Most of us are well aware by now that plastics are littering our oceans. But despite the ubiquity of this trash, we still know worryingly little about how animals interact with it.

It now seems that the tiny crustaceans known as krill – the basis for many of the ocean's food webs – can digest microplastics into even smaller fragments. On the surface, this seems like they are doing a service, but in reality they may be making it easier for other organisms to consume and spread to further environments.

“This reveals a previously unidentified dynamic in the plastic pollution threat, with the implication that biological fragmentation of microplastics to nanoplastics is likely widespread within most ecosystems,’’ explains Bengtson Nash, co-author of the latest study in Nature Communications, in a statement.

The work was carried out at Australia’s Griffith University, where researchers were studying whether microbeads had a toxic effect on krill in a laboratory setting. To see how different concentrations of plastics had an impact, they kept groups of krill in separate aquariums and fed the animals a mix of either 80 percent algae and 20 percent microplastics or 20 percent algae and 80 percent microplastics, and noted any changes.

They found, for the first time, that the krill were not only ingesting the plastics, but actually breaking it up into smaller parts before excreting it. They observed the crustaceans consuming bits of plastic 31.5 microns in diameter and turning it into fragments of less than one micron. Considering this was carried out on "fresh" plastic, the researchers expect that plastics in the wild are even easier to digest as they have already been degraded to some degree by UV light.

It turns out that the mouth parts and stomach of the krill act in effect like a pestle and mortar, grinding up the tiny pieces of microplastic, which then became enmeshed in their mouths, stuck in their oesophagus, and lodged in their stomachs. Considering other species of plankton also have a similar digestive process, they suspect that many other types of zooplankton will likely be degrading microplasics in a similar way.

Interestingly, they found that when plastic concentrations were high, the krill became less efficient at breaking it down, but this effect is unlikely to be present in the oceans today. “Current contamination levels in the Southern Ocean are theoretically low enough to promote efficient digestive fragmentation by krill species, and in a global context, possibly for other zooplankton with sufficiently developed gastric mills,” says co-author Dr Amanda Dawson.

This means there's still the same degree of plastics in the oceans, it just might be that it's being shredded into ever finer pieces and becoming more pervasive.