A giant larvacean (the blue, tadpole-like animal) beats its tails, pumping water and microplastic beads (red dots) through...

Giant plankton eat and transport plastic through the ocean

Of the 8.3 billion tons of plastic ever made, more than half winds up in the environment, especially the ocean. Marine critters not only eat this pollution, but they can also dictate the plastic’s final destination, scientists from the Monterey Bay Aquarium Research Institute reported Wednesday.

Their study found that bizarre, tiny creatures called giant larvaceans ingest and transport plastic from the ocean surface to the seafloor. The research exposes the potential for the distribution of plastics and their toxins throughout the food web given the keystone role of plankton play in ocean ecosystems.

“The biggest habitat on the planet is the deep sea,” said Anela Choy, an oceanographer and a lead author of the study published in Science Advances. “Looking at how plastics make their way into deep sea food webs, which are directly connected to humans, is a really important step in understanding the overall impact of plastic in the ocean.”

Giant larvaceans, despite the name, are zooplankton about the size of your pinky finger and no bigger than the palm of your hand. Transparent and shaped like a tadpole, these zooplankton erect exquisite dwellings for themselves out of mucus. The larvaceans feed themselves by beating their tails and filtering seawater through large snot houses, which are more than three feet in diameter.

Their filtration skills are stunning. A single larvacean can clear 11 gallons per hour, and when their populations are at peak density, they can clear the equivalent of 500 Olympic-sized swimming pools per hour. When these abodes become too clogged with ocean gunk, the larvaceans simply drop them and build a new one.

This GIF shows the inner filter of a giant larvacean, a member of the genus Bathochordaeus. Photo by MBARI

This GIF shows the inner filter of a giant larvacean, a member of the genus Bathochordaeus. Photo by MBARI

Choy and Kakani Katija, a bioengineer at MBARI, started their project by checking if larvaceans could even consume plastic. They used an underwater drone to disperse one liter of microplastics — pieces similar in size to grains of sand — near 25 larvaceans living in the Monterey Bay off the coast of California. Since the microplastics were color-coded by size and larvaceans are transparent, the researchers could easily track the fate of the plastics.

Eleven larvaceans took up the plastics into their mucus houses, and six ingested the colored beads into their guts. With assistance from cameras on the underwater robot, Choy and Katija examined the guts and fecal pellets of five plastic-eating larvaceans. They found microplastics of all sizes in the feces of four of the five larvaceans — one animal was lost during collection.

This study addresses a big missing piece in microplastics research, said Chelsea Rochman, an ecologist at the University of Toronto who was not involved in the study, namely how much plastic exists in mid-space between the ocean surface and the seafloor and how it gets there.

The larvaceans’ poop sinks swiftly to the seafloor. The team estimated their feces descend at nearly 1,000 feet per day, and a previous study found the discarded mucus dwellings fall even faster — around 2,600 feet per day.

Because of these speedy sinking rates, the jettisoned debris is unlikely to break down or be intercepted by other animals, the researchers reported. Discarded larvacean homes and bodily waste thus act as a fast means for transporting plastic from the upper regions of the water column to the deep sea.

By extension, the creatures may ultimately act as a distribution point for inserting plastics into the food chain. Many bottom dwellers feed on the discarded larvacean houses and stools, while the larvaceans themselves are prey for carnivorous plankton and fish. The latter go onto feed bigger organisms, like whales and humans.

Though larvaceans are numerous, they are not the only filter feeders that inhabit the space between the ocean surface and the seafloor. Other organisms probably contribute to the distribution of plastics within the marine environment, Katija said, though further testing is needed to confirm this idea.

Microplastics can also be toxic, but Choy said more research is needed to understand how these plastics move through the food web and the extent of the risk to the seafood we eat.