Picture this: It’s a warm, spring day in May 2021. You are at a local seafood restaurant overlooking the Chesapeake Bay and your order of raw oysters arrives delicately placed on a layer of ice. Your waiter reviews the type of oysters you ordered. He says, “Running clockwise, you have Pemaquid, Blue Point, and PEI.” Before the waiter steps away, he asks, “Would you like extra microplastics added to your oysters?” Dumbfounded, you reply, “Extra?”

This Twilight Zone-like scenario is not totally fictional. Oysters are a keystone species in the environment, meaning they are the backbone of ecosystems. They are heroes in a small shell. In addition to where they sit on the food chain, oysters can filter up to 50 gallons of water per day, cleaning the surrounding water of chemicals and pollutants. However, this means they inevitably suck up more than they bargained for. Scientists have discovered that oysters contain microplastics, plastic pieces that measure less than 5 mm in size in one dimension (similar to the size of a sesame seed). Oysters suck in microplastics and, sometimes, they never pass them out.

Oysters are not alone. Microplastics can be mistaken for food and consumed by varying marine organisms at each level of the food chain from plankton to whales. Jambeck et al. estimates that between 4.8 and 12.7 million tons of plastic flow into the oceans each year from coastal regions. While the vast majority of microplastics pass harmlessly through marine organisms, a few are retained within the gut or body of the animal.

Our dependence on seafood means microplastics impact us, too. Yet, despite the ubiquity of microplastics in our food, water, and even air, very few countries have passed policies to prevent microplastic leakage from a variety of sources, including packaging, sewage sludge, and textile production. This is in part because there is still a great need for more conclusive research to understand the source, fate, and distribution of microplastics in the environment.

Can’t Shuck Microplastics

In 2016, the U.S. aquaculture industry produced $1.5 billion, representing 21 percent of the value of the nation’s total seafood production. Whether you personally enjoy oysters or not, they play a major role in the seafood economy as the most highly valued and expensive shellfish due to booming consumer demand.

With growing concerns of plastic pollution, researchers have looked for signs of plastics in sea life. Studies have shown that both wild and commercially sold oysters can have microplastics in them. Because oysters are filtering creatures, the working theory is that they are more at risk for ingesting microplastics. When discarded, plastic bottles or wrappers break down into microplastics and they can cause more widespread issues, including being mistaken for food.

Scientists are still trying to understand the impacts of microplastic ingestion; thus far, experiments have been conducted in both naturalized observation and laboratory studies with varied results.

Some laboratory studies on potential threats to oyster health have shown that polystyrene microbeads can cause feeding modifications and reproductive disruption in oysters, with significant impacts on offspring and reproduction, which could have population-wide effects on this vital species. One concern, however, of most laboratory based studies is that the oysters are fed significantly higher microplastics than what they would typically ingest in the natural environment. Of those studies done using environmentally relevant concentrations of microplastics, the researchers did not document any major threat to the survival of oysters. This could be a result of oysters, and other bivalves, being capable of selective feeding, meaning that particles of a certain size range and shape may be released by the bivalve prior to ingestion, thus limiting the potential consequences. But without proper mitigation measures in place, microplastic concentrations in the natural environment will continue to rise and the impacts on oysters may ultimately reflect those observed in the laboratory-based studies.

Everything Filters Up the Food Chain?

Much as the potential dangers of microplastics on oysters are still uncertain, research into the effects of microplastics on human health is also still in its infancy. The first concern is how much microplastic we are ingesting through our seafood. For example, a group of Belgium researchers calculated that in European countries with high shellfish consumption, consumers may be ingesting up to 11,000 microplastic particles (ranging in size from 5 – 1000 mm) per year, compared to 1,800 in countries with low shellfish consumption. This means that annually, each person ate the equivalent weight of one standard AA battery in microplastics.

While scientists have documented microplastics everywhere and in many foods, scientists still need to explore the fate of microplastics in the human body following ingestion, such as whether they get metabolized and excreted or if they persist. Three billion people in the world depend on wild or farmed seafood for at least 20 percent of their protein intake, underscoring the importance of research that could help clarify the potential ramifications of microplastics on seafood security and allow regulators to create food safety risk analysis tools to evaluate hazards to consumers. Scientists should parse out seafood that humans eat whole (e.g., mollusks and crustaceans) and those that are consumed partially. Humans eat the whole oyster compared to just the filet of the fish—when fish uptake microplastics, the particles end up in the gastrointestinal tract of the fish, which people generally do not consume.