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Tiny bits of plastic get to the seafloor by hitching rides on snot palaces

Thanks to the snot-dwelling larvaceans Bathochordaeus stygius

The filter-feeding larvacean Bathochordaeus stygius.
Video: Monterey Bay Aquarium Research Institute

Scientists have discovered a new way for tiny pieces of plastic to move around the ocean: through the poop and snot made by a peculiar sea creature that’s half the size of your hand. The new research shows the important role that small, unassuming sea critters can have in transporting plastic waste from the ocean surface all the way to the seafloor.

The animals described in the study, published today in Science Advances, are called Bathochordaeus stygius. These are so-called larvaceans that live inside snot “houses” used to filter seawater for food. As the creatures filter water, they also filter tiny pieces of plastic that litter the oceans. Some of this plastic is eaten by the animals, and then discarded through poop, which sinks to the seafloor, researchers found. Other pieces of plastic get stuck to the snot house — and end up sinking to the seafloor when the houses get clogged and the animals drop them. Thus, through poop and snot, plastic waste reaches the deepest corners of our oceans.

Microplastics traveling to the guts of a filter-feeding larvacean observed in the study.
Video by Katija et al., Sci. Adv. 2017;3: e1700715

Plastic debris is everywhere: it can be found in sediments at the bottom of the ocean, floating at the surface, washed up on remote islands, inside fish and birds, and even frozen inside Arctic ice. Microplastics — pieces smaller than 0.19 inches — are particularly dangerous: because chemicals like PCBs accumulate on plastic debris, if these microplastics are eaten, they can end up poisoning the animals. A study, for instance, found that fish that ate microplastics had liver problems. But there’s a lot we still don’t know about their health effects — especially on people who eat fish — and even where most microplastics are and how they move around the ocean.

“We’re really at the tip of the iceberg in understanding really where these plastics are winding up,” says study co-author Kakani Katija, the principal engineer at the Monterey Bay Aquarium Research Institute (MBARI).

To answer some of those questions, researchers at MBARI looked into B. stygius, which are known to eat particles that are about the same size as microplastics. In a study published earlier this year, some of the same researchers showed how these animals feed: through their snot houses, which can be as large as three feet, they filter an average of about 11 gallons per hour, or roughly two of those big water jugs that get delivered to your office. When the houses get clogged, the animals just drop them. The old mucus dens then sink to the seafloor at a speed of over 2,500 feet per day, and here, they become food for other bottom-dwelling sea creatures.

For today’s study, the researchers studied 25 B. stygius off the coast of Monterey using a remotely operated vehicle. They released tiny pieces of plastics — between 10 and 600 micrometers in diameter — in the water, to see whether the larvaceans ate them. (The microplastics had a fluorescent coating, so they could be easily observed.) The majority of the animals were found to ingest the plastic, and then poop it out within about 12 hours, Katija says. The tiny pieces of plastic also stuck to the gooey snot houses. It’s unclear whether the plastic harmed the animals, but Katija says none of them died and they continued behaving normally.

The remotely operated vehicle used in the study, alongside a floating larvacean.
Video by Katija et al., Sci. Adv. 2017;3: e1700715

Because both the poop and the snot houses are known to sink to the seafloor once they’re discarded, that means that microplastics encased in them sink as well. And because both the poop and the snot are eaten by small crustaceans, sea cucumbers, and other creatures living at the bottom of the sea, we can assume that the microplastics are being ingested by other animals. (Whether this ends up harming the sea creatures is still up for debate.)

“The point of the study was to shed some light on the fate of microplastics in open ocean food webs,” say study co-author Anela Choy, a postdoctoral fellow at the Monterey Bay Aquarium Research Institute, “not just plastics that are found at the very surface of the ocean but that encompass the whole water column, which is the biggest habitat on the planet.”

The study doesn’t solve the entire mystery of where microplastics are and what happens to them in the ocean, but it’s one revealing piece of the puzzle. Other smaller larvaceans are known to also ingest plastics, Katija says, and they also might be sinking microplastics to the seafloor through their poop.

To answer these questions, we might just need to analyze more deep-sea mucus and feces. Sign me up!