As they filter seawater through their gills, oysters and other shellfish are ingesting the microplastics that are accumulating throughout the oceans.
And as we eat those shellfish, we may occasionally ingest at least a few of those tiny particles ourselves.
These revelations have become part of a new but growing research field: What and how much plastic is in shellfish?
What could that mean for human health?
And, likely more importantly, what it could mean for human health as the amount of plastics in the oceans continues to grow?
“The things we don’t know far exceed the things we know. What we do know is there is a good deal of microplastics out there in the environment,” said Evan Ward, PhD, a marine sciences professor at the University of Connecticut who is studying what plastics Long Island Sound oysters are ingesting.
Tiny particles becoming big problem
Microplastics can be the size of plankton and can be confused for food by marine animals.
They come in large part from the degradation of larger pieces of plastic, which have formed giant tangles of trash in each of the world’s oceans.
Other microplastics start out small, such as microbeads and microfibers, which slough off synthetic fabrics like fleece.
The concentration of plastics in the water varies, although it tends to be higher near the shore and near urban areas.
That also happens to be where most oysters and other shellfish are raised and harvested.
One recent study, for instance, found New York’s Hudson River contained, on average, one microfiber per liter of water. That means there’s 300 million microfibers dumped into the Atlantic per day.
A 2014 study found the concentration of microplastics in water around Vancouver Island was, in some places, as high as 9.2 particles per liter.
Plastics can filter through gills
At least some of these plastics are making their way into shellfish.
The sea creatures are filter feeders that pass seawater over gills, filtering out plankton and other microscopic particles — including microplastics.
An oyster, on average, processes about 5 liters of water per hour.
“So if they’re feeding for 20 hours, that’s about 100 liters per day for one oyster,” Ward told Healthline.
If there’s, say, one particle of microplastic in every other liter, that could mean an oyster would be ingesting 50 particles of microplastic a day.
Researchers have already determined that most of those particles are passed through the oyster and expelled.
But some of them are confused for food and retained.
And some of those oysters become food for humans.
Low impact… for now
A study released earlier this year found people in Europe are consuming as much as 11,000 particles of microplastics a year by consuming shellfish and fish.
Almost all of them pass through the body, but about 1 percent is retained and accumulates in the body’s tissues.
It’s unlikely there are any impacts on people, though — at least not yet.
“The level of microplastics that we’re talking about, I doubt there’s any human health impacts at this time,” Ward said. “On a typical day, when you put on your polo shirt while standing over your coffee cup, there’s a rain of microplastics into your coffee.”
He said he guesses “there’s a hell of a lot more” microplastics we ingest from those floating around our homes and landing in our food than we’d get from eating oysters.
But the reason why research is still necessary is because we don’t know what microplastic concentrations are going to look like in the future — other than that there will be more and more.
“This is the time to start working on it,” Ward said. “We don’t need to wait until there’s a thousand particles in oysters.”
The first step is determining which particles oysters are more likely to try to digest.
The day after Healthline spoke with Ward, he and other researchers were heading out to Long Island Sound — wearing 100-percent cotton lab coats with no plastic microfibers — to collect oysters and find out what’s in their gut.
As part of a new research project, his team will be trying to determine what types of plastics oysters are ingesting, and thus what types might be passed on to humans.
“If we can determine which microplastic size, shape, and type are most likely eaten then we can say, down the road, maybe we should have restrictions on these types of plastics, at least near the marine environment,” Ward said.
In preliminary research, he has found that plastic fibers are more likely to be spit out and bead-shaped plastics are more likely to be retained. Microbeads, found in products like cosmetics and toothpaste, were banned from products in the United States in 2015.
“If we find there’s a lot of microplastics out there in the type that shellfish are more likely to eat, then that’s a problem because over time microplastics in the environment are going to go up,” Ward said. “It’s going to increase without a doubt in the future. … The question is how concerned do we need to be down the road, knowing that the amount of microplastics is going to increase.”
The European study, for instance, predicted that by the year 2100 people could be ingesting 780,000 particles of microplastic per year, absorbing about 4,000 particles into the body.