At first glance, the microscopic creatures known as tardigrades don't appear that hardy. Also known as water bears and moss piglets, tardigrades sport blobby physiques, lumbering gaits and perfectly circular mouths stuck in the middle of their otherwise-featureless faces. Sort of like an alien teddy bear.
Yet scientists have long marveled at the resilience of these microscopic animals. They can famously withstand extreme heat, extreme cold, extreme high pressure, extreme low pressure, dehydration, starvation, radiation and air deprivation. They can even survive and reproduce in the vacuum of space.
How is this possible? For clues, scientists behind a recent study in the journal Communications Biology analyzed four tardigrades that have been preserved in amber — the only known fossilized tardigrades in the entire world. In the process, they helped unlock some of the mysteries behind the cuddly critters' legendary toughness.
Using a type of laser microscopy usually employed for studying cell biology, the researchers were able to examine extinct tardigrade species like Beorn leggi (named for a "Lord of the Rings" character) in surprising detail.
"Before our publication, we only know how two of these fossils are related to living tardigrades," the study's corresponding author, Dr. Marc A. Mapalo from Harvard University's Department of Organismic and Evolutionary Biology, told Salon. "In our paper, we redescribed the other two fossils and we are finally able to determine how they are related to other tardigrades. Because of this, we are able to determine what changes could have occurred between the fossils and their close relatives."
Additionally, scientists were able to estimate when specific groups of tardigrades evolved, importantly among them their living descendants that can perform cryptobiosis, or entering a state of extreme physical inactivity where their metabolism is almost at a standstill. Water bears usually enter cryptobiosis in response to facing what other organisms might consider a harsh environment.
"Currently, the science indicates that water bears have several modes of cryptobiosis, including responses to lack of water, high salinity or osmotic pressure, the presence of reactive chemicals, freezing temperatures, among others," Dr. Derrick R. J. Kolling, a chemist at Marshall University who was not involved in the study, told Salon. He added that water bears can retract their limbs and shorten their bodies to enter a state called "tun" when they are particularly stressed out. Scientists have long been fascinated at the diverse ways tardigrades adapt to adverse conditions — yet until the study came out, they scratched their heads at figuring out how they evolved these abilities.
"Our study highlights the importance of resolving the taxonomic relationships of these crown-group fossils," the authors write in the study's conclusion. "Finding more tardigrade fossils will enable the reconstruction of more accurate timelines that will open the clade for comparative analyses. By doing so, we will be able to understand the evolution of tardigrade characters, such as inferring when their cryptobiotic ability evolved and estimating their molecular and morphological rates of change over time."
Dr. Diane R. Nelson, a professor emerita of biology at East Tennessee State University who also was not involved in the study, said this research is "significant" because it provides this useful context in understanding water bear origins despite the intrinsic logistical difficulties involved in acquiring relevant raw data.
"It describes a new genus and species, but places them in the phylogeny of tardigrades," Nelson told Salon. "With only [four] fossil tardigrades identified in amber, because tardigrades do not fossilize well and so far have only been found preserved in amber, our knowledge of tardigrade evolution has been difficult to discern."
Not everything about tardigrade fortitude is a complete mystery, however.
"We know that tardigrades have multiple intrinsic proteins for cryptobiosis that allow the tardigrades to dehydrate or freeze without damage to the cell membranes and enhance their survival from extreme conditions that kill most living organisms," Nelson said. "They also can repair damage to their DNA caused by radiation and other extreme conditionals or chemicals."
While these skills may seem like superpowers, doctors hope that some day they can be used to develop medical treatments.
"If we can learn all the 'secrets' of cryptobiosis (how to prevent cellular damage), we would be able to preserve human organs (hearts, kidneys, etc.) so they can be stored successfully and then later transplanted into another human when needed," Nelson explained. Learning from water bears could also help humans with "knowledge of preserving/restoring cell membranes" and thereby "help prolong the shelf-life of medications such as antibiotics so they could save human lives."
Tardigrade science may even help us travel beyond Earth, given space is an extremely dangerous place for the human body. Some scientists hope that humans may be able to travel to far off worlds because of the evolutionary strategies of our tiny neighbors.
"Although this application would be in the distant future, protecting human DNA and proteins from the inhospitable conditions — notably, cosmic radiation — of space would make travel away from Earth more feasible," Kolling said. "Findings concerning deleterious (or beneficial) effects of reaction-oxygen species on mitochondria or cellular signaling via reversible cysteine oxidation both provide information relative to the study of myriad human processes and afflictions."
At the same time, Kolling urges tardigrade fans to remain grounded in their excitement about what humanity can learn from them. After all, the fundamental trick to cryptobiosis is a complete state of inactivity, which jokes about laziness aside is not physically possible for humans.
"Tardigrades are fascinating animals and while they are famous for their robustness, it’s prudent to remember that they are extremotolerant, not extremophilic," Kolling said. "They can survive transient periods of extreme stress, but during these exposures, they enter a suspended state in which metabolism is throttled down or perhaps turned off, so they do not eat, reproduce, etc., which are essential for survival."
While this may not be the most satisfying conclusion when studying tardigrades, the silver lining is that there is still much more for people to discover about these micro animals.
"As mentioned, there are only very few fossil tardigrades known so far," Mapalo said. "I am sure that there are many more out there waiting to be discovered. The highest chance is that we can find them in amber fossils. This is why, when I get the chance, I always tell people who work with amber to also check for tardigrades in their sample."
