Not too many years ago, Maryland, Virginia and the federal government joinedforces to tackle a nettlesome problem that has plagued Chesapeake Bay sinceIndian times: jellyfish.

The idea was to isolate a vulnerable stage in the life span of stinging Chesapeake sea nettles and chemically eradicate the pests, which blossom in the waters from Baltimore to Norfolk in high summer like waterborne poison ivy.

Sounds simple enough, except that no one ever found the vulnerable stage, which came as no great surprise to the scientists involved.

"I think Congress had the idea maybe we could stamp them out," said Ronald Larson, North America's reigning dean of jellyfish studies, "but the truth is, most biologists knew from the start it was hopeless."

They knew it because they knew the jellyfish is one of the oldest multicellular creatures in the animal kingdom, and it didn't get that way by being vulnerable. Older even than humble flatworms, jellyfish have been around hundreds of millions of years. The likelihood that they would fall for some trick cooked up by man, a veritable newborn, was remote.

What scientists found, instead of a chink in the sea nettle's armor, was a creature so reproductively bulletproof it survived almost without a hitch the most devastating natural disaster in the bay's modern history, Hurricane Agnes, in June 1972.

Agnes dumped enough rain to turn the nettles' prime spawning area in the bay into a freshwater lake. It was a sea change that might have decimated the species, which needs brackish water to spawn. But while Agnes sent oyster populations plummeting and wiped out huge beds of sea grasses, which never recovered, the jellyfish reproductive polyps, which cling to oyster shells and rocks on the bottom, simply did what had come naturally for eons: Facing hard times, they rolled up into tight, impervious, cyst-like balls and waited for conditions to improve.

"I think every polyp in the northern half of the bay encysted that year," said David Cargo, who counts jellyfish for the University of Maryland Chesapeake Biological Laboratory on Solomons Island. "I guess I counted three jellyfish all summer."

When the floods receded and salt water returned, the polyps reopened like flowers in the sun and started churning out baby jellyfish as if nothing had ever happened. By the following year, counts were up to near normal.

"When we first got into the business, we thought the polyp stage would be a weak link," said Cargo. "But the closer we looked, the tougher the polyps proved to be and the more we realized we couldn't do much to destroy that stage and eradicate the sea nettle from the bay."

So Maryland, Virginia and the federal government folded the tent. The primordial, purplish, pulsating jellyfish had won, and scientists went home with a new respect for the resiliency and fecundity of the bay's resident nuisance, bane of swimmers, crabbers and water-skiers.

Even in slow summers, when heavy spring rains keep sea nettle production down, the bay supports billions of them in high summer, more than enough to dissuade people from swimming or even wading, which infuriates beach concessionaires, waterfront property owners and boaters.

Along the Chesapeake's eastern and western shores, beaches lie deserted as throngs zoom past in their cars, headed for the ocean, where jellyfish are rarer and generally less noxious.

Now jellyfish season is upon us again. So what exactly are these strange, annoying, primitive nuisances, and why are they wrecking our fun?

Sea nettles are 97 percent water, which makes them hard to stamp out from the get-go. Worse, they come in two forms, each of which reproduces with great success -- the harmless, tiny polyps that live out of sight anchored to the bay bottom, and the dreaded medusas, which swim around trailing their long tentacles and stinging everything in range, including innocent humans.

While the polyps live several years, medusas live only a few months, usually July through September or October, when the change in water temperature kills them.

What they lack in longevity, medusas make up for in unwanted abundance. Some years it seems like there are more medusas in the bay than water. In 1969, Cargo counted 5,000 during one stroll down a 300-foot dock at Solomons.

Despite their brief lives, medusas mate like mad, the males setting off a shower of sperm that finds the females. The females then emit embryos smaller than salt grains, which swim to the bay bottom, attach to hard objects and form the hardy, one-eighth-inch-long, tentacled polyps, or embryos. An individual oyster shell might hold as many as 1,000 polyps.

Four or five times a summer, starting in May, the medusa polyps spin off five to 10 tiny, jellied disks that within a month become full-size, sexually mature medusas. They are so successful at this job that by August, you look over the side of a boat and it's like watching the umbrellas of Cherbourg from above as thousands of medusas pulsate along on their aimless, pain-inflicting journeys to who-knows-where.

They are beautiful in their way, all instinct and survival strategy. The pulsing of the umbrellas is their locomotion, while behind each creature trail several dozen frail, stinging tentacles that immobilize prey on contact, plus four larger, central oral arms that propel the stunned victims to digestive organs under the umbrella.

At full size, the umbrella is five to nine inches across and the tentacles up to four feet long. The entire creature, including the umbrella, is coated with the stinging cells -- minute, coiled darts that fire instantly when they brush anything, shooting out enzymes and protein that produce a toxic reaction. Tentacles detached from the medusa will sting even after the jellyfish is washed up on a beach, dead.

A sea nettle has no brain, just a sensory package that directs it to swim around until it finds rich territory in its quest for microscopic foods. The stingers know not what they do. "There is no volition involved," said Cargo. "If they touch something, they sting it."

Mostly, sea nettles eat copepods -- microscopic animal plankton that thrive in the bay, the "primordial soup" at the prehistoric heart of all animal life. But they also stun and gobble shellfish larvae, small worms, small fishes and jelly-like creatures called sea walnuts.

Sadly, almost nothing eats them. Scientists suspect turtles desperate for a meal will munch medusas, and a small bottom-feeder called the nudibranch is known to graze on the polyps. But as a rule, there is so much water and so little meat in a sea nettle, practically anything else is preferred food, which makes the nettle almost predator-free.

The effect of a nettle sting on man is not much more than a painful annoyance, usually a red welt that goes away in an hour or two. In 20 years of study, Dr. Joseph Burnett, head of the University of Maryland School of Medicine's dermatology department, found only one case of serious infection and one of allergic reaction to sea nettle stings, neither of which was life-threatening.

This is a far cry from more serious stinging jellyfish, like the box jelly of northeastern Australia, blamed for five deaths last year, or the oceanic Portuguese man-of-war, which "stings like crazy," according to Burnett, and has tentacles up to 40 feet long.

The Chesapeake sea nettle, by contrast, "is a nuisance thing," said Burnett, "and a big problem only because of the number of incidents. As far as deaths or serious incidents, we don't know of any."

There are some who grow positively eloquent in praise of the jellyfish:

"It's a beautiful animal, a graceful swimmer," argued Larson, now a postdoctoral fellow at the Harbor Branch Oceanographic Institution in Fort Pierce, Fla., "and if people knew how primitive yet sophisticated in behavior it is, and how well adapted to its environment, they might appreciate it more and appreciate how lucky they are to have it in their backyard."

"I'm amazed at the grace and beauty when you look at him," said Burnett. "He has a well-fixed evolutionary niche, and he's very efficient in living."

There are even those who believe the stinging nettle is something of a blessing to the Chesapeake.

"One summer I was cruising with my wife and some friends," said Bill Brener, a fisherman, boater and lover of the bay, "and we anchored in an isolated section of the Little Choptank River.

"Sitting there with beautiful beaches all around and no one in sight, not a building around, it occurred to me that if the bay didn't have these jellyfish, you could swim there, and if you could swim there, the place would look like Ocean City.

"Personally," said Brener, "I decided I liked jellyfish better than condominiums." ::

SEX AND THE SINGLE SEA NETTLE

The sea nettle is capable of reproducing both sexually and asexually. Sexual reproduction takes place between male and female medusas. To reproduce asexually, sea nettle polyps form tubular projections that extend out and down and settle on a hard surface, where they grow. The polyp can also form into disk-like buds, which begin to pulsate, then break free in the water. The sea nettle is not a species that is in danger of extinction.

During sexual reproduction, male medusas set off a sperm shower that finds the female medusa and enters through her gastric cavity. The female medusa (A) releases from its ovary the fertilized embryo (B), which is carried by the currents to a hard surface (C), where it settles and develops into a polyp (D). These polyps are capable of asexual reproduction (not pictured here).

When the temperature and time are right, the polyp divides into buds (E) that break free to become adults (F) that, in turn, develop into sexually mature sea nettles (A).