HORN POINT -- The still water by the pier at Horn Point Environmental Laboratory near Cambridge is white with undulating sea nettles, concentrated in ungodly numbers along the shore by the wind and the Choptank's steady currents.
"Oh, you wouldn't want to swim in the Choptank now, no indeed," chuckled Dave Nemazie, a University of Maryland graduate student conducting jellyfish research at the lab. "You can go out in the middle of the river and see them everywhere."
Thanks to hot, bone-dry weather over the vast watershed of the Chesapeake Bay in recent months, the sea nettle -- a species of jellyfish known as Chrysaora quinquecirrha -- has arrived in force, weeks early and farther up the bay and its tributaries than usual.
"The numbers are higher than I expected, and they were extra large early in the season," said veteran sea nettle researcher David Cargo, who retired last year after more than 25 years as a research associate at the university's Chesapeake Biological Laboratory in Solomons.
Mr. Cargo saw his first nettle in the Patuxent River the last week in May, and Jennifer Purcell -- a University of Maryland assistant professor who heads the jellyfish research at Horn Point -- noticed her first on June 3 or 4, "at least a couple of weeks early."
Maryland environmental scientists who regularly monitor Chesapeake water quality have spotted the stinging summer scourge as far north as Still Pond and Betterton in Kent County, across the bay from Aberdeen Proving Ground.
"Normally, they might get up to Tolchester Beach and Hart-Miller Island," said Sally Bowen, a section chief with the state's monitoring program. "This year, they're already four or five miles farther north than that."
And sea nettle infestations are severe at beaches and waterfront property throughout the Annapolis area, including Sandy Point State Park near the Bay Bridge, where they arrived in mid-June to torment swimmers two to three weeks earlier than usual. "They're terrible this year," said Sergio Galindo, head lifeguard at Sandy Point. "On a scale of 1 to 10 for sea nettle activity, we should be at 2 about now, and we're at 8. And just because they got here early doesn't mean they'll disappear sooner."
Long-time observers recognize the familiar rhythms of the sea nettle, an ancient, remarkably adapted creature that proliferates when water temperature and salinity in the bay are favorable and quietly retreats to bide its time when they aren't.
In June 1972, the massive freshwater runoff from Hurricane Agnes diluted bay water so dramatically that the nettles virtually disappeared for two years, Mr. Cargo said. But they quickly rebounded and several times have approached the "100 percent year" of 1969, when they were so abundant that a swimmer risked getting stung wherever he took a dip in the bay.
The ideal salinity for reproduction and growth ranges from 10 to 18 parts of salt per 1,000 parts of water, Mr. Nemazie said. They avoid environments with fewer than 5 parts per 1,000 and more than 28 to 30 parts per thousand, the approximate salinity at Ocean City. Water in the open ocean measures about 35 parts per thousand.
This year, salinity has soared in the upper bay because of the diminished freshwater inflow.
At Betterton, where salinity is normally 4 to 5 parts per 1,000, last week's measure was 7 parts per 1,000, and the sea nettles have opportunistically expanded their normal range.
"People look at these things and remark about how primitive they are, unchanged for millions of years," said Mr. Nemazie. "But jellyfish figured it out so well, they never had to evolve into anything else."
Sea nettles feed on microscopic animal plankton known as copepods -- plus small worms, fish larvae and gelatinous creatures called comb jellies , or ctenophores -- by stinging them and gently wafting the stunned prey toward the mouth with its tentacles.
A nettle is covered with cup-like cells containing tiny, coiled darts that fire a poison at the slightest touch.
In humans, the sting can produce red, itching welts on the skin, with the potential for respiratory problems, severe swelling, even shock in susceptible individuals.
But the transparent, wispy white jellyfish -- with "umbrellas" up to 10 inches across and tentacles stretching to 4 feet -- can shrink in size when food is scarce, and they have no known predators in the bay to limit their numbers.
A quarter-century ago, there was great optimism that sea nettles could be controlled by poisoning the animals during the "polyp" stage in which they pass the winter on the bottom of the bay, firmly attached to oyster shells.
The polyps form in late summer when the female's eggs are fertilized by sperm from the male, and they can remain dormant if conditions are hostile in the spring. With favorable salinity and temperatures, juvenile jellyfish emerge as vegetative buds on the polyps.