Tardigrades can hitch-hike on snails to travel longer distances

Tardigrades may hitch-hike on passing snails to travel relatively long distances, according to laboratory studies.

Sometimes called water bears, tardigrades can survive extreme environments in a dried-out state called a tun. But because they are so small, they can only walk a short distance by themselves. This creates a mystery, though, because tardigrades are found across the world, with a genetic diversity of more than 1400 species.

Many small animals can travel long distances by clinging to the body of a larger, more mobile creature. This behaviour has never been observed in tardigrades, so Milena Roszkowska and Zofia Książkiewicz at the Adam Mickiewicz University in Poland decided to investigate.

The pair used lab studies to test whether snails might transport tardigrades. In one box, they placed a number of tardigrades belonging to the species Milnesium inceptum by themselves. In a second container, there were tardigrades plus a species of snail (Cepaea nemoralis) that occurs in their natural habitat, while in a third box there were tardigrades, snails and moss, where tardigrades often reside in nature.

After three days, the researchers counted how many tardigrades remained in their original location and how many had moved, and whether they were alive or dead. They found that living tardigrades only left their starting location in the boxes where snails were present without moss. They speculate that this might be because tardigrades are picked up passively by passing snails, and that this process is more unlikely if the tardigrades are embedded in moss.

“This emphasises the role of the fine-scale dispersion of tiny animals,” says Roszkowska. “Short-distance transportation of invertebrate animals may have a significant impact on their genetic diversity.”

The researchers found that the tardigrades in moss didn’t travel any more than the control group, suggesting that the effect only happens in certain natural scenarios.

They also found that some of the tardigrades died from contact with the snail’s mucus. But tardigrades’ ability to reproduce asexually means only one needs to survive the journey to establish a population in a new area. This means snail travel could still be a viable means for tardigrades to populate novel habitats.

While the study demonstrates that snail-based transport is possible for some tardigrades, the researchers still don’t know whether the tardigrades are transported this way in nature – or how often. One variable is that the damp habitats tardigrades and snails need for survival may change with time. “It depends on the weather conditions. In wet years, tardigrades may be transferred by snails more often than in dry years,“ says Książkiewicz.

“The fact that it is a potential does make it interesting,” says Sandra McInnes at the British Antarctic Survey. “It’s the sort of thing that somebody hasn’t done before. This experiment is a pilot study to see whether it’s feasible and they’ve proven that it is.”

Future experiments, says McInnes, should now try to observe this happening in natural habitats.

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