Imagine trying to figure out the details of a jellyfish's life by studying the formless blob that it quickly becomes out of water. How would you determine what this creature looked like when it was alive? What clues would tell you how it caught its food? How could you fathom the way it moves through water? To learn how a particular type of animal lives, you must watch it—as it eats and is eaten, hides from predators, mates, and tends its young. What do you do, however, when the animal lives far below the ocean's surface?

The study of jellyfish evolved with the technology that gave scientists access to the deeper waters of the ocean. In the 1800's and early 1900's, groups of scientists set sail to explore the world's oceans. Using towed nets, these scientists collected at least half of the known robust (durable) species of jellyfish. Sometimes their nets brought up specimens from as far down as the abyssal plain (flat, deep stretches of the sea floor). Scientists also learned that, in certain places, they could effortlessly collect jellyfish species that normally live in the deep. Near Nice, off the coast of France, for example, seasonal upwelling currents deliver these deep-sea creatures to the surface. With the development of scuba equipment in the 1940's, biologists could closely observe jellyfish up to 30 meters (98 feet) below the surface.

The use of submersibles in science, beginning in the 1950's, revolutionized the study of jellyfish. These piloted or remotely operated vehicles (ROV's) provided scientists with exciting opportunities to study deep-dwelling jellyfish, especially the more fragile species, in their own environments. These vessels often record the details of jellyfish life with high-quality video cameras and other photographic equipment. Submersibles also can carry containers and other devices for collecting jellyfish alive and unharmed. For example, the reversible suction sampler—a sort of underwater vacuum cleaner—carefully sucks in a jellyfish as well as some of its surrounding water for transport to a ship.

Jellyfish, also called jellies, are transparent or semitransparent invertebrates (animals without backbones), most of which swim freely in the ocean. They get their name from the gelatinous (jellylike) material, called mesogloea, that lies between the two layers of cells that make up their body. This material, which serves as a skeleton, may be thick or thin and varies in density from floppy to stiff. Jellyfish live in all the oceans, from the tropics to the Arctic and Antarctic, and at all depths.

Scientists recognize two main groups of jellyfish—cnidarians (pronounced ny DAIR ee uhnz) and ctenophores (TEEN noh fohrz or TEHN uh fohrz). Cnidarians include the most familiar type of jellyfish, the bell-shaped creatures known to biologists as medusae. Medusae swim by expanding and contracting their body, like an umbrella opening and shutting. This jet action forces water out from beneath the body, and the medusa moves forward, bell first.

Although most medusae cannot see and do not have teeth or brains, they are efficient predators. Covering their tentacles are stinging cells called nematocysts. When touched, the nematocysts explode, driving tiny poisoned barbs like harpoons into the unlucky sea creature that came in contact with them. The poison can paralyze or kill the victim. The jelly then uses its tentacles to pass the prey to its mouth. Although all medusae are toxic to their prey, only a small number can seriously harm people.

Ctenophores, another major group of jellies, are also known as comb jellies. The body of a ctenophore may be shaped like a ball, a belt, or a thimble. A few deep-sea ctenophores can grow to 91 centimeters (3 feet), but most are closer to the size of a thimble. The comb jelly gets its name from the eight bands of comblike organs on the sides of its body, which consist of tightly packed rows of cilia (tiny hairlike structures). Ctenophores move through the water by beating their combs in a coordinated way. Most ctenophores catch prey using colloblasts, sticky structures covering their tentacles. For some ctenophores, transferring food to the mouth is an acrobatic exercise. They spin while contracting the muscles of their tentacles so that they become wrapped in their tentacles. The tentacles eventually sweep across the mouth, transferring the food.

The deep-sea environment of the jellyfish is relentlessly carnivorous, scientists have found. As a submersible maneuvers through the deeper areas of the ocean, its lights reveal jellyfish swimming or drifting, each with its tentacles expanded into a miniature “drift net” for capturing prey. The largest of these nets can extend up to several meters into the dark water. The shape of each net depends on the species and its hunting style. The jellies that create these nets are so abundant that some researchers have estimated that jellyfish may account for up to 40 percent of the biomass (total weight of living matter) of the open ocean.

Submersible research has also revealed that many jellyfish and most ctenophores are bioluminescent (able to produce light). Scientists have observed an array of lights in blues, greens, and reds, which seem to vary by habitat and, to some extent, by depth. The light may serve to scare off predators, though scientists are not always certain of its function because they rarely see jellyfish bioluminescence in nature.

Advances in molecular biology have led to important findings about the evolution of jellies and the genetic relationships between various jelly groups. For example, scientists studying jelly DNA (deoxyribonucleic acid; the material that determines heredity) have learned that medusae and ctenophores are not as closely related as their appearance and behaviors might suggest. They actually belong to separate phyla (primary divisions of the animal kingdom), which may be even more different than previously thought. Over time, the animals evolved in such ways that they now seem quite similar. In addition, some scientists, including biologist Kevin Peterson of Dartmouth College in Hanover, New Hampshire, have theorized that jellyfish were not just simple forerunners of more complex and varied animals. They argued that the jellyfish's anatomical and genetic complexity indicate that its evolutionary history is as elaborate as that of other animals.

Scientists also have made exciting discoveries about jellyfish anatomy and behavior. They understand that though jellies do not have a brain to receive and interpret sensory information, their nerve cells can still communicate in an organized way. For example, some species react to light, moving down through the water as daylight grows and up as the light fades. The jellyfish may be moving along with small plankton, which they eat.

Some of scientists' most important recent findings have highlighted the importance of medusae and ctenophores in marine ecosystems (the living things in a particular place and their interrelated physical and chemical environment). For example, an increase in jellyfish blooms (sudden population explosions) may signal environmental problems. In some places, blooms seem to be occurring because pollution—in the form of sewage or runoff from farms and cities—has improved feeding conditions for certain jellyfish species.

In 2004, plate-sized Sanderia malayensis bloomed for the first time in the estuary of China's Yangtze River, whose flow has been radically changed by the construction of the Three Gorges Dam to the southwest. (An estuary is the broad mouth of a river where its fresh water meets the salt water of the sea.) The medusae have seriously disrupted summer trawl fishing in the estuary, one of China's richest fishing grounds. In some areas, jellyfish have blanketed the water's surface.

Other jellyfish blooms have resulted from invasions by exotic (foreign) species that escaped their natural predators. Among these is an American ctenophore that has nearly destroyed commercial fishing in the Black and Caspian seas. In the eastern Mediterranean, the invasion of a stinging jellyfish from the Red Sea has severely affected swimming beaches.

Some scientists have also linked jellyfish blooms to overfishing. As large predator fish disappear from an ecosystem, some scientists suspect that local jellyfish benefit from a more abundant food supply, and their populations explode. Understanding the impact of these blooms on marine ecosystems ranks as one of scientists' most important tasks.