Tardigrades May Be Immune to Swallowing Microplastics

Rocks are gradually ground down into sand and carried out to sea. Human waste follows a similarly unavoidable route: as plastic breaks apart, it becomes microplastics that end up in the ocean.

Like natural sediment, these minute synthetic particles find their way into the stomachs of animals that inhabit marine environments.

Yet one minuscule creature, renowned for reviving after space travel, freezing, boiling, suffocation and even radiation bombardment, appears to possess another remarkable defence. In a study involving several groups of tiny animals, tardigrades seemed unaffected by the ingestion of microplastics.

That is particularly unusual because every other small creature examined in the research consumed the particles.

Tardigrades and microplastics in beach sediments

A research team headed by zoologist Flávia de França of the Federal University of Pernambuco, Brazil, gathered meiofauna samples from shallow sediment on a sandy beach along Brazil's north-eastern coast at low tide. Meiofauna are intertidal invertebrates measuring roughly 45 micrometres to 1 millimetre.

The samples contained 5,629 individual organisms. These included nematodes, segmented worms, flatworms, bristleworms, seed shrimps, hairybellies, mites, crustaceans and crustacean larvae - as well as the star of the meiofauna world, the tardigrade.

The animals were housed in tanks intended to replicate their natural conditions as closely as possible. Each tank also contained 100 grams of sediment with differing amounts of plastic particles.

The fluorescent polystyrene microspheres had a radius of about one thousandth of a millimetre. Technically, this places them in the 'nanoplastics' category, but the researchers chose not to distinguish them from larger microplastics because the size boundaries for these particles remain under debate.

Why tardigrades did not swallow the particles

The analysis showed that all of the tiny animals ingested microplastics apart from the tardigrades, which appeared to be unusually capable of avoiding the questionable particles.

They were not entirely free of plastic, though. Microplastic particles were found on the outside of more than half the waterbears, especially on their ‘legs’ - more accurately termed locomotory appendages.

"The absence of microplastic ingestion by Tardigrada likely links to the structure of their feeding apparatus, which includes a mouth tube with a stylet used to pierce and suck rather than ingest prey organisms whole," the authors write.

It is worth noting that tardigrades are highly diverse, with many species possessing differently shaped body structures. This means that not every tardigrade may be shielded from disintegrating human rubbish in this manner.

Although it is intriguing to identify another danger that tardigrades apparently withstand, the study also draws attention to the more serious consequences of microplastics for this small yet important level of life on Earth.

Microplastics across the meiofauna food chain

The researchers consider this to be the first evidence that Turbellarians, or flatworms, and Gastrotrichs, or hairybacks, consume microplastics.

"Turbellarians can prey on nematodes, copepods, and even individuals of the same taxonomic group," the authors write. "It is therefore plausible that at least part of the ingested microplastics derived from their prey, indicating that microplastics can indeed be transferred up the trophic chain."

Hairybacks, by contrast, were thought to have eaten the microplastics after mistaking them for food. This may be an even greater hazard because bacteria rapidly colonise plastic surfaces, potentially making them resemble an appetising chocolate coating to a hairyback.

The research was driven by the question of how microplastics affect species abundance and diversity at this scale. Although the connection between plastic concentration and ingestion was relatively simple - more plastic particles meant more consumption by the creatures - the ecological effects were less straightforward.

"We observed that environmentally relevant concentrations [of microplastics] resulted in a decrease in total meiofauna density and species richness," the authors report, but "a much higher microplastic concentration did not differ from a control without microplastic addition."

The extensive consequences of humanity's dependence on plastic are concerning. Still, it offers a small measure of comfort that some marine tardigrades appear to remain plastic-free.

This research was published in PeerJ Life and Environment.