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Tardigrades, or water bears, are among the smallest and most resilient creatures on the planet. Although the largest of these microscopic four-legged creatures measures no more than 1.5mm, they have been shown to withstand extreme temperatures and pressures, air deprivation, radiation and starvation. Not only that, they also successfully survived the voids of outer space.
However, their survival largely depends on “slow and controlled dehydration” which, if precipitated, could prove fatal to the body. This makes the ability to move an important factor in managing dehydration and rehydration, as well as finding mates, food, and predator escape.
Their slow gait, which earned them their name (Tardigrada means slow in Italian) from the 18th century, has been the subject of research for some time. A recent study has shown that their gait resembles that of insects much larger than the tardigrade and whose skeletal structures are remarkably different. In addition, there is also a considerable difference in the environment of arthropods and tardigrades.
Tardigrades navigate in a heterogeneous environment and their locomotion must do the trick. Different situations also require different speeds, so the capabilities of the locomotives must also be adapted to this. These environments are made up of aquatic (marine and freshwater) as well as land.
Our article on #tardigrade walk (with @duranigrade, Deborah Johnston, and @DJCohenEtAl ) is now available in @PNASNews : https://t.co/XDzBurTwTq
Follow this little guy, strolling on his way to an explanatory thread 🧵[1/n] pic.twitter.com/vVPaK8yKBu
– Jasmine Nirody (@jasnir_) August 30, 2021
In order to better understand this, the study examined the march of Hypsibius pattern, a species of tardigrades, on two different polyacrylamide gel substrates through a light microscope under different environments. It has been observed that the way a tardigrade coordinates the movements of its legs is very similar to that of insects like bedbugs (Carausius morosus). Neither the tardigrade nor the insects show distinct gaits, but instead show a continuous and smooth transition through different speeds. A good example of a distinct and unobtrusive gait for different speeds would be that of a horse: its “step” is distinct from a “trot”, which is again distinct from a “gallop”.
Common neural network
Most soft animals like worms lack legs and their movement style lacks regularity. One of the questions the study focuses on is why would a sweet little animal like the tardigrade need paws – and walk like insects – in the first place? After all, most small, skeletal organisms have a mode of locomotion other than walking. The authors put forward two theories for this. One is that the similarities between the tardigrade gait and that of arthropods come from a common neural network.
Soft-bodied worms (nematodes and annelids) and exoskeletal insects (arthropods) share the ventral spinal cord (VNC), the equivalent of the spinal cord in vertebrates.
While most worms (the study cites the example of the velvet worm) have a remarkably different CNV than arthropods, tardigrades, on the other hand, have a “surprisingly similar” CNV to that of insects.
In insects and tardigrades, the VNC is divided, with each division controlling a pair of limbs and the right half controlling the left and vice versa (“contralateral projection” in scientific jargon). Worms, on the other hand, have an unsegmented VNC.
Parallel evolution
The second theory is that the walking styles of tardigrades and arthropods (insects) are an example of convergent, but independent, evolution in which two species evolve to develop similar physical characteristics.
In other words, this implies that the common underlying neural circuits mentioned above might not have a common ancestor, and the similarity of walking patterns might have evolved in parallel with each other. It’s “intriguing,” the article says, because arthropods and tardigrades have seemingly diametrically opposed ecologies, skeletal structures, and size differences.
So does this imply that there may be more to the obvious crawling evolution? “The preference for tetrapod-like coordination in tardigrades and much larger species like the stick insect indicates [its] importance in… species that regularly navigate variable and three-dimensional terrain, ”the document states.
– The author is an independent scientific communicator. (mail[at]ritvikc[dot]with)
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