Why tiny tardigrades walk like insects 500,000 times their size

Pudgy, ungainly tardigrades are among the smallest legged animals on Earth, and these microscopic water bears lumber around like chubby-thighed toddlers. But most creatures as small as tardigrades don't even have legs, so scientists recently analyzed tardigrades in motion to better understand how they use their limbs. 

Tardigrades, also known as moss piglets, have segmented bodies and four pairs of legs. They scoot through deep sea sediments and sandy river bottoms, and scurry over lichens and moss on land, scampering toward prospective mates and food or away from predators. 

Footage of scuttling tardigrades in the species Hypsibius exemplaris revealed that their movements closely resembled locomotion in insects about 500,000 times their size, despite being separated by around 20 million years of evolution and belonging to a different phylum. The step patterns of insects and other arthropods (invertebrates with segmented bodies and jointed legs) change when the animals speed up, and tardigrades' steps follow similar patterns when they walk faster, the new study found.

Related: 8 reasons why we love tardigrades

Tardigrades, of which there are about 1,300 known species, are notorious for being hard to kill; they can survive exposure to extreme temperatures, solar radiation and the vacuum of space. But few studies have examined these hardy creatures in more ordinary circumstances, and prior to the new study, scientists knew next to nothing about how tardigrades walk, said lead author Jasmine Nirody, a researcher and independent fellow at the The Rockefeller University Center for Studies in Physics and Biology in New York City.

Most microscopic, soft-bodied animals don't have legs, so it's also difficult to observe exactly how such tiny animals move. By analyzing walking tardigrades, literally one step at a time, the researchers also hoped to uncover clues about locomotion in general on a very, very small scale, Nirody told Live Science. 

"We saw tardigrades as giving us this porthole into both of these things that we don't know that much about," Nirody said.

Nirody's team looked at adults in the species H. exemplaris, which measure up to 0.02 inches (0.5 millimeters) long. All eight of their legs are structurally similar, but the pair closest to their rear ends has fewer muscles than the others. While this pair of legs plays some part in locomotion, most of the hard work is divided among the other six limbs, the scientists reported Aug. 31 in the journal Proceedings of the National Academy of Sciences (PNAS).

At first, the researchers tested tardigrades on slick glass slides, but they found that the water bears had a hard time propelling themselves over the slippery surface. Walking was easier for the tardigrades when they could dig in and push off with their claws. So for the rest of the experiments, the tardigrades trotted over gel that yielded to their claw pressure, according to the study. 

Unlike bigger animals that can be prodded into walking or running, tardigrades are too small for researchers to prompt their movement, Nirody said. So the scientists set up microscopes and cameras in the lab, let the tardigrades loose ... and then waited. 

"You get hours and hours of footage," Nirody said. "And I watched all of it."

Slow steppers

The phylum name Tardigrada (tardigrades are the sole member) comes from the Latin "tardigradus," or "slowly stepping," and tardigrades in the study lived up to that name. When moving at a leisurely pace, they traveled about half their body length per second — approximately 0.01 inches (0.25 mm) — and at faster speeds, they covered about two body lengths per second.

And when the tardigrades shifted gears between slow and fast walking, they smoothly transitioned to a new step pattern, as many arthropods do, rather than shifting into a new gait — in which the body's center of gravity also changes — as is common in animals with backbones.

When arthropods (and tardigrades) walk slowly, they lift one foot at a time. As they speed up, they lift two feet that are diagonal from each other across the body. Faster speeds make the animals shift to a new pattern in which three feet are off the ground at once: a front foot and a back foot on one side of the body, and a foot in the middle on the other side. 

"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 said. And in the experiments, the tardigrades demonstrated that they followed the same pattern of which legs were airborne when other legs were on the ground.

But why do tardigrades walk like arthropods? It could be that the groups share a common ancestor that was wired to walk this way. However, it's also possible that arthropods and tardigrades evolved this stepping pattern independently, after their lineages diverged, according to the study.

"What that means is that despite having completely different body structures, body sizes and environments that they're moving through, there's something about this particular coordination scheme that's efficient across all of these conditions," Nirody said. 

Originally published on Live Science.