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 such as 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 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 reproduction. Oysters that were exposed to microplastics produced fewer and smaller egg cells and slower sperm. Exposed oysters also produced fewer larvae 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. A second theory is the microplastics somehow had a negative effect on the oysters’ production of hormones.