Scientists Discover Tiny Tardigrades Trot Around Like Insects

The microscopic organism’s gait may have evolved to adapt to unpredictable terrains

A microscopic image of a water bear standing on a gel-like surface. The image is in grayscale.
Tardigrades use their claws like grappling hooks and pull their bodies forward to move. Jasmine Nirody

Tardigrades, also known as water bears, are durable, microscopic aquatic animals about the size of a period at the end of a sentence. They have evolved to thrive in environments as varied as wet leaves, Antarctic mosses, underwater volcanic vents and soils. The tiny squishy organisms can survive harsh stressors like freezing temperatures, lethal doses of radiation, dehydration and even impact speeds of up to 1,845 miles per hour. The organisms are not only incredibly adaptable, but also one of the only soft-bodied animals known to walk. So how do they propel themselves? Researchers have found that their gait is similar to that of insects and changes depending on their environment's conditions, reports Jacinta Bowler for Science Alert. The findings were detailed in a study published last month in the Proceedings of the National Academy of Sciences.

Water bears have four pairs of legs and segmented bodies that they use to scoot themselves towards food, mates or away from predators, Live Science's Mindy Weisberger reports. Usually, such small organisms have difficulty walking because viscous and inertial forces make it harder for them to navigate some environments. In other words, microscopic organisms feel like they are moving through a large pool of honey or peanut butter, explains Jasmine Nirody, a biophysicist and the study's first author to Gizmodo's George Dvorsky.

To analyze how tardigrades walk, researchers observed various adults in the species, Hypsibius Dujardin. They placed them on different surfaces to study how their bodies moved in various conditions, reports Hannah Seo for Popular Science. Researchers carefully observed the placement of their feet as they moved from one direction to another. The tardigrades use their claws like grappling hooks and pull their bodies forward, explains Nirody to Gizmodo.

Researchers noted the surface mattered. The water bears relied on a stiffer platform to walk. Without it, they can't propel forwards. Tardigrades also had difficulty walking on slippery glass slides. The tiny organisms needed a surface where they could dig and push off with their claws. Gel substances of varying stiffness allowed the water bears to trot along the surface, Live Science reports.

"Tardigrades have a robust and clear way of moving—they're not these clumsy things stumbling around," said Nirody in a statement. "If you watch tardigrades under a light microscope for long enough, you can capture a wide range of behavior," she added. "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."

While on the gel substrate, the tardigrades moved half their body lengths at a time. When sprinting, they moved at two body lengths per second. Even if the water bear's speed changed, their gait stayed the same, per Gizmodo.

Their movements resembled the locomotion of insects even though they are separated by 20 million years of evolution. When arthropods walk slowly, they lift one foot at a time but as they speed up, their step pattern changes. When scurring away, insects lift two feet diagonal from each other at a time, and at even faster speeds, the step pattern changes where three feet are off the ground at once. When water bears change speeds, their step patterns also are very similar to those of arthopods, per Live Science.

"These patterns are tightly regulated by speed, they transition nicely between five legs on the ground, four legs on the ground, and then three legs on the ground as they get faster," Nirody expains to Live Science.

According to a statement, the research team suspects that tardigrades share locomotion abilities with insects because they may share common ancestors with animals like fruit flies and ants. Another possibility is that there is no ancestral connection but the animals have similar gaits because they are evolutionarily advantageous.

By detailing how the tardigrade's move, the study could help scientists design small nano-robots that scoot to deliver drugs around the human body or carry cargo to hard-to-reach areas, per Gizmodo.