Tardigrades Seem Immune to Swallowing Microplastics

Rocks erode into sand that eventually reaches the sea. In much the same way, the ocean becomes the unavoidable final destination for breaking-down human waste: microplastics.

Like naturally occurring sediment, these minute synthetic particles find their way into the stomachs of animals that inhabit the marine world.

Tardigrades and microplastics

Yet one microscopic creature, famous for reviving after exposure to space, freezing, boiling, suffocation and even radiation bombardment, appears to possess another remarkable defence. In a study examining several tiny animals, tardigrades appeared not to ingest microplastics.

That is particularly unusual because every other small organism assessed in the research consumed the particles.

Zoologist Flávia de França of the Federal University of Pernambuco in Brazil led a team that gathered meiofauna samples - intertidal invertebrates measuring roughly 45 micrometres to 1 millimetre - from the shallow sediment of a sandy beach on Brazil's north-eastern coast during low tide.

The samples contained 5,629 individual animals, spanning nematodes, segmented worms, flatworms, bristleworms, seed shrimps, hairybellies, mites, crustaceans and crustacean larvae. They also included the celebrity of the meiofauna world: the tardigrade.

Testing marine meiofauna in sediment

The animals were placed in tanks intended to replicate their natural conditions as closely as possible, with 100 grams of sediment containing differing concentrations 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, although the researchers chose not to distinguish them from larger microplastics because the particle-size boundaries remain under debate.

Analysis showed that every tiny animal swallowed microplastics apart from the tardigrades, which appeared to be unusually adept at avoiding the questionable particles.

They were not entirely free of plastic, though. Microplastic particles were found on the body surfaces of more than half the waterbears, especially on their 'legs' - more scientifically termed their 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. Consequently, this feature may not shield every tardigrade from humanity's fragmented waste.

Microplastics move through the food chain

Although it is entertaining to find another hazard that tardigrades seem able to withstand, the research draws attention to the more troubling consequences of microplastics at this small yet important level of Earth's ecosystems.

According to the authors, this is the first evidence showing that Turbellarians (flatworms) and Gastrotrichs (hairybacks) eat 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."

By contrast, the researchers believe hairybacks consumed the particles after mistaking them for food. The danger may be greater because bacteria rapidly colonise plastic surfaces, potentially making them resemble an appetising chocolate coating to a hairyback.

A central question behind the research was how microplastics affect species abundance and diversity at this scale. Although ingestion followed a fairly direct pattern - higher numbers of plastic particles led to greater consumption by the animals - 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 our own species' reliance on plastic are concerning. Still, there is modest reassurance in knowing that some marine tardigrades remain plastic-free.

This research was published in PeerJ Life and Environment.