Jellyfish Are Technically Classified as "Cnidarians"
Named after the Greek word for "sea nettle," cnidarians are marine animals characterized by their jelly-like bodies, their radial symmetry, and their "cnidocytes," cells on their tentacles that literally explode when stimulated by prey. There are about 10,000 cnidarian species, roughly half of which are anthozoans (a family that includes corals and sea anenomes) and the other half scyphozoans, cubozoans and hydrozoans (what most people refer to when they use the word "jellyfish"). Cnidarians are among the oldest animals on earth; their fossil record stretches back for almost 600 million years!
Jellyfish Are Among the World's Simplest Animals
What can you say about animals that lack a central nervous system, a circulatory system, and a respiratory system? Compared to vertebrate animals, jellyfish are extremely simple organisms, characterized mainly by their undulating bells (which contain their stomachs) and their dangling, cnidocyte-spangled tentacles. Their nearly organless bodies consist of just three layers-the outer epidermis, the middle mesoglea, and the inner gastrodermis-and water makes up 95 to 98 percent of their total bulk, compared to about 60 percent for the average human being.
Jellyfish Begin Their Lives as Polyps
Like all animals, jellyfish hatch from eggs, which are fertilized by males after females expel the eggs into the water. After that, though, things get complicated: what emerges from the egg is a free-swimming planula, which looks a bit like a giant paramecium. The planula soon attaches itself to a firm surface (the sea floor, a rock, even the side of a fish) and grows into a stalked polyp reminiscent of a scaled-down coral or anenome. Finally, after months or even years, the polyp launches itself off its perch and becomes an ephyra (for all intents and purposes, a juvenile jellyfish), and then grows to its full size as an adult jelly.
Some Jellyfish Have Eyes
Weirdly, box jellies, or cubozoans, are equipped with as many as two dozen eyes-not primitive, light-sensing patches of cells, as in some other marine invertebrates, but true eyeballs composed of lenses, retinas and corneas. These eyes are paired around the circumference of their bells, one pointing upward, one pointing downward-giving some box jellies a 360-degree range of vision, the most sophisticated visual sensing apparatus in the animal kingdom. Of course, these eyes are used to detect prey and avoid predators, but their main function is to keep the box jelly properly oriented in the water.
Jellyfish Have a Unique Way of Delivering Venom
Most poisonous animals deliver their venom by biting--but not jellyfish (and other cnidarians), which have evolved specialized structures called nematocysts. There are thousands of nematocysts in each of the thousands of cnidocytes (see slide #2) on a jellyfish's tentacles; when stimulated, they build up an internal pressure of over 2,000 pounds per square inch and explode, piercing the skin of the unfortunate victim and delivering thousands of tiny doses of venom. So potent are nematocysts that they can be activated even when a jellyfish is beached or dying, which accounts for incidents where dozens of people are stung by a single, seemingly expired jelly!
The Sea Wasp Is the Most Dangerous Jellyfish
Everyone worries about black widow spiders and rattlesnakes, but pound for pound, the most dangerous animal on earth may be the sea wasp (Chironex fleckeri). The biggest of all box jellies-its bell is about the size of a basketball and its tentacles are up to 10 feet long-the sea wasp prowls the waters of Australia and southeast Asia, and is know to have killed at least 60 people over the last century. Just grazing a sea wasp's tentacles will produce excruciating pain, and if contact is widespread and prolonged, a full-grown human victim can die in as little as two to five minutes.
Jellyfish Move by Undulating Their Bells
Jellyfish are equipped with hydrostatic skeletons, which sound like they might have been invented by Iron Man, but are actually an innovation that evolution hit on hundreds of millions of years ago. Essentially, the bell of a jellyfish is a fluid-filled cavity surrounded by circular muscles; the jelly contracts its muscles, squirting water in the opposite direction from where it wishes to go. (Jellyfish aren't the only animals to possess hydrostatic skeletons; they can also be found in starfish, earthworms, and various other invertebrates.) Jellies can also move along ocean currents, thus sparing themselves the effort of undulating their bells.
One Species of Jellyfish May Be Immortal
Like most invertebrate animals, jellyfish have very short lifespans: some small species live for only a few hours, while the largest varieties, like the lion's mane jellyfish, may survive for a few years. Controversially, one Japanese scientist claims that the jellyfish species Turritopsis dornii is effectively immortal: full-grown individuals have the ability to revert back to the polyp stage (see slide #5), and thus, theoretically, can cycle endlessly from adult to juvenile form. Unfortunately, this behavior has only been observed in the laboratory, and T. dornii can easily die in many other ways (say, being eaten by predators or washing up on the beach).
A Group of Jellyfish Is Called a "Bloom" or "Swarm"
Remember that scene in Finding Nemo where Marlon and Dory have to thread their way through a jellyfish traffic jam? Technically, this type of aggregation is known as a bloom or a warm, and consists of hundreds or even thousands of individual jellyfish. Marine biologists have noticed that jellyfish blooms are getting bigger and more frequent, which may be an indicator of pollution and/or global warming (blooms are more likely to form in warm water, and jellyfish can also thrive in oxygen-depleted marine environments that comparably sized invertebrates have long since fled).