What lurks beneath?

MARLBOROUGH -- When conservation officials drew down the water at Fort Meadow Reservoir in Marlborough to kill invasive weeds this winter, lake residents were shocked by what they saw.

Asiatic clams -- thousands of them -- spread out across the lake bottom.

"I've never seen anything like it," said Lee Thomson, who has lived alongside the reservoir for more than 40 years. "There were areas where you couldn't see any bottom."

The Asiatic clam is one of the latest invasive species to arrive in the state, having suddenly turned up in five ponds and, in some places, multiplying like mad.

Once-smooth beaches could become littered with their sharp, tiny shells, which range from a half-inch to 1 1/2 inches across. Wildlife officials fear that they could overwhelm native mussels and fish. And when they spread to waterways that feed power and waste-water plants, as they have in Connecticut, they can clog intake valves and force costly repairs. Similarly, they plague boaters by jamming equipment.

"This is a species well adapted to invasion," said Jay Cordeiro, a zoologist with an international non profit group, NatureServe, which tracks endangered species.

The clams are hermaphrodites -- meaning each possesses both male and female reproductive capabilities. Under the right conditions, a single clam can release hundreds or even thousands of baby clams a day, and up to 70,000 a year. Spawning can continue throughout the summer months in Massachusetts, and during colder seasons, eggs and sperm can remain in the clam's tissue, ready for release as soon as spring arrives.

Cordeiro said the record accumulation of Asiatic clams is 130,000 of them clustered in a cubic yard; their numbers could easily reach into the billions in a small lake, he said. They thrive in sandy lake bottoms. And eradicating them is mission impossible.

If one or two survive, Cordeiro said, they are "enough to start all over again. Unless you get rid of every one, you're going to have the same problem."

No one is sure how the Asiatic clam arrived in the United States. The US Geological Survey suspects they were brought to North America by Asian immigrants who grew them here for food. They were first found in California in the 1930s and spread through the state's canals, making their way eastward.

Popular as bait and among aquarium hobbyists, they eventually spread through the South.

An aquarium provider was offering clams online this week for $1.60 each.

They are sold as "pygmy" or "gold" clams, valued for their prolific nature, classic shape, and smooth purplish interior.

Until recently, they had been recorded only as far north as Connecticut, and biologists had hoped that was the extent of their cold comfort zone. The clam thrived in the warm water generated by the Connecticut Yankee nuclear power plant. The plant spent millions to fix the problem and has chlorinated the water in the Connecticut River to kill the bivalves.

Massachusetts has enacted laws making it illegal to place any live shellfish in state waters without a permit, and asks residents to dispose of shellfish waste in the trash, rather than tossing into a waterway. But such measures have not stopped the Asiatic clam.

Documented locations

State wildlife officials have documented the clams in six freshwater bodies: the Charles River in Watertown, Congamond Pond in Southwick, Long Pond in Freetown, Tispaquin Pond in Middleborough, Webster Lake in Webster, and Fort Meadow Reservoir in Marlborough.

Michelle Robinson, an aquatic biologist for the state Department of Conservation and Recreation, tracks the species in Massachusetts. She said the clam has not survived much in the Charles River, where salty tidal flows appear to keep it in check. By comparison, Tispaquin Pond in Middleborough and Webster Lake, south of Worcester, have "huge" infestations.

The clams eat plankton in vast quantities, leaving less for other species. Of the 12 species of freshwater clams and mussels native to Massachusetts, seven are protected due to threats of endangerment. The Natural Heritage & Endangered Species Program, which charts endangered species across the state, said just one nonendangered species of native clam lives in Fort Meadow Reservoir. The Charles River and Webster Lake have clam species that are listed as of "special concern" due to threats of endangerment.

And even if the clams die in a cold weather draw-down, their rotting shells can produce ammonia in high enough quantities to be lethal to other fish and wildlife, officials said. Anglers could notice a drop in the number of fish species.

Robinson said the only way to control Asiatic clams is to stop them from spreading. She is also on the lookout for the zebra mussel, a similarly prolific invasive species that has spread as far north as a lake on the Massachusetts-Connecticut border.

The state employs six workers to monitor boat ramps in various communities, reminding visitors to wash the bottom of their boats thoroughly to prevent the spread of invasive species, such as the weed milfoil or zebra mussels, between waterways. Two are assigned to boat ramps on Long Pond and Webster Lake. Several years ago, a few communities installed shoreline boat washes to ensure that plants and animals weren't spread, but the idea never took off.

Even the most vigilant sportsman can't prevent the Asiatic clam from hitchhiking. Its larvae are invisible to the naked eye.

State officials have posted signs at most lakes cautioning fishermen to empty their bait buckets on dry land far from the shoreline to stop an inadvertent spread.

"It could be in tons of ponds," Robinson said, "and just not detected yet."

The first sighting

Robinson recalled the day she discovered the first Asiatic clam in Marlborough's Fort Meadow Reservoir.

It was the summer of 2005. She had been kayaking along the shoreline, looking for evidence of the spread of milfoil, when the glint of a floating shell caught her eye.

"I was hoping at the time that it was maybe just a fluke," she said.

She brought the sample back to her office, where it was confirmed as an Asiatic clam shell.

So a few months later, she returned to the Marlborough reservoir, a 308-acre recreational lake that is no longer a source of drinking water for the city. She and a friend snorkeled along the lake's extensive shoreline, looking for evidence of the clam's spread. They found nothing.

But at a community meeting last year to discuss the milfoil dilemma, several residents brought with them handfuls of shells, wondering what they were.

Marlborough conservation officer Priscilla Ryder knew. But it wasn't until a draw-down of the lake to kill invasive milfoil that the community saw the scope of the invasion.

"They were everywhere," Ryder said. "We thought we had two spots."

Cold temperatures appeared to have killed most of the clams, Ryder said, judging from the number of open shells. Those shells either have or will wash up along shorelines this spring and summer. Beachgoers should expect it, Ryder said. If the population continues to thrive, residents will have to get used to raking their beaches to keep sand underfoot.

Although the clams are edible, no one interviewed said they would consider dining on the tiny critters.

That includes Lee Thomson, the Fort Meadow lake commissioner of 35 years.

"I'm not a clam lover," he said.

Especially now.

If you suspect you have spotted an Asiatic clam, contact Michelle Robinson at the Department of Conservation and Recreation at michelle.robinson@state.ma.us.

Megan Woolhouse can be reached at mwoolhouse@globe.com.