Dave Seitler Dragged Up a Clam That Isn't Supposed to Be Here

The clams came up in the drag with the quahogs, the way everything comes up in the drag with the quahogs — the good ones, the broken ones, the odd rock, a crab that has made a bad decision. You sort it standing up, by feel, a few thousand times a season, and after enough years your hands know the shape of a quahog better than your eyes do.

Which is why Dave Seitler noticed.

This was May of last year, off Jeremy Point, on ground he works. The shells in his palm were the wrong shape. Oval where a quahog is heart-heavy. And the ridges ran the wrong way — not the tidy concentric rings a quahog wears like a tree stump, but a crosshatch, lines crossing each other, going two directions at once.

He called them weird clams. He took a picture with his phone, the way anybody would, and sent it to his friend Owen Nichols, who runs marine fisheries research at the Center for Coastal Studies over in Provincetown.

Nichols went through the regional shellfish guidebooks. Nothing. He went through them again. Then he pulled down a 1962 illustrated volume called Seashells of the World, by the malacologist R. Tucker Abbott — the kind of book that sits on a shelf for thirty years being useless right up until the afternoon it isn't — and stopped on a color plate.

"I landed on a beautiful color image of Manila clams," Nichols said, "and I thought, 'Well, that sure looks like what Dave found in Wellfleet.'"

Unlikely. Not impossible. Just unlikely, because Manila clams live in the northwest Pacific — Russia down through Japan and Korea and China, south to the Philippines — and in Puget Sound, and in France, and in the Venice Lagoon, and on menus most everywhere in the northern half of the world. Not here. The northwest Atlantic was the last major stretch of the Northern Hemisphere without an established population of them.

Chatham had them first, and nobody knew what they were either

Two years before Seitler's drag, harvesters in Chatham started bringing Renee Gagne, the shellfish constable, things she couldn't name.

"Some of my harvesters started to bring these bizarre-looking clams," she said.

She sent them along to the Cape Cod Cooperative Extension, where they sat in the queue that all strange things sit in, and it took until 2025 for somebody to put a name on them.

Meanwhile, up in Boston Harbor, a marine ecologist named Aly Putnam was standing on Spectacle Island in front of a scatter of shells trying to work out whether they meant anything at all — or whether she was looking at the remains of somebody's takeout. About forty of them turned up in a few minutes on a field trip last July. That is either a discovery or a picnic, and telling the difference is most of the job.

Both she and Nichols eventually wrote to James T. Carlton, an emeritus professor at Williams College who has spent a career on invasive species, and Carlton did the useful thing: he introduced them to each other.

"Find me living clams," he told them — especially babies, especially anything showing it had reproduced.

"When I learned about what each group was working on," Carlton said, "I realized that this was a golden opportunity to not only combine forces but also to catch a detailed snapshot of the moment a new invasive species establishes itself."

That is a rare thing to catch. Usually you find out an animal has moved in the way you find out about a leak — from the damage, years late.

Their paper ran in Biological Invasions on the Fourth of July. It documents the first reproducing population of Manila clams in the northwest Atlantic, from Cape Cod up to Salem Sound. The adults they measured here ran 31 to 71 millimeters — the biggest just shy of three inches, where a quahog goes substantially larger. Then they went and sieved the sediment down in Boston Harbor and pulled up settlers half a millimeter long, and juveniles between one and seven.

That is the whole ballgame. Full-grown clams could have been dumped off a boat or tipped out of a bucket. Half-millimeter clams were born here.

What to look for, standing on the flats at low water

Turn the shell over. The lunule — that little dished area beside the hinge — is oval on a Manila clam and heart-shaped on a quahog. Run a thumbnail around the inside margin: smooth on a Manila, grooved on a quahog. But the fastest tell is the outside, and you can do it at arm's length. A quahog's ridges run parallel to the edge, one inside the next. A Manila clam's cross.

They have a short siphon, so they can't reach far, so they don't go deep — about eight inches down, in the upper half of the intertidal, in sand and gravel and mud. Shallow enough that a hard winter can kill them. Shallow enough, also, that a quahog drag working the flats off a tidal spit like Jeremy Point turns out to be very close to a purpose-built device for finding them.

Nobody can say yet how they got here. Ballast water is one candidate. The live seafood trade is the other. Asked which, Nichols said it was "really hard to say," which is what an honest scientist says in August of the first year. Vania Rivera León, a staff scientist at the Center for Coastal Studies, has been pulling DNA out of the Wellfleet samples to try to find the source population.

In the Pacific Northwest the answer turned out to be oyster seed. The clams came in by accident in the 1930s, hitchhiking in shipments from Japan, and within a decade were all over the Strait of Georgia. France imported them from Puget Sound in 1972 on purpose. They are in the Mediterranean now.

The question underneath all of it

Wellfleet ranked first in Massachusetts for the value of its shellfish landings in the most recent state data, more than $9.6 million in 2024. The town calls shellfishing its largest year-round industry and figures it employs about a quarter of its working-age population. Last year the wild quahog harvest came to 1,723 bushels, up sixty-one percent in a single season.

That is Seitler's fishery. So the question everybody circles back to is whether this new clam is competition or dinner.

What nobody can point to is a study putting Manila clams directly against quahogs. Not one. In some parts of the world Manila clams have shouldered native bivalves aside; in others they've simply moved in and shared the neighborhood. Putnam is running an experiment right now on whether a green crab would rather eat a Manila clam or a quahog. So far all she will say is that they'll eat them.

"I can tell you that they definitely will eat the Manila clam."

Which is not a small sentence on this harbor. European green crabs were recorded in Massachusetts by 1817 — ballast is among the proposed routes — and they are widely blamed for taking down Maine's soft-shell clam industry. Last year the Wellfleet Shellfish Department ran fifteen traps and pulled more than 38,000 green crabs out of the water, roughly double the year before. Carolina Bastidas of MIT Sea Grant has floated the possibility that a new clam could take some of that pressure off the natives simply by giving the crabs something else to chew on. Seitler has wondered the same thing.

He has also noticed something more immediate.

"They're worth more than the hard clams," he said.

Which runs straight into the part nobody has answered. Massachusetts propagation rules plainly restrict bringing in and cultivating non-native shellfish — that permit exists for exactly this reason. What is far less clear is what happens when a licensed harvester's drag comes up with one that is already here. The state's minimum-size rule for hard clams is written to the genus Mercenaria, and a Manila clam is not a Mercenaria. No regulation anyone has produced squarely answers whether a Wellfleet dragger can land a wild one and sell it to a dealer.

Alexis Neffinger, who works on coastal habitat for the state's Office of Coastal Zone Management and sits on the working group that now meets monthly about all this, put the timing about as plainly as it can be put.

"Sometimes these things go undetected until they're everywhere. Right now, we're really at the emerging understanding of this bio invasion."

The state added Manila clams to MIMIC's 22-species monitoring list this year — one of four species added in 2026. And the scientists would like a hand. If you turn one up raking, dragging, or just walking the flats at dead low, photograph it. Note whether it was alive or dead. Estimate the length. Count how many. Live ones under an inch and a quarter are the ones they most want to hear about. Absences count too — if you looked hard at a spot and came up with nothing, that is data nobody else can give them.

Report a clam | [email protected], or post it to iNaturalist
Identification poster and photos: seagrant.whoi.edu/manila-clams

Seitler is still dragging for quahogs off Jeremy Point. Most of what comes up is quahogs.