They may be primitive, but jellyfish have much to teach us about survival in both ocean depths and outer space
- Cheryl Lyn Dybas
- Aug 01, 1992
During the dead of a July night on the Chesapeake Bay, most hands of the research vessel Cape Henlopen are below deck, asleep in their humid bunkrooms. But not Jennifer Purcell. The 36-year-old oceanographer is sitting under the ship's bright deck lights, trying to stay awake.
Finally, just before 1 A.M., she sees what she's been waiting for: stinging sea nettles, hundreds of them, undulating slowly toward the ship. Four inches wide and ghostly white, each jellyfish trails 24 tentacles--a foot-long mass of colorless streamers that can deliver a nasty sting to exposed human skin.
"As long as I'm not planning to take a midnight swim, the appearance of all these jellies is a lucky break," says Purcell, a biologist at the University of Maryland Horn Point Environmental Laboratory in Cambridge. She's on board the Cape Henlopen as part of her research into the role of jellyfish in the interaction of predators and prey in the sea.
Jellyfish feed in shallow waters by day and normally sink to deeper regions by night. But for now, the ship's bright lights have lured microscopic crustaceans and mollusks--and therefore the jellies that eat them--to the surface. Purcell leans over the gunwale, dip net in hand, and scoops up some nettles for further study in the laboratory.
"They may seem threatening," explains Purcell, "but the nettles are performing an important service to the bay by controlling numbers of their prey." For example, fishermen once thought sea nettles were eating oyster larvae. But that's not so, says the scientist, pointing out that the jellies are actually helping the shellfish by preying on microscopic creatures that do eat the oysters' young. "We ought to give jellyfish a break!"
Washed up on a beach, jellyfish look like ugly gobs of slime. But in the sea, these fragile creatures shimmer with jewellike radiance. Their real beauty, though, is their crucial contribution to the functioning of marine ecosystems. "We're just beginning to discover how important jellyfish are to the web of life in the sea," says Purcell.
Researchers are finding that jellyfish may help shed light on the complex relationships among creatures in the oceanic food chain. They may also foretell changes in the environment and even show scientists how people would fare as residents of another realm--outer space.
Jellyfish can live practically anywhere there's water: under the ice in Arctic and Antarctic seas, or even in some North American freshwater lakes and streams, where one tiny species is found. No one knows exactly how many species of jellyfish inhabit the Earth; scientists are still discovering new ones. But some experts say there may be as many as 2,000.
The creatures range in size from the 8foot-wide lion's mane jelly, found off the Atlantic and Pacific coasts of this country, to the angled hydromedusa, a half-inchlong creature that clings to eelgrass fronds in shallow Atlantic and Pacific waters. With tissues made up of 95 percent salts and water, jellyfish are the ocean brought to life.
Most jellyfish are propelled by the rhythmic contraction and expansion of an umbrella-shaped saucer, or bell, as well as by winds, currents and tides. Nearly all are part of a drifting community of organisms called plankton, a term derived from the Greek word for wanderer. Marine scientists refer to animal drifters with gelatin-like tissues as gelatinous zooplankton. This group encompasses the familiar bell-shaped jellyfish, related walnut-shaped comb jellies and similar creatures such as siphonophores, which are chains or colonies of jelly animals.
Many jellyfish live for just one summer. Before they die in September, females of most species release hundreds of eggs into the water, then males release sperm. The resulting larvae swim to the bottom and attach themselves to a hard surface. In spring they bud into tiny jellyfish, and the cycle begins anew.
Jellies prey on a host of microscopic and larger sea creatures, and are themselves food for sea turtles, most of which are endangered, and fish such as the giant ocean sunfish. In Alaskan waters, saucerlike moon jellyfish provide nourishment for green sea urchins, crabs and burrowing anemones, among others.
Bell-shaped jellyfish dangle streams of stinging tentacles as a means of capturing food. The tentacles contain millions of microscopic darts, each filled with a fast-acting toxin and ready to strike at the merest touch. Some jellyfish can inflict serious wounds on unwary swimmers. Contact with the more than 150 stinging tentacles of a large lion's mane, for example, can cause severe burning and muscle cramps, depending on the victim.
Only one jellyfish inhabiting U.S. waters is consistently dangerous to anyone who comes in contact with it--the Portuguese man-of-war. This 12-inch-long, 5-inch-wide jelly co drifts along the surface, using its 2 balloonlike float as a sail to catch prevailing winds. Its tentacles can inflict burns and blisters long after the creature has washed up dead on a beach. (Worldwide, there are fewer than five documented human deaths annually from man-of-war stings.)
Jellyfish are "simple" creatures, in that they don't have highly specialized organs. Yet more than 600 million years ago they were the first animals to develop nerve and muscle cells. That's just what makes them so attractive to Dorothy Spangenberg, a researcher at Eastern Virginia Medical School in Norfolk.
Through a NASA-sponsored research project to put jellyfish into space, Spangenberg is studying how animals react to a weightless environment. "Because jellyfish were probably the first animals to organize individual nerve and muscle cells into a group of cells that could sense the presence or absence of gravity--'gravity receptors'--they're an excellent choice for a study of this kind," explains Spangenberg, escorting a visitor through the laboratory where she tends thousands of tiny jellyfish in a row of aquariums.
At NASA's direction, Spangenberg cultured more than 60,000 moon jellyfish, 2,700 of which were chosen for a nine-day flight aboard the shuttle Columbia in spring of 1991. The creatures traveled in plastic bags inside an incubator. Since the jellies' return to Earth, the biologist has examined their tissues to find out if they functioned normally in the weightlessness of space. "I'll know something more about the answer to that question in a few months," she says. Information from the biologist's experiment may be useful during flights when other animals, or even people, are sent into space.
Spagenberg calls NASA's decision to support her research "truly pioneering and imaginative." But the government's attitude about jellyfish wasn't always so enlightened: In 1966, Congress passed the Jellyfish Control Act, which appropriated nearly $5 million for studying ways to "control and eliminate jellyfish and other such pests in our coastal waters."
David Cargo well remembers the time when federal research money was available to "anyone with a proposal to eradicate jellyfish from America's swimming holes." Cargo, who recently retired from the University of Maryland Chesapeake Biological Laboratory in Solomons after 25 years of studying jellyfish, says research projects funded by the act--including an effort to develop a jellyfish "pesticide"--were set up in a number of states.
"But even then I wondered if we'd end up producing a biological wasteland if we put jellyfish-killing chemicals into the marine system," says Cargo, who still returns to his lab to conduct an annual jellyfish survey. "I used to walk up and down this dock every day, counting jellies as I went, so I could give people around the Chesapeake some idea of how 'bad' the problem was going to be. Now," he says, "I think we scientists need to make more of an effort to hear just what these creatures are trying to tell all of us about our environment."
But finding out just what jellyfish have to tell us is no easy task. The creatures rarely survive being hauled out of the water in collecting nets, the primary research tool in Dave Cargo's day. Today, scientists can observe fragile jellyfish in their element by using advanced instruments such as the remote-controlled Plankton Imagery Camera.
Towed behind a ship, the camera glides along like a trailing baleen whale, clicking photos of jellylike sea creatures at a rate of one per second as they pass through its mouthlike opening. "This camera allows us to see jellyfish undamaged and alive," says John Olney, an oceanographer at the Virginia Institute of Marine Science in Gloucester Point, "and to identify creatures so delicate that they'd be nothing more than goo if we tried to bring them up in a net." With instruments attached to the camera that measure water temperature, salinity, depth and oxygen concentration each time a picture is taken, scientists have unprecedented access to a wealth of information about the environment in which a creature lives.
Every summer, Olney and a team of researchers set out from the marine science institute to study jellyfish aboard the Bay Eagle, one of the few vessels equipped with the Plankton Imagery Camera. From the pictures, says Olney, scientists hope to estimate the abundance of jellyfish in various parts of the ocean. That knowledge, in turn, may help them answer such questions as whether jellyfish are for some reason attracted to coastal areas of the country where polluted runoff boosts concentrations of nitrogen and other elements.
Among the most common subjects to pass through the imagery camera are comb jellies, luminescent drifters named for the eight rows of shimmering "combs" that line their translucent bodies. The "teeth" of these combs are actually tiny vibrating hairs that propel the creatures through the water. At night, comb jellies glow fluorescent green or flash eerie blue light when disturbed by boats or swimmers. Like fireflies on land, they produce their otherworldly glow by a chemical reaction involving a light-emitting enzyme.
Comb jellies don't have, or need, the stinging tentacles of bell-shaped jellyfish. Instead, they use adhesive cells to snare prey, simply ingesting whatever small creatures they come in contact with as they float along. Vacuum cleaners of the sea, swarms of comb jellies can devour whole patches of freshly spawned fish eggs in a matter of hours.
In the Chesapeake, a species called Leidy's comb jelly reaches its greatest abundance about the same time bay anchovies are spawning millions of eggs, explains Edward Houde, a biologist at Chesapeake Biological Laboratory. Houde has teamed up with John Olney to find out whether Leidy's comb jelly, sometimes called the sea walnut, is regulating numbers of bay anchovies by eating "excess eggs"--those that would hatch into more fish than the Chesapeake could sustain. Says Houde, "Bay anchovies may owe the continued health of their population to these grazing comb jellies."
Other fishes, including the small, silvery butterfish, also owe a debt to jellyfish. A young butterfish relies almost totally on a host jellyfish for the first few months of its life, taking up residence within the jelly's tentacles from the moment it hatches until it migrates out to sea in the fall. The fish depends on its host as a food collector and for protection from harsh sunlight and predators. As for the jelly's tentacles, the fish is either agile enough to avoid them or somehow immune to their sting.
Leatherback and loggerhead sea turtles likewise remain unaffected by jellyfish stings, according to biologist Peter Lutz of Florida Atlantic University in Boca Raton. The researcher has conducted extensive studies of sea-turtle predation on jellyfish, and as yet has "no answer to how turtles can chomp away on jellyfish and not get sick--especially when you consider that jellies make up over 90 percent of some sea turtles' diets." Lutz calculates that sea turtles eat their weight in jellyfish every day, which for some translates into a half-ton helping of Jell-O that bites back.
Perhaps the most unusual relationship between marine animals and gelatinous zooplankton is the dependence of some deep-sea fishes on a group of creatures called siphonophores. Six miles off the coast of Central California, biologist Bruce Robison of the Monterey Bay Aquarium Research Institute studies these relatives of the Portuguese man-of-war with the help of "Ventana," a remote-controlled, underwater robot equipped with a television camera.
Deep-sea siphonophores are translucent organisms resembling long strands of rope. Each is actually an association of specialized polyps, functioning as a single organism and sharing housekeeping duties. Some polyps exist solely to propel the creature. Others collect food with short, curtainlike tentacles, while still others defend the group or take care of reproduction. Chains of these colonies, some of them 100 feet long, slowly drift wherever currents take them.
In regions where ocean depths reach several thousand feet, an entire category of marine animals lives in close association with siphonophores. Like trees and rocks on land, siphonophores serve as substrata for fish and other deep-sea denizens, which hide behind, nibble food from and even hitch rides on the gelatinous clusters. Says Robison, "We're finding out that we've grossly underestimated the ecological significance of jelly animals like siphonophores."
Visitors to the Monterey Bay Aquarium can see siphonophores through a video link to Ventana, which transmits eerie images of drifting wraiths from waters hundreds to thousands of feet deep. Earlier this year, the aquarium opened "Planet of the Jellies," the first jellyfish exhibit to feature deep-sea species, some trailing lacy tentacles, others pulsing with rainbow bands of light.
Aquarium officials hope that seeing these creatures up close will convince people that jellyfish are among the sea's greatest treasures. If nothing else, says Robison, "they offer us a window into a world we know very little about."
Virginia writer Cheryl Lyn Dybas traveled on several research ships for this article.