Sea nettles, the nasty jellyfish that sting unsuspecting swimmers, will make their presence known in Maryland waters in great numbers for the next two months, experts say.

The sea nettles began appearing two weeks ago and have been sighted as far north as the Chesapeake Bay Bridge near Annapolis.

"It looks like it's going to be a heavy infestation," said David Cargo, a researcher at the University of Maryland's Chesapeake Biological Laboratory in Solomons who has studied sea nettles for 25 years. "We have already stopped our noontime swimming at Solomons."

The sea nettle is one of many types of jellyfish found on the East Coast. They live in the bay year-round, spending most of their time as harmless polyps, clinging to pilings and shells.

When water temperatures rise and the salinity increases, the nettles shift into the "medusa stages," growing caps and tentacles, said Bill Golds- borough, a senior staff scientist at the Chesapeake Bay Foundation in Annapolis.

Rangers said three dozen people have reported being stung at Sandy Point State Park, and jellyfish have been seen floating on the water at Point Lookout State Park in St. Mary's County, but people have been venturing into the bay anyway.

Generally, nettles' stings cause burning and itching that subsides after 30 minutes or so.

Cargo said victims of nettle stings should wash the area with as little agitation as possible.