Tardigrades are squidgy, microscopic animals with eight legs and a penchant for watery habitats. It's tempting to consider them cuddly, especially given their nickname ‘water bears’; however, it's hard to hug an animal measuring less than 1 mm. Despite their soft exterior, they are famed for their hardened resistance against the harshest possible conditions. Freeze them, heat them, dry them out, irradiate them – tardigrades take it all in their stride. For this reason, they can be found in every corner of Earth and even in space, with the latest batch of tiny tardigrade astronauts sent to the International Space Station earlier this year.

But being small has its challenges. For a tardigrade, taking a walk through a droplet of water is like you or me trudging our way through a bucket of mud. Despite this, tardigrades appear to quite happily walk and swim, using their tiny clawed feet to propel themselves forward. Given that their habitats are complicated three-dimensional environments and they have limited brain space, how are tardigrades able to negotiate their microscopic worlds? Jasmin Nirody from the University of Oxford, UK, together with Daniel Cohen and Lisset Duran from Princeton University, USA, and Deborah Johnston from the University of Rochester, USA, were curious and so began investigating how tardigrades would fare when given a variety of different surfaces to walk over.

Using a light microscope and video camera, Nirody and colleagues designed a performance arena for the terrestrial species Hypsibius exemplaris and first observed the tardigrades walking freely over a stiff gel-like substance, taking note of the animals’ speed and pattern of their footfalls. They found that the tardigrades’ feet on the left and right sides moved independently from each other, but their feet on a given side were tightly coordinated according to two basic rules: first, that if one leg was swinging forward, the adjacent leg in front or behind had to be on the ground; and second, that any given leg would only lift off the ground once its rear neighbouring foot touched down. Nirody and colleagues had seen this pattern before in much larger animals, such as fruit flies, and they suspect that these walking rules are governed by simple neuronal circuits composed of a series of on–off switches.

Having watched the tardigrades fare well on the stiff gel surface, Nirody and colleagues then decided to give the mini beasts a bit of a challenge; could a tardigrade with a soft body walk on soft ground? This time, Nirody provided the animals with a softer, more jelly-like surface to walk on and the tardigrades adapted their walking style by adjusting the synchronisation between their left and right sides. More incredibly, their new stepping pattern resembled the gallop seen in dung beetles moving over shifting sands. Nirody and colleagues suggest that perhaps this is a strategy used by both arthropods and tardigrades when navigating tricky terrain.

The tardigrades’ performance and similarities with insects have two possible explanations, which are equally absorbing. Either insects and tardigrades share a common ancestor which has ‘gifted’ them with similar patterns of coordination or, despite their huge difference in size and anatomy, tardigrades and insects have independently arrived at similar strategies to propel their bodies in challenging environments. With tardigrades now living in orbit, ‘one small step’ carries new meaning.

J. A.
L. A.
D. J.
Tardigrades exhibit robust interlimb coordination across walking speeds and terrains
Proc. Natl. Acad. Sci. USA
. doi:10.1073/pnas.2107289118