Have you seen tardigrades walk? Scientists decode why they trot like insects

Tardigrades and arthropods are an example of convergent, but independent, evolution whereby two species evolve to develop similar physical characteristics.

Pictured here in a color-enhanced electron micrograph, a millimeter-long tardigrade crawls on moss. (Nicole Ottawa & Oliver Meckes / Eye of Science / Science Source Images/NASA)

Tardigrades, or water bears, are some of the smallest – and most resilient – creatures on the planet. Though the largest of these microscopic four-legged creatures are no bigger than 1.5 mm, they have been found to withstand extreme temperatures and pressures, air deprivation, radiation, starvation. Not only that, they have successfully survived the vacuums of outer space as well.

However, their survival depends extensively on a ‘slow and controlled dehydration’, which, if rushed through, could prove fatal for the organism. This makes the ability to move around an important factor in managing dehydration and rehydration as well as finding mates, food, and escaping predators.

Their slow gait, which earned them their name (Tardigrada means slow stepper in Italian) as long back as the eighteenth century, has been a matter of inquiry for quite some time. A recent study has found that their gait resembles that of insects that are much bigger than the tardigrade, and have skeletal structures that are remarkably different. Furthermore, there is also a considerable difference in the environment of arthropods and tardigrades.

Tardigrades navigate a heterogeneous environment and their locomotion needs to fit the bill. Different situations also call for different speeds, so locomotive capabilities need to be attuned to that as well. These environments consist of aquatic (both marine and freshwater) as well as terrestrial terrains.

In order to further understand this, the study had examined the walk of Hypsibius exemplaris, a species of tardigrades, on two different substrates made of polyacrylamide gel through a light microscope under different environments. It was observed that the way in which a tardigrade coordinates its leg movements is very similar to that of insects like stick bugs (Carausius morosus). Neither the tardigrade nor do insects show distinct gaits, but instead, show a smooth continuous transition across different speeds. A good example of a distinct, discrete gait for different speeds would be that of a horse: its ‘walk’ is distinct from a ‘trot’, which is again distinct from a ‘gallop’.

Common neural network

Most soft animals like worms are devoid of legs and their style of movement lacks regularity. One of the questions that the study engages with is why a soft, small animal like the tardigrade would need legs – and walk like insects – in the first place? After all, most small organisms lacking a skeleton have a manner of locomotion other than walking. The authors put forward two theories for that. One is that the similarities in the tardigrade gait and that of arthropods stem from a common neural network.

Soft-bodied worms (nematodes and annelids) and insects with an exoskeleton (arthropods), share the ventral nerve cord (VNC), the equivalent of the spinal cord in vertebrates.

While most worms (the study cites the example of the velvet worm) have the VNC remarkably different from that of arthropods, tardigrades, on the other hand, have a VNC ‘strikingly similar’ to that of insects.

In both insects and tardigrades, the VNC is divided, with each division controlling a limb on the same side i.e. right hemiganglia corresponding the right limb. Further, the right and left hemiganglia are connected to each other, “which are necessary to coordinate inter-leg stepping patterns,” says lead author of the paper, Jasmine Nirody in an email to 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 whereby two species evolve to develop similar physical characteristics.

In other words, this implies, the common underlying neural circuitry mentioned above might not have a common ancestor, and the similarity in walking patterns might have evolved parallel to each other. This is ‘intriguing,’ the paper states, as arthropods and tardigrades have seemingly diametrically opposite ecologies, skeletal structures, and size differences.

So, does this imply that there may be more to the evolution of four-legged walking that meets the eye? “The preference for tetrapod-like coordination in both tardigrades and far larger species like the stick insect point to [its] importance in… species that regularly navigate variable, three-dimensional terrain,” the paper states.

– The author is a freelance science communicator. (mail[at]ritvikc[dot]com)

In an earlier version of this article it was incorrectly mentioned that in insects and tardigrades, the right half of the VNC controls the left limb and vice versa. The error is regretted.

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