Allison Cochran
The tardigrades have been given some endearing nicknames, water bear and moss piglet being two of them. Could “mini-astronaut” be joining the list? Tardigrada is a phylum within Ecdysozoa, presently sister to the clade containing velvet worms and Arthropoda. The animals themselves are incredibly small, but visible to the naked eye. The details of their features can be revealed under a microscope. They have a thin, chitinous exoskeleton which is occasionally molted. Their body is elongated with four pairs of unjointed lobopods with claws at their ends (Figure 1). They have a pair of specialized feeding structures called “stylets,” which they can use to pierce plant cell walls. These stylets have been adapted for their herbivorous lifestyle in which they feed on moss, hence the nickname “moss piglet”. Of the invertebrates, they have comparatively more developed structures such as a complete gut and “well developed nervous system” (Weronika et al. 2016).
Their ecological range can be described as “ubiquitous in every biome on Earth” (Nelson et al. 2019). Since they have such a small exoskeleton, they “must be surrounded by a film of water to be active” (Nelson et al. 2019). They can be found in marine, freshwater, and terrestrial habitats (Nelson et al. 2019). They can also have many different kinds of adaptations to thriving in a marine habitat. Additionally, tardigrades inhabit every corner of moving and standing water in freshwater habitats (Nelson et al. 2019). In terrestrial habitats, tardigrades can be found on plants that rely on diffusion such as mosses and lichens (Nelson et al. 2019). It has been difficult to accurately describe the range and role of tardigrades due to the lack of thorough study in their ecosystems (Nelson et al. 2019). The study of tardigrades has also evolved as a result of integrating molecular information into taxonomic considerations (Nelson et al. 2019). Their worth of study can be associated with their ubiquity, potential survival in space travel, and possible gateway to astrobiological avenues.
One interesting mechanism they use to overcome stressful environments is called “cryptobiosis.” The tardigrade can enter a “tun” state in which they dehydrate themselves to <1% of their normal water content and reduce their metabolic rate to 0.01% of its regular rate (Figure 2). When they are in their tun state, they can survive extreme conditions such as extreme temperatures, and can survive “from nine to 20 years in natural conditions” (Weronika et al. 2016). Cryptobiosis is used regularly by tardigrades as a mechanism to survive dry seasons. It is now being tested as a tool for surviving extraterrestrial atmospheric conditions. In an article titled, “Tardigrades in Space Research – Past and Future,” the authors write, “To survive exposure to space conditions, organisms should have certain characteristics, including a high tolerance for freezing, radiation, and desiccation” (Weronika et al. 2016). Tardigrades can be described as the “perfect model organism for space research” due to their ability to withstand stress and acclimate to different climates (Weronika et al. 2016). The paper also describes the way in which tardigrades’ source of food are equally resistant to harsh conditions, meaning the odds of life surviving in an extraterrestrial environment is possible (Weronika et al. 2016). The allowance of “travelling long cosmic distances” and withstanding long periods of stress through cryptobiosis could show researchers how life might find a way on other planets or moons (Weronika et al. 2016).
References:
Schill, R. O., Guil, N., Bartels, P. J., & Nelson, D. R. (2018). Tardigrade Ecology. In Water bears: The biology of tardigrades (Vol. 2, pp. 163–210). essay, Springer Nature Switzerland.
Weronika, E., & Łukasz, K. (2016). Tardigrades in space research – past and future. Origins of Life and Evolution of Biospheres, 47(4), 545–553. https://doi.org/10.1007/s11084-016-9522-1
Møbjerg, N., & Neves, R. C. (2021). New insights into survival strategies of Tardigrades. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 254. https://doi.org/10.1016/j.cbpa.2020.110890
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