Oysters dine on ocean plastic

The tiny mollusks take in microplastics, which may harm their ability to reproduce

oyster bed

The oysters on these rocks can filter up to 189 liters (50 gallons) of water per day, but they may be taking in some unwanted plastics along with that water. 

JesseW900/ Wikimedia Commons/CC BY-SA 4.0

When someone throws a plastic bag on the beach, it doesn’t become a memory. Instead, as it sits in the sun, it turns brittle and breaks into little pieces. Blown by the wind, these can end up in the water where those tiny bits can cause big problems for many ocean critters. Those plastic bits can be small enough to be vacuumed up by oysters. As the animals take in water filled with plastic, the oysters eat more and reproduce less, a new study finds.

It offers yet another reason to keep plastic out of our oceans — and the animals that live there.

Oysters may seem rather lazy, sitting on the seafloor with their shells open. In fact, they are working hard. Each is filtering water through its gills to glean food. “An oyster can filter up to 5 liters of water per hour,” explains Rossana Sussarellu. That’s about 1.3 gallons per hour. Sussarellu is a marine biologist at the French national institute for oceanography, known as IFREMER, in Nantes.

All of that filtering helps clean the water. The beds where oysters live — full of live animals and the shells of oysters past — also serve as a home for other species. And people find oysters quite tasty. That’s one reason “the oyster is the most farmed mollusk in the world,” says Sussarellu. Another reason: The pearls these mollusks can make are worth a lot of money.

So keeping oysters healthy has many benefits.

When what oysters eat isn’t food

As oysters run water past their gills, they’re hoping to catch phytoplankton (FY-toh-plank-tun) for lunch. These bite-size organisms float in the ocean. But oysters will filter pick up anything from the water that’s the same size as their food — including microplastics. These are tiny bits of plastic five millimeters (0.2 inch) or smaller. Some of those pieces started out small, such as the tiny microbeads in scrubby face washes. Others are the broken shards of what had been plastic bottles or Styrofoam takeout containers.

In 2010, scientists estimated between 4 million and 12 million tons of plastic enter the oceans each year.  

To find out just how eating microplastics might affect oysters, Sussarellu and her colleagues set up six tanks. Each held a small collection, or reef, of 40 oysters. Three tanks got seawater full of tiny algae for the oysters to filter out and dine on. These tanks served as a control — that part of the experiment where nothing changes from normal.

The other three tanks got yellow-green microplastics pumped into their water in addition to the algae. The plastics were two and six micrometer beads of polystyrene. It’s a type of plastic that people normally see in Styrofoam food containers or in take-out cups used for hot drinks.

Sussarellu and her colleagues fed algae and microplastics to their oysters for more than two months. Along the way, they measured the animals’ growth. They even monitored how much the mollusks ate and looked at where the brightly-colored plastics went by occasionally looking at the animals’ flesh through a microscope. And the oysters gobbled up plastic right along with algae.

“They’ve got such simplistic feeding,” explains Matthew Cole, who was not involved in the study. He studies marine biology at the University of Exeter and Plymouth Marine Laboratory in England. The rule of thumb for oysters, he explains, is “If they can consume it, they will.” But if they can’t use it as food, they will excrete it.

And the plastics did go right through the oysters and out their feces. The shellfish also ended up eating more algae than normal. It might be that they need a little extra energy to fuel the extra work of filtering out and excreting the unwanted plastics, Sussarellu suspects.

Microplastics didn’t seem to directly affect the dining oysters. It didn’t make them smaller or weaker. But it had a notable effect on their offspring. When adults were ready to reproduce, the females produced fewer eggs. The males also produced sperm that swam more slowly, making it less likely they would meet and fertilize an egg, leading to a new baby oyster.

Finally, when the oysters did successfully reproduce, the young exposed to microplastics grew more slowly than those that had grown up in clean water. The scientists published their findings February 1 in the Proceedings of the National Academy of Sciences.

Picking up plastic for oyster protection

Oysters aren’t the only animals snacking on plastics. Corals dine on them, too, as do the plankton floating in the water. The plastic has even floated up North to ocean ecosystems in the Arctic.

On December 28, the United States passed a law that bans all microbeads from facewash products. It directs companies to stop making cosmetics with the beads in them by July 1, 2017. Sales of cosmetics containing them will be illegal six months later.

That’s a good first step, says Sussarellu. But the microbeads are also, well, just a micro part of the problem, she adds. “The main source of microplastics is probably discarded plastics, bigger plastics that fragment,” she explains. Like a plastic bag left on the beach, which could break into thousands of tiny pieces.

The best approach is to “try and prevent plastic getting into the oceans in the first place,” says Cole, because “once it’s in the ocean it’s very difficult to get rid of.”

“Try and set yourself a challenge of not using plastic for one day or one week,” Cole recommends. “Write down or record the items you can do without that are made of plastic.” Then think about plastic in your home, he says, and how to reduce the amount you use.

People should also pick up and dispose of larger plastics properly, before they ever break into tiny pieces, he adds.

“Beach cleans,” says Cole, “are perhaps the most effective way of removing plastic” from the environment. Volunteers can sign up to be part of the Ocean Conservancy’s International Coastal Cleanup. Every September the Conservancy organizes a worldwide day to clean beaches. The group also advises cleaners about what to put in their cleanup kits, so they can clean beaches any time of year.

But it’s not just beaches that need cleaning. Cole notes that plastics can also end up in rivers, lakes and estuaries. Cleaning the nearest stream bed can still prevent plastics from ending up in oceans — and in oyster guts.

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Power Words

(for more about Power Words, click here)

control     A part of an experiment where there is no change from normal conditions. The control is essential to scientific experiments. It shows that any new effect is likely due only to the part of the test that a researcher has altered. For example, if scientists were testing different types of fertilizer in a garden, they would want one section of it to remain unfertilized, as the control. Its area would show how plants in this garden grow under normal conditions. And that give scientists something against which they can compare their experimental data.

estuary The mouth of a large river, where it empties into the ocean and freshwater and saltwater mix. estuary  The mouth of a large river, where it empties into the ocean and freshwater and saltwater mix. Such regions are often nurseries for young fish.

feces   A body’s solid waste, made up of undigested food, bacteria and water. The feces of larger animals are sometimes also called dung.

marine biologist   A scientist who studies creatures that live in ocean water, from bacteria and shellfish to kelp and whales.

micrometer  (sometimes called a micron) One thousandth of a millimeter, or one millionth of a meter. It’s also equivalent to a few one-hundred-thousandths of an inch.

microplastic  A small piece of plastic, 5 millimeters (0.2 inch) or smaller in size. Microplastics may have been produced at that small size, or their size may be the result of the breakdown of water bottles, plastic bags or other things that started out larger.

phytoplankton  Sometimes referred to as microalgae, these are microscopic plants and plant-like organisms that live in the ocean. Most float and reside in regions where sunlight filters down. Much like land-based plants, these organisms contain chlorophyll. They also require sunlight to live and grow. Phytoplankton serve as a base of the oceanic food web.

plankton  A small organism that drifts or floats in the sea. Depending on the species, plankton range from microscopic sizes to organisms about the size of a flea. Some are tiny animals. Others are plantlike organisms. Although individual plankton are very small, they form massive colonies, numbering in the billions. The largest animal in the world, the blue whale, lives on plankton.

polystyrene   A plastic made from chemicals that have been refined (produced from) crude oil and/or natural gas. Polystyrene is one of the most widely used plastics, and an ingredient used to make a widely used white, rigid foam (often sold under the name Styrofoam).

sperm   The reproductive cell produced by a male animal (or, in plants, produced by male organs). When one joins with an egg, the sperm cell initiates fertilization. This is the first step in creating a new organism.

Bethany Brookshire was a longtime staff writer at Science News Explores and is the author of the book Pests: How Humans Create Animal Villains. She has a Ph.D. in physiology and pharmacology and likes to write about neuroscience, biology, climate and more. She thinks Porgs are an invasive species.