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CONCORD, Mass. — Gwen Acton thought the dime-sized translucent pods she saw on her Sunday swim in Walden Pond were strange, beautiful seeds that had drifted down to the water surface from some flowering plant.

But as she cut through the water in a crawl stroke Monday she noticed their numbers had ballooned. She saw them everywhere. And they were pulsating.

“I said to myself: ‘Oh, no. I am surrounded by thousands and thousands of jellyfish,”’ Acton said.

She was.

A deeply mysterious species - freshwater jellies - has bloomed in one of the nation’s most visited ponds. The organisms rarely cause health problems in humans, but the discovery has set off a flurry of interest at the New England Aquarium, where scientists have unsuccessfully attempted to breed the elusive creatures.

It is not that the tiny jellyfish are rare; after probably hitching a ride to the United States in the late 1800s on Asian water hyacinth or other ornamental plants, the jellyfish are believed to have spread to lakes and ponds throughout the country because of activities of fishermen and waterfowl.

But because the tiny jellies can lie in a dormant state for years - perhaps decades - and bloom en masse suddenly before disappearing just as quickly, people rarely come across them or do not know what they are looking at when they do. In Massachusetts, where officials began keeping track of the species about five years ago, they have been recorded in about five or six lakes and ponds across the state.

But no one has reported them before in Walden Pond, and Acton, who lives nearby in Concord, has not seen them in the 15 years she has been swimming there. The Walden Woods Project, which curates the most extensive research collection by and about Henry David Thoreau, conducted a preliminary search of his work just in case he might have noticed them during his time at Walden, and found no mention.

“Jellyheads like me knew these guys are around, but they can go for years and years and years without anybody seeing them,” said Steve Bailey, New England Aquarium’s curator of fishes. “They are a wickedly cool critter that is still a mystery to us when so many other things have been demystified.”

When Peter Davenport of the state Department of Conservation and Recreation, and Tony LaCasse of the aquarium, dipped nets from a canoe in the dark Walden water Thursday morning, they came up empty. But they had proof the jellies existed: On Wednesday, a visitor armed with goggles and a net dove 5 feet below the water surface and scooped up scores of them, which park workers promptly transferred into a plastic bag.

On Walden’s shore Thursday, visitors ogled the undulating creatures and repeatedly asked one critical question: Do they sting?

Although the organisms’ tentacles have thousands of stinging cells to immobilize the tiny plankton they eat, most people are unaffected by them. Some people, like Acton, say they cannot feel them while swimming, but others have said they can, and have reported mild itching, red spots, and, in some cases, a slight numbness, said Terry Peard, a retired biology professor from Indiana University of Pennsylvania.

Peard, who studies the organisms, first heard of freshwater jellies in the late 1980s, became interested in them, and hosts www.freshwaterjellyfish.org in part to keep track of public sightings in the United States.

People rarely see the jellyfish because they spend much of their life out of sight. Tiny stalked forms of the jellyfish, known as polyps, attach to underwater surfaces such as plants, rocks, and tree stumps to feed and reproduce asexually during spring and summer, developing colonies.

But some offspring of the polyps develop into full-blown jellyfish that can grow to the size of a quarter. When or why is a mystery. The blooms take place in warm water, usually above 70 degrees Fahrenheit, and some at Walden speculated this summer’s soaring temperatures may have contributed to the bloom.

Scientists also believe other factors, including the abundance of microscopic animals they eat and the acidity of the water, contribute to the jellies’ growth. They are not linked to poor water quality, and do not appear to harm native species.

“I start getting sightings in July and go to October” across the country, Peard said, noting this is a prime time for sightings.

A worker saw some in a pond at Nickerson State Park in Brewster on Aug. 21, and in New Hampshire, the Department Of Environmental Services has received 10 calls this week on them.

“They are very cool to look at,” said Jody Connor, director of the center that monitors lakes and ponds, and who has been tracking the jellyfish since the early 1980s. “For a while we used to put them in beakers, but they don’t last long.”

Peard said this year’s reports from New England do not appear to be any more frequent than previous years. Yet because the creatures are so elusive and public reporting of them is sporadic, it is hard to track any definite trends in blooms, he said. “We really don’t know much about them,” Peard said.

Acton, the Walden swimmer, said the jellies she saw were gorgeous, like “underwater snowflakes.” But that didn’t stop her from being worried when she realized what they were.

“No matter where I was swimming, I knew I was going to touch them, but then I realized I hadn’t been stung so far,” Acton said. She got out of the water and asked a woman whether she had seen jellyfish. The woman looked at her somewhat oddly and said there is no such thing as freshwater jellyfish. Acton researched them when she got home, found they were real, and telephoned the aquarium.

Thursday, as LaCasse and Davenport searched for the jellyfish, swimmer Jean Weicker was asked whether he had seen any of the creatures. Weicker chuckled at what he thought was a joke.

Yeah, sure, he replied. And there are sharks in there, too.