The ocean is a wondrous place, inhabited by a seemingly endless supply of bizarre and fanciful animals, from trap-jawed worms to tiny, jellyfish-wrangling lobsters. Take a dive in the open oceans of California's Monterey Bay, and you might bump into the gooey membrane of Bathochordaeus stygius, the giant larvacean.
Larvaceans are an important part of marine ecosystems – they belong to a group of life called tunicates, with a simple spinal cord but no backbone – and this "giant" species is actually no bigger than your finger. Its big claim to fame is the vast mucous "house" it builds around itself, which can be over a metre (three feet) across! This eye-catching bubble not only protects the little tadpole-like animal inside, but it also takes in and filters tiny particles floating in the water, including microscopic organisms, bits of food, and – unfortunately – plastic.
The thought of plastic pollution typically brings to mind images of animals caught in plastic strapping or choking on discarded trash, but the most devastating form of plastic pollution is tinier than a grain of sand. Pieces smaller than five millimetres are called microplastics, and they're incredibly abundant in the world's oceans, easily entering animals' bodies to cause digestive blockage or poisoning.
"Globally, plastic waste is now a pervasive feature of nearly all marine environments, ranging from coastal bays and estuaries to large oceanic gyres and down to the abyssal seafloor," say researchers from the Monterey Bay Aquarium Research Institute (MBARI) in a newly published study in the open-access journal Science Advances. They note that the amount of plastic entering the oceans could, over the next decade, rise to 250 million metric tons per year in the United States alone.
But despite rising concerns about plastic pollution, not much is really known about how these little particles move from one part of the ocean (like the surface, where they enter the water) to another (like the deep sea). The scientists who conducted this study wondered if plastics are cycled throughout the seas by animals in the same way food and nutrients are. To find out, they turned to their favourite giant, snotty particle-collector.
Floating alongside its goo-bubble, the larvacean flaps its tail vigorously in a sort of underwater ribbon dance, creating a flow of water that collects tiny bits of flotsam inside its big filter. This made it the perfect candidate for the study since the detritus these animals normally collect is about the same size as many microplastics particles. And because larvacean bodies are transparent (how cool is that?), the researchers could observe how ingested plastic moves through their system. What's more, these are some of the world's most impressive filter-feeders, pumping through an average of 40 litres of water an hour!
Sure enough, when plastic pieces (half a millimetre or smaller in size, in this study) were introduced into the water, most of the larvaceans gobbled them up. They didn't seem harmed by the plastic, but once their digestive tracts were done confirming these were definitely not food, the plastic went the way of all waste: out the back.
In the ocean, the fate of faeces is to fall. The researchers even undertook the fascinating task of measuring how fast these "microplastic-rich faecal pellets" sank through the water column (about 300 metres a day, if you're wondering). By hitching a ride on this descending poop elevator, microplastics make their way from the upper layers of the ocean down to the sea floor.
And it's not just droppings these tunicates are dropping. Collecting sea-junk all day clogs up those mucous bubbles pretty quickly, so the larvaceans simply drop them and start making new ones on a regular basis. These falling globs carry plenty more plastic pieces stuck inside, following the same path as the poop (but faster, around 800 metres per day, while we're keeping track) down to the bottom of the ocean.
Normally, these sinking deposits would be a great source of nutrients for animals lower down in the water column. But with plastic inside, the faeces and gunk-blobs are also providing a consistent way for dangerous microplastics to travel to deeper ecosystems, or – if they're gobbled up before they get all the way down – to spread throughout the food web.
Many questions remain. How often do larvaceans normally ingest plastic? Are other filter-feeders providing similar transportation for pollutants down the water column, particularly those living higher up, where microplastics tend to be most common? What other ways are there for microplastics to move through the oceans? There's a lot more research to be done to truly understand the scale of impact that plastic pollution has on marine ecosystems.
While scientists are seeking answers to those big questions, the rest of us can do our part by brushing up on the subject of ocean plastic pollution, and by watching the mesmerising movements of these beautiful globby ocean creatures, to remind us what we're fighting to save.
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Top header image: Katija et al.
Deep Ocean
Predator vs Prey
Human Impact
