LUBEC – Here on the eastern edge of the United States, generations of clam diggers have dug deep into the mud flats for soft-shell clams.

These days, the digging isn’t so good. The clam harvest Down East has dropped sharply in recent years, and clam diggers say one big reason is an onslaught of moon snails.

The snails are mucus-oozing creatures that emerge at night, drill into clams and other shellfish with sandpaper-like tongues, eat the meat inside and leave behind empty shells.

“It’s like science fiction,” said Brian Beal, a marine ecology professor at the University of Maine at Machias who has studied clams and snails for decades.

For 30 years, Bob Varney, 55, has earned his living on Lubec’s clam flats. The work is difficult and dirty, bringing barely $50 a day.

These days, Varney typically harvests 55 to 60 pounds in four or five hours. Not long ago, he could rake up double that, but the clams are spread thin nowadays. A growing number have drill holes in their shells from the moon snails.

What concerns him most are the snails’ volcano-shaped egg cases — some of which hold more than 1 million eggs. They show up nearly everywhere he digs.

“We didn’t have anything like this even two or three years ago,” Varney said, taking a break from digging. “They’ve just escalated.”

Moon snails are native to the region, ranging from Canada’s Gulf of St. Lawrence to North Carolina. In Maine, their numbers go in cycles from area to area. Biologists say there was a surge in the 1970s and 1980s.

For now, there’s no concern that the creatures will spread in abundance beyond far eastern Maine, one of the most productive clamming areas in New England. Thirty miles away, clam diggers aren’t seeing the snails, Beal said.

Some cultures consider the snails a delicacy, but in Maine they’re just a nuisance. Beal said they may be more numerous in eastern Maine because of the decline of green crabs, which prey on the snails and clams.

The downturn has been severe in Cobscook Bay, a sprawling body of water against the Canadian border that’s known for its 20-foot tides, turbulent waters and rich ecosystem.

The harvest in Lubec and other towns on the bay fell from about 800,000 pounds in 2006 to about 100,000 in 2009, according to the Maine Department of Marine Resources.

Partly to blame are widespread outbreaks of “red tide,” naturally occurring algae that produces a toxin that makes clams unsafe to eat and halts the harvest, Beal said. But the rise of moon snails is the No. 1 reason.

On a recent day on a flat in Lubec, Beal and some clammers found dozens and dozens of the snails and their small egg cases, which look like small pieces of rubber littered on the mud.

Harmless-looking and slow-moving, the creature is deadly when it gets hold of a clam under the mud, enveloping it with its slimy, mucus-oozing foot and softening the shell with an enzyme secretion. In just a day, the snail can drill a hole in the softened shell and eat the clam inside.

“They just go in and slurp it up,” Beal said.

On the flats, clammers have a hard enough time without the snail invasion. They work in the wet and cold, sometimes sinking knee-deep into Jello-like mud pockets that can suck their legs down and trap them until they wiggle free, sometimes leaving a rubber boot behind.

Lately, diggers have been getting less than $1 a pound, but prices will probably be double that in August, when demand is highest.

In Lubec, a town of 1,500, the value of the harvest has fallen from $566,000 four years ago to just $39,000 last year, according to the state.

David Case, 34, said it’s barely worth his while to dig clams these days. But there’s little else for him to do in the area to make a buck, and he still has to put food on the table.

Dennis Huckins, 56, said the moon snail is a crook, taking money out of clam diggers’ wallets.

Clam diggers have been removing snail egg cases — more than 90,000 so far — from the flats. Beal records their findings and the diggers usually dump the snails on their gardens as fertilizer.

Some people think the moon snails could be processed, packaged and sold.

“There’s a market for moon snails somewhere in the world,” Beal said. “Why not market ours?”