Seeing the bottom of the Potomac River through a facemask is a little like looking through a microscope. It’s a new world down there, and you never know what you might find.

I was looking sponges. The previous year they were right here, at the head of Watkins Island, near Pennyfield Lock. It was apparently the first sighting of sponges in the river’s main stem. What happened to them? Would they come back?

I examined the bottom minutely, admiring the shimmering pebbles and the green patina on the mussel shells. I poked at anything that looked even the slightest bit spongy.

Then I saw something that made me come to a full stop. Wedged among the pebbles was a pair of tiny white tubes, then another, and another. Hundreds of them. I had found a colony of freshwater clams.

This was interesting, but not because clams are unusual in the Potomac. Far from it. The shells of this little bivalves literally pave parts of the river in Montgomery County. Along some stretches of shoreline, their shells crunch under every footstep.

But I had never seen these little creatures actually at work. And here they were, drawing water through one tentacle-lined tube, retaining nutrients other matter, and then expelling the same water through the other tube.

It struck me that they might be a little like the Higgs boson of the Potomac. Even though most people don’t know anything about them, their vast numbers could give them a decisive role in the river’s ecology. Are they another of Charles Darwin’s humble creatures that rule the earth, such as the  barnacles that were the subject of his first book and earthworms which he lovingly saved for his last?

Oysters of the Upper Potomac. It seems so, at least according to biologist Harriette Phelps, a professor emeritus at the University of the District of Colombia. Phelps is widely known for her work with this clam species in the tidal Potomac.

She told me the clam’s scientific name is Cobicula fluminea. Its common name will have to wait, so as not to give away the second part of the story.

“Hard working Corbicula,” she calls them. Put a layer of them on the bottom of a pail of murky water, and an hour and a half later the water will be clear. Multiply this effect by the countless millions of clams in the river, and you have what Phelps calls a “key species.”

Forget the feisty bass, the graceful heron, the majestic eagle. One of the real drivers of life in the river is this humble mollusk, most of them no bigger than a quarter. Darwin would have understood.

Corbicula almost seems designed with the express purpose of purifying water. The particles it extracts from the river are not simply cast back again. Instead, they are combined with mucus to build up the sediment in which they live.

Does this sound familiar? Elsewhere on this site I wrote how oysters were the cheapest and most efficient water purifier system in the Chesapeake Bay, until they were nearly wiped out.

Clearing up the water kicks off a chain of happy events. More sunlight penetrating the water fuels the growth of aquatic plants. With oxygen-producing plants come fish and birds, and pretty soon, the river environment becomes whole again.

Phelps saw this happen in the tidal Potomac.  There’s “absolutely every reason” to assume that Corbicula is providing the same service in our upstream portion as well, she says.

Certainly something good is going on out there. In some places in the river, plants are so thick that you can barely plow your kayak through them. In the channels through the vegetation you see darting shoals of minnows and bass slipping back into the cover. Big catfish chug along the bottom like monster tadpoles.

A dark secret. But these sunlit waters contain a dark secret.  Corbicula is not a native of the Potomac region or anywhere else in North America. It originally came from eastern Asia, and is know by the popular name Asian clam. It arrived first on the West Coast around 1930, then showed up in the Potomac in 1977.

Once in the Potomac the clams rapidly increased their numbers, carpeting many parts of the river bottom. This would seem to qualify Corbicula for membership in that increasingly familiar “genus” of the world of advocacy ecology called “invasive.” Here we have another foreign creature that has invaded our own natural communities, like a shiploads of Vikings burning monasteries, slaughtering townspeople, and carrying off treasure.

But not according to Phelps. “Corbicula were not invaders,” she said emphatically. “They occupied areas that were previously unoccupied.” And like many immigrants, they got their start by doing jobs that natives wouldn’t or couldn’t do themselves.

In the tidal Potomac, Corbicula inhabits the upper layer of the river bottom, she explained. The estuary’s native clam lives deeper down. “Even with Corbicula, the native clams are all over,” said Phelps.

Paradoxically, Phelps worries that this “invasive” species might disappear. It already has in her former study areas in the tidal Potomac. The same could happen in the upper river as well.

If Corbicula disappears, the river’s ecological clock could start to go in reverse. With no more clams filtering out so much stuff that gets washed into the river, the water could again lose its clarity, and with it, many of its plants, fish, birds, and other life forms.

The clams’ disappearance would also be felt by one group of local people whose ties to Corbicula extend to long ago and far away. You can sometimes see them, the men bent over the water, raking out clams with fan covers, the women sitting in the river, plastic baskets resting on their outstretched legs, picking the clams out from the stones and empty shells.

These people are also immigrants, harvesting the same clams in the same way they they and their families have done for many generations in their southeast Asian homelands. Following age-old traditions they will cook the clams in a wok with ginger, chile peppers, basil, and soy sauce.

Evidently it’s an acquired taste. “They’re good,” says Phelps, “but not really clam-like to me.  I like clams that are saltier.”