Giant Larvaceans Sweep Up and Poop Out Plastic Waste in the Oceans

The plankton species appears responsible for transferring plastic pollution from near the surface of the ocean to the sea floor, but it remains unclear if that’s a positive or negative mechanism.

A type of giant Pacific plankton scoops up tiny plastic beads from the ocean and poops it out in pellets that sink to the ocean floor, shifting that pollution toward the depths, scientists at California’s Monterey Bay Aquarium Research Institute reported today.

While most plankton are so small they’re invisible to the naked eye, species known as giant larvaceans can grow as long as 10 centimeters (4 inches). They surround themselves with a wide net of translucent mucus up to 1 meter (3.25 feet) across, which they use to capture and filter food from the ocean around them. That wide net, called a house, allows them to sweep dozens of liters of water an hour before eventually being discarded.

Kakani Katija, the principal engineer at the research institute, and her colleagues used a submersible robot to feed plastic pellets to a giant larvacean species in Monterey Bay, about 75 miles south of San Francisco. Then they scooped up the plankton, took it back to the lab and watched what happened next.

“The animals basically pooped out the microplastics into really tight compact little pellets,” Katija said. “Both the pellets and the houses, despite having plastics in them, would sink to the bottom of the containers.” 

Katija and her colleagues published their findings the research journal Science Advances. It will take more study to figure out whether the plastics excreted by the larvaceans were broken down at all after being ingested, she said.

Plastic pollution is a major problem in the marine environment, with an estimated 8 million tons of the stuff ending up in the oceans every year. Much of that takes the form of beads smaller than 5 millimeters (three-sixteenths of an inch) that have been used in commercial products or result from the breakdown of larger chunks.

They’re easily consumed by fish and other sea creatures, so they’re also likely to get passed up the food web to humans. And other recent studies have found them not only bobbing in the waves, but embedded in the sea floor. The Monterey study was aimed at helping figure out how that happened.

“Somehow, someway, these plastics are getting from the surface where we are down to the bottom of the ocean,” Katija said. “That mechanism wasn’t well understood. So what we found was there might be this biological transport mechanism that can aid the transport of these plastics from the near surface to the bottom. Whether that’s a positive or a negative thing, that’s hard to say.”

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Animal species living at the bottom of the ocean eat both the fecal pellets and the larvaceans’ discarded houses, which Katija called “utterly beautiful and structurally complex.”

“What that means is there are these different avenues, these different pathways by which plastics can enter food webs in the ocean,” she said. “The big question that we need to start considering is that if animals are eating these plastics, potentially we could be eating them too, because we eat some of these marine animals.”

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