Among the most extraordinary animals on earth, jellyfish (Cnidarians, scyphozoans, cubozoans, and hydrozoans) are also some of the most ancient, with an evolutionary history stretching back for hundreds of millions of years. Found in all oceans of the world, jellies are made up of 90 to 95 percent water, compared to 60 percent for humans.

Fast Facts: Jellyfish Scientific Name: Cnidarian; scyphozoan, cubozoan, and hydrozoan

Cnidarian; scyphozoan, cubozoan, and hydrozoan Common Name: Jellyfish, jellies

Jellyfish, jellies Basic Animal Group: Invertebrate

Invertebrate Size: Bell diameter of two-tenths of an inch to over six and a half feet

Bell diameter of two-tenths of an inch to over six and a half feet Weight: Under an ounce to 440 pounds

Under an ounce to 440 pounds Lifespan: Vary between a few hours to a few years

Vary between a few hours to a few years Diet: Carnivore, Herbivore

Carnivore, Herbivore Habitat: Oceans throughout the world

Oceans throughout the world Population: Unknown

Unknown Conservation Status: Not Evaluated

Description

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 anemones); the other half are 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 come in a wide variety of shapes and sizes. The largest is the lion's mane jellyfish (Cyanea capillata), which can have a bell over six and a half feet in diameter and weigh up to 440 pounds; the smallest is the Irukandji jellyfish, several species of dangerous jellyfishes found in tropical waters, which measure only about two-tenths of an inch and weigh well under a tenth of an ounce.

Jellyfish lack a central nervous system, a circulatory system, and a respiratory system. Compared to vertebrate animals, they 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. Water makes up 95 to 98 percent of their total bulk, compared to about 60 percent for the average human being.

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.

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—this gives 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.

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Species

Scyphozoans, or "true jellies," and cubozoans, or "box jellies," are the two classes of cnidarians comprising the classic jellyfish; the main difference between them is that cubozoans have boxier-looking bells than scyphozoans and are slightly faster. There are also hydrozoans (most species of which never got around to forming bells and instead remain in polyp form) and staurozoans, or stalked jellyfish, which are attached to the seafloor. (Scyphozoans, cubozoans, hydrozoans, and staurozoans are all classes of medusozoans, a clade of invertebrates directly under the cnidarian order.)

Diet

Most jellyfish eat fish eggs, plankton, and fish larvae, converting them to energy in an alarming pattern known as an energy-loss pathway. That kind of pathway consumes energy that would otherwise be used by forage fish who can be eaten by top-level consumers. Instead, that energy is being communicated to animals which eat jellyfish, not part of the higher food chain.

Other species, like upside-down jellies (Cassiopea species) and Australian Spotted Jellyfish (Phyllorhiza punctata), have symbiotic relationships with algae (zooxanthellae), and they obtain enough carbohydrates from them to not need additional food sources.

Lion's mane jellyfish (Cyanea capillata) eating Sarsia tubulosa. Cultura RF/Alexander Semenov/Getty Images

Behavior

Jellyfish practice what is called vertical migration, arising from the ocean depths to the surface in large aggregations known as blooms. In general, they bloom in the spring, reproduce in the summer, and die off in the fall. But different species have different patterns; some migrate once or twice a day, and some migrate horizontally following the sun. The jellies most injurious to humans, the Irukandji species, undergo seasonal migrations which bring them into contact with swimmers in the tropics.

Jellyfish spend all of their time is seeking food, escaping predators, or finding a mate—some set a trap with their tentacles arranged in a spiral pattern, an impenetrable curtain for their prey, or array their tentacles in a big field around their bodies. Others simply drift or swim slowly, dragging their tentacles behind them like a trawler net.

Some species are pleustonic, meaning they live at the air/water interface year round. Those include the sailing jellies, like the Portuguese man-of-war, the Blue Bottle, and the By-the-Wind Sailor Jelly (Velella vellal), which has an oblong blue raft and a silvery vertical sail.

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, 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 (such as being eaten by predators or washing up on the beach).

Reproduction and Offspring

Jellyfish hatch from eggs which are fertilized by males after females expel the eggs into the water. 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 anemone. 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.

Humans and Jellyfish

People worry 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 its sting is known 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 human adult can die in as little as two to five minutes.

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 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.

Threats

Jellyfish are prey for sea turtles, crabs, fish, dolphins, and terrestrial animals: There are some 124 fish species and 34 other species that are reported to feed either occasionally or mainly on jellyfish. Jellyfish often establish symbiotic or parasitic relationships with other species—the parasitic ones are almost always detrimental to the jellyfish.

Many species—sea anemones, brittle stars, gooseneck barnacles, lobster larvae and fish—hitch rides on jellyfish, finding safety from predators in the folds. Octopuses are known to use jellyfish tentacle fragments on sucker arms as added defensive/offensive weaponry, and dolphins tend to treat some species like underwater frisbees. Jellyfish have been considered a delicacy for human diets since at least 300 CE in China. Today, fisheries raising jellyfish for food exist in 15 countries.

But jellyfish may have the last laugh. Far from being a threatened species, jellyfish are on the increase, moving into habitats that have been damaged or destroyed for other marine creatures. Increased blooms can have negative impacts on human economic activities, clogging cooling water intakes at coastal power plants, bursting fishing nets and contaminating catches, killing off fish farms, reducing commercial fish abundance through competition, and interfering with fisheries and tourism. The primary causes for habitat destruction are human over-fishing and climate change, so the reason for the uptick in jellyfish blooms can be assigned to human interference.

Alastair Pollock Photography/Getty Images