SEM image showing Milnesium tardigradum active state. Credit: PLoS ONE (7(9): e45682. doi:10.1371/journal.pone.0045682
When scientists first discovered the tardigrades' unique lumbering gaits, which were 0.02-inch long and plump, in the 18th century, scientists called them "water bears". However, their plodding gait raises questions about how tardigrades evolved to walk.
Terrorigrades, small and delicate animals, rarely have legs and don't even bother walking. Round worms, for example, are similar in size and shape, but they slither over unpredicted substrates. The water bear is a unique micro-animal that scientists had to give its own phylum. It uses eight long legs to propel itself through freshwater sediment and desert dunes.
A new study published in PNAS examines the gaits of tardigrade water bears. It shows that their walking style is very similar to that of insects half their size. This discovery suggests that there may be a common ancestor, or an evolutionary advantage that allows water bears to walk like larger, more robust insects.
Jasmine Nirody is a Rockefeller Center for Studies in Physics, Biology fellow. "Tardigrades have an extremely robust and clear method of moving. They are not these clumsy things that stumble around in the desert or in leaf litter." "Several interesting evolutionary questions can be posed by the similarities in their locomotive strategy to that of larger arthropods and insects."
Tardigrades move in a way that is very similar to the movement of insects half their size. Credit: Jasmine Nirody.
Smooth runners
Nirody and his colleagues were the first to determine how water bears move and run. Nirody states that you can see a variety of behaviors if you keep your eyes on tardigrades. We didn't force them into doing anything. Sometimes they'd just be super chill and want to walk around the substrate. Sometimes they would see something they liked and want to run towards it.
Nirody discovered that water bears move at a speed of half a meter per second when they are most relaxed. Their loping strides allowed them to move two times as fast at full throttle. The surprise was when she saw how water bears' feet touch the ground as they gain momentum. Contrary to vertebrates which have different gaits for each speedpicture, horses' hooves when it transitions from walking to galloptardigrades are more like insects. They move at increasing speeds with the same basic stepping patterns.
Nirody states that vertebrates can switch from running to walking when they do so. "All stepping patterns in arthropods follow the same continuum," Nirody says.
Ancient coordination
Why does tardigrades have a common locomotive strategy with larger, more hard-bodied insects like the flies?
Another explanation could be that tardigrades, which were long thought to fit neatly within no existing taxonomy may have shared common ancestors and even a common neural network with insects like fruit flies, honey bees, and other segmented, scurrying creatures. Some scientists actually advocate that tardigrades be classified within the proposed panarthropod Clade, which would give common shelf space to insects and crustaceans, velvetworms, water bears, and other scurrying creatures.
Another possibility is that arthropods and tardigrades do not have an ancestral connection. However, it is possible that different groups of organisms evolved the same running and walking strategies. The best way to navigate unpredicted terrain with a microscopic body, is to plod like an aquatic bear.
Both possibilities are equally fascinating to Nirody. She says, "If there's an ancestral neural system that controls all panarthropod walking," we have much to learn." "On the other side, if arthropods or tardigrades came up with this strategy independently, then it would be interesting to learn more about why this strategy is so appealing for different species in different environments.
The implications of the findings for evolutionary biology and animal locomotion are not the only ones. However, they could have implications for soft and microscale robotics.
Scientists may be able design robots that fit into tight spaces and operate at the microscale by studying the evolution of small animals to move in difficult environments. Nirody states that we don't know enough about the extremes in locomotion to design a small walker or what soft-bodied animals should do.
"Tardigrades provide a key porthole into microscale, soft-bodied locomotion."
The research was published by Proceedings of the National Academy of Sciences.
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