change. So many jellyfish threaten the fishing industry, by preying on fish normally collected by humans. It also creates an obvious nuisance to tourism: jellyfish stings are very painful, and some can be deadly. The venom of some species is as toxic as a cobra's, and can cause death in under three minutes.
Although they have little nutritional value, some varieties are collected as food. Mostly this takes place in southeast Asia, where they are cured with salt to produce a crunchy, crispy texture. In China they are often served raw, with oil dressing or as a salad with vegetables . The cannonball jellyfish is a favorite delicacy, because of its meaty body and because its toxins are harmless to humans.
Biologists are especially interested in a species called turritopsis nutricula, because it appears to be immortal. This jelly, found in the Caribbean Sea, can change from a medusa back to the polyp stage, allowing it to enter another life cycle, and another after that. Some individual turritopsis might be thousands of years old. Researchers think humans may one day be able to live up to 500 years if this process can be duplicated.
Most famous of all is the giant, dangerous, Portuguese Man-Of-War. It is common in tropical seas, floating on the surface with a brightly colored, gelatinous mass. This is actually a colony of hundreds or thousands of individual animals, living together for mutual defense. They have no independent means of propulsion, and travel either by floating in the currents or by catching wind with their billowing sails. When threatened, they can deflate the airbag and briefly submerge. The tentacles typically grow to over 60 feet in length, and have sunk ships by getting tangled underneath them.
Finally, it is a myth that urine cures a jellyfish sting. So don't try it.
There's a cool, two minute video worth checking out below:
This is my second blog post on animal life. For the first one, on sponges, click here.