, 16 August 2017
Small filter-feeding animals in the world’s oceans take in bits of plastic and excrete them in pellets that sink to the ocean floor.
The feeding behaviour of these creatures, known as larvaceans, may transport vast amounts of microplastics from the upper layers of the ocean down into the depths. And it could be why surveys are finding far less plastic floating in the oceans than expected.
The removal of plastic from surface waters might sound like a good thing, but this isn’t necessarily the case. “It means plastic is a much bigger problem than just at the surface,” says Kakani Katija of the Monterey Bay Aquarium Research Institute in California. “It has the potential to affect the inhabitants at various depths throughout the ocean.”
It could also affect us, says her colleague Anela Choy. We eat a lot of animals that live on the seafloor, such as crabs.
Transporters of plastic
While terms like the “Great Pacific Garbage Patch” conjure up visions of floating islands of rubbish, most of the plastic in the oceans consists of tiny pieces invisible to the human eye.
To find out what happens to the plastic, Katija and Choy studied larvaceans: filter-feeding animals that are distantly related to vertebrates. They focused on giant larvaceans (Bathochordaeus stygius). The bodies of these tadpole-like creatures are just a few centimetres long, but the mucus “houses” they secrete can be a metre across.
Previous studies by Katija have shown these abundant animals filter immense amounts of seawater each year. “They are incredibly important organisms in mid-waters,” she says.
The pair used a remotely operated vehicle to squirt tiny plastic pellets near individual giant larvaceans and watched what happened. The observations were made at depths of between 200 and 400 metres.
Some of the plastic pellets stuck to the larvaceans’ mucus houses, which are regularly discarded. Others were ingested and incorporated into fecal pellets. Both the discarded houses and fecal pellets sink to the sea floor, but may be eaten by other animals on the way down.
Fecal pellets that contain buoyant plastic sink more slowly and are thus more likely to be eaten, says Matthew Cole of the University of Exeter, UK. His team has shown this in lab experiments with copepods, another kind of abundant small marine animal.
“And then the plastic has the chance to be eaten again and cause an effect, perhaps, to another animal,” Cole says. It is unclear how microplastics affect the organisms that eat them.
Source : New Scientist