Tardigrades utilize a tetrapod-like stepping pattern remarkably similar to that observed in insects, despite significant disparities in size and skeletal structure between the two groups, according to new research led by scientists from Rockefeller and Princeton Universities.
Hypsibius exemplaris. Image credit: Tarushika Vasanthan & Jonathon Stone, doi: 10.22120/jwb.2020.96855.1037.
The vast majority of animals need to move to survive. Tardigrades, though famed for their slow and unwieldy gait, are no exception.
One of the smallest legged animals, tardigrades rely on their locomotive abilities to escape from predators and to find food and mates.
The first observations of tardigrades in the 18th century CE centered around their distinctive gait.
They were described, in quick succession, as ‘water bears’ and ‘il tardigrado’ due to their slow, lumbering style of walking.
Beyond these initial characterizations, however, not much is known about how tardigrades move about in their environment.
“Tardigrades have a robust and clear way of moving — they’re not these clumsy things stumbling around in the desert or in leaf litter,” said Dr. Jasmine Nirody, a researcher at Rockefeller University and the University of Oxford.
“The similarities between their locomotive strategy and that of much larger insects and arthropods opens up several very interesting evolutionary questions.”
In the study, Dr. Nirody and her colleagues first determined how individuals of the tardigrade species Hypsibius exemplaris walk and run.
“If you watch tardigrades under a light microscope for long enough, you can capture a wide range of behavior,” Dr. Nirody said.
“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.”
The researchers found that tardigrades walk with an average speed of 163 μm/s (0.48 body lengths per s; range: 79.1 to 263.5 μm/s).
But the surprise came when the scientists observed how feet of Hypsibius exemplaris contact the ground as they gain momentum.
Unlike vertebrates, which have distinct gaits for each speed — picture a horse’s hooves as it transitions from a walk to a gallop — tardigrades run more like insects, scurrying at increasing speeds without ever changing their basic stepping patterns.
“When vertebrates switch from walking to running, there is a discontinuity. With arthropods, all stepping patterns exist along the same continuum,” Dr. Nirody said.
Why do tardigrades share a locomotive strategy with much larger insects?
One possible explanation is that tardigrades, long assumed to fit neatly into no existing taxonomy, may share common ancestors with insects such as fruit flies, ants, and other segmented scurrying creatures.
Another possibility is that there is no ancestral connection between tardigrades and arthropods, but that the unrelated groups of organisms independently arrived at the same walking and running strategies because they were evolutionarily advantageous.
“If there is some ancestral neural system that controls all of panarthropod walking, we have a lot to learn,” Dr. Nirody said.
“On the other hand, if arthropods and tardigrades converged upon this strategy independently, then there’s much to be said about what makes this strategy so palatable for species in different environments.”
The findings were published online in the Proceedings of the National Academy of Sciences.
Jasmine A. Nirody et al. 2021. Tardigrades exhibit robust interlimb coordination across walking speeds and terrains. PNAS 118 (35): e2107289118; doi: 10.1073/pnas.2107289118