Tiny bits of plastic in the ocean could have a serious impact on the ability of oysters to reproduce, a new study suggests, affecting everything from the movement of their sperm to the growth of their babies. The study is just the latest in a long string of research on the harmful effects of plastic on marine organisms.

The problem is that when plastic is dumped into the ocean, it doesn’t exactly decompose — rather, it breaks down into smaller and smaller pieces. Once it reaches a certain size (fewer than 5 millimeters in diameter, to be exact), it’s referred to as “microplastic.”

In the past decade or so, microplastics have captured special attention over the unique threats they pose to marine life. Being so tiny makes them easy to accidentally ingest, particularly for filter feeders like clams, mussels, sea cucumbers and many marine worms, who feed by straining tiny organisms out of the water. Now, in a study just published in the Proceedings of the National Academy of Sciences, more than a dozen researchers from institutes in France and Belgium have teamed up to investigate what kinds of effects microplastics may have on oysters.

The researchers conducted their experiment in a laboratory setting using Pacific oysters, a species native to the Asian Pacific coast, but now introduced and cultivated throughout much of the rest of the world. In the lab, the researchers exposed adult oysters to two different sizes of microplastics: 2 micrometers and 6 micrometers. These are on the small side for microplastics — a micrometer is just one thousandth of a millimeter.

First, the researchers found that while the oysters ingested both sizes of microplastics, they consumed far more of the larger ones — possibly because these microplastics were closer to the size of the plankton that the oysters are typically most efficient at filtering.

The researchers then observed the oysters’ physiological responses to ingesting the microplastics. The most obvious effect was on the animals’ reproduction. Oysters that were exposed to microplastics produced fewer and smaller egg cells and slower sperm. Exposed oysters also produced fewer larvae — about 41 percent fewer, in fact — and their offspring tended to grow more slowly.

There are a couple of reasons the microplastics may have produced these effects, the researchers said. First, the plastics may have interfered with the oysters’ digestive processes after being ingested, possibly affecting the oysters’ ability to gain energy from their food. By using models to examine the ways oysters use up their energy, the researchers suggested that exposed oysters are forced to allocate more energy toward body maintenance and growth and away from reproduction in order to survive.

A second theory is that the microplastics somehow had a negative effect on the oysters’ endocrine function — essentially, the production of hormones that regulate all kinds of body processes, including sexual function and reproduction. The researchers observed that certain genetic information in charge of regulating hormones was expressed differently in exposed oysters, suggesting that endocrine disruption may have been a factor.

While the experiment was conducted in a laboratory setting, the researchers noted that the concentrations of microplastics used in the study are comparable to concentrations that have been observed in the kinds of natural settings where oysters live (although the sizes of the pieces of microplastic may vary). They also point out in the paper that “assuming no waste management infrastructure improvements, the cumulative quantity of plastic waste available to enter the marine environment from land is predicted to increase by an order-of-magnitude by 2025.”  

Indeed, the amount of plastic in the ocean is already an alarming problem. A 2015 paper published in Science estimated that anywhere from 4.8 million to 12.7 million metric tons of plastic were dumped into the ocean in 2010 alone. And if the amount of plastic entering the water continues to increase, as the paper predicted, it could be bad news for the oysters’ ability to reproduce effectively and maintain their populations — which, in turn, could ultimately threaten oyster fisheries.

Oysters are just one of many different types of organisms at risk, and the effects of microplastics may vary from one animal to the next. Research has suggested, for instance, that microplastics can change the feeding behavior of mussels, transfer pollutants into the bodies of marine worms and affect the growth and reproduction of algae and zooplankton. And larger bits of plastic have also been known to turn up in the bodies of other types of marine animals, such as seabirds and turtles.

So the paper highlights just one more problem in an already long list of ways that plastic in the ocean is harming ocean life and disrupting marine ecology. The researchers suggest in the paper that their conclusions can be taken as a kind of “early warning system” about the dangerous effects of these plastics in the water, and note that their research can help supply “the necessary data to limit the impact of the microplastic legacy in decades to come.”