Tardigrades are among the smallest animals, but are very different from other microscopic creatures – most notably their slow walking style that earned them the nickname “water bears”. A new study explains why creatures a third of a millimeter (0.015 inches) long evolved to walk, where most mobile organisms of similar size prefer to slither.
Sometimes known as “moss pigs”, tardigrades are nature's ultimate survivors, capable of being frozen, placed in a vacuum, or exposed to 600 atmospheres pressure all by turning to glass and coming out alive. It stands to reason their method of locomotion has also been refined by evolution for survival – but microbiologists have been unsure of the benefits of limbs on a soft-bodied animal.
Researchers spent time just watching tardigrades of the species Hypsibius dujardini move naturally under microscopes and describe their walk in Proceedings of the National Academy of Sciences. They noticed of all the macroscopic creatures their movements most closely resemble those of insects.
"We didn't force them to do anything. Sometimes they would be really chill and just want to stroll around the substrate. Other times, they'd see something they like and run towards it," study author Dr Jasmine Nirody of Rockefeller University and the University of Oxford said in a statement.
The authors found tardigrades maintain the same style of movement irrespective of their speed, swinging three groups of two limbs together, like ants and crustaceans. This contrasts with the way a horse, for example, will shift from a walk to a trot to a gallop as it decides to pick up the pace.
The similarity between water bears' walk and that of insects might seem unremarkable from our lofty heights, despite tardigrades having eight legs, but there is a scale factor of around 100 times in each dimension between them. Moreover, insects are hard-bodied, while tardigrades are almost the only soft creature with legs.
This means either tardigrades – that have their own phylum because they're so different from other living things – share a common ancestor with insects, or they evolved the same type of walk independently. Considering the differences in scale and habitat between insects and tardigrades, this would indicate there are some substantial advantages to this time of movement.
The paper does not favor either option, but the authors believe each would have important implications if the correct answer can be identified. Moreover, whichever turns out to be true, tardigrades could serve as models for robots designed to move over rough surfaces at very small scales.
"We don't know much about what happens at the extremes of locomotion – how to make an efficient small walker, or how soft-bodied things should move," Nirody said.
Naturally, tardigrades' walk is not identical to that of insects, and one noticeable difference is that their hind legs move differently to the front three pairs. “This is in accordance with the hypothesis that the posterior legs of tardigrades are used primarily for grasping rather than propulsion in forward locomotion,” the paper notes. Why you'd use your back, rather than front legs this way only tardigrades know.
Tardigrades have adapted to a wide range of environments including seas, lakes, and wet forests. However, the polished slides that formed one of the substrates Nirody had them walk across proved a challenge. They did much better on soft gels where they could engage their tiny claws.