The Asian clam, a Far Eastern bivalve given to indiscriminate breeding and general ecological clutter, has invaded the Potomac River and is now marching on Washington.

This is serious business, say aquatic biologists, because the libidinous little mollusk likes to swim into intake pipes and clam up the water and cooling systems of power companies, factories and towns.

Localities in California already spend thousands of dollars each year scraping clam colonies out of their water pipes, where they can reach a density of 163,000 per cubic foot.

"They're a very expensive addition to the environment," said Paul Dresler, the U.S. Geological Survey biologist who discovered the clamity on the Potomac. "They'll settle almost anywhere."

A dark, olive-colored shellfish about the size of a 25 cent piece when full grown, the Asian clam (Corbicula manilensis) was first introduced to this country in 1938 by Chinese immigrants who hoped to raise it as a food source.

From the Columbia River in Washington State, it has gradually migrated down the West Coast, across Central America and up the East Coast by means not yet completely understood.

It reached the James River about 10 years ago, Dresler said, and appears to have started up the Potomac about 1975.

The problem with the Asian clam, apparently, is that nobody much eats it except in Asia.

"Catfish, carp and red-ear sunfish feed on it occasionally," said Ralph Sinclair, an Asian clam expert with the U.S. Environmental Protection Agency in Cincinnati, "but it really has no major predator here. It's an invader in an area where the food chain is already pretty much established. It seems to have settled into an ecological niche that was basically uninhabited."

Asians keep the Asian clam under control by eating it -- in such quantities that Asian governments have passed laws to protect the beleaguered bivalve.

"They've overfished the beds," Sinclair said, "particularly in the Philippines and along the Pearl River near Canton."

Theoretically, Americans could do the same and thus solve the problem. But things are rarely that simple.

"I've eaten them and they taste all right," said Sinclair. "But in this country they seem to cluster most in marginal rivers where, like any shellfish, they concentrate the pollutants in their own meat. I certainly wouldn't eat any from the Ohio."

Unlike American clams, the Asian variety goes through a microscopic larval stage when, for 48 hours, it can swim through almost any commercial intake filter and attach itself to the inside of the pipe.

There it rapidly grows nd multiplies until the pipe is completely clogged, and engineers must kill the clams with chloring and then somehow flush out the shells.

As annoying as the clam itself is the sediment it produces: roughly five grams for every gram of clam.

Subscribers to the Corbicula Newsletter, which traffics in clam lore, will learn that the Potomac's "patchy" population of 62 clams per square foot ranks it as a relatively minor trouble zone so far.

The big league lies in Europe, domain of the dreaded Zebra clam, about which biologists swap horror stories in seven languages.

"Heaven help us if that one ever gets here," said Sinclair.