Tardigrades: The Animals That Defy Nature

Picture an animal that can live anywhere: hot springs to solid ice, mountaintops to the deepest sea levels, spanning a temperature range of -458 °F to 302 °F. Imagine that this animal can survive in outer space, live through global mass extinctions, and persist for 30 years without food or water. Sounds like science fiction? Well, these animals are real, and they’re known as tardigrades.

Tardigrades are micro-animals, meaning they are so small that they can only be seen with the help of a microscope. These organisms were first discovered in 1773 by Johann Goeze, who named them “little water bears.” Three years later, Lazzaro Spallanzani named them “Tardigrada” (slow stepping)  because of their slow gait. 

When viewed under a microscope, they look like bears lumbering around in the water. They sport four pairs of claws; some species’ claws even resemble bear claws. Interestingly, their hind-most legs are attached backwards, as seen in Figure 1. These ones don’t help with walking; they are used for grasping surfaces.

Figure 1: An adult tardigrade can grow up to 2.1 mm in length. They are also known as moss piglets and can be easily found by soaking a piece of moss in water.

Tracing the lineage of these animals has proved to be quite a challenge. Due to their small size, it is difficult to find tardigrade fossils. Additionally, the few fossils that have been found reveal an animal that differs from living tardigrades in several ways. The current consensus seems to be that these creatures are closely related to arthropods, a phylum of animals that includes insects, spiders, shrimp, and crabs.

Despite their reputation, not all tardigrades are resilient. The aquatic ones do not have many defense mechanisms because they live in relatively stable environments. On the other hand, terrestrial tardigrades encounter fluctuating temperatures and moisture levels, which have caused them to adopt a more hardy disposition.

In fact, tardigrades have been known to pioneer new locations because they can withstand environments most other organisms can’t. How do these animals adapt to such a wide range of conditions?. 

First and foremost, tardigrades are endurance champions. They survive extreme conditions by suspending their metabolic activities, bringing them to a state known as cryptobiosis. When hospitable conditions return, they restart their life processes. Intriguingly, they can recover even after being in a deep freeze for 30 years! They’re so durable that they are one of the few groups that have survived all five of the Earth’s mass extinctions. That includes the extinction of dinosaurs and several marine animals. Not bad for a tiny bug.

Tardigrades can reach their cryptobiotic state because of their unique biology. Their bodies contain high levels of a sugar that forms a glassy armor on their surface and unique proteins that shield their DNA from radiation. These proteins are known as Dsup, short for damage suppressor, and with their protection, tardigrades can withstand 1000 times more radiation than other animals. They can also survive losing up to 97% of their water content as they shrivel up to form a “tun.” The lost water is replaced with trehalose or other biomolecules that act as hydration buffers and prevent the cell from drying out.

Figure 2: An active, hydrated tardigrade (left) and a tun (right). Tardigrades can enter into this state in response to drying out, freezing, getting exposed to high levels of environmental toxins or low levels of oxygen.

Due to their resilient characteristics, tardigrades are perfect model systems for space research. Astrobiologists are interested in studying the endurance limits of life forms in their investigation of the possibility of colonizing other planets. Since it is too dangerous to test the effects of long-term space travel on humans, these organisms serve as a perfect template for such experiments.

In 2007, scientists conducted three projects to test the survival capabilities of tardigrades in space. They looked at three distinct conditions: on board the spacecraft, in open space, and in other extreme stress conditions, including exposure to radiation and microgravity. They found that these animals could survive in the vacuum of space and were only moderately affected by solar radiation, indicating that they could potentially survive long distance space travel.  

In 2011, scientists sent tardigrades to the International Space Station to study whether the stresses of living in space would damage their DNA. In this case, the cosmic radiation and the microgravity conditions on board did not affect them, further supporting that these animals can be used for interplanetary flights. 

Scientists are excited about the potential of tardigrade research in space. For instance, determining the limits to tardigrades’ resilience in space could give us a better idea of how far they can travel before they start dying. This, in turn, helps us understand the potential obstacles humans will face if they ever embark on such long journeys.  Furthermore, the resiliency of tardigrades makes it possible to use them to potentially colonize planets. With tardigrades as a base, it would be possible to add more life on top that could feed on the animals and create a habitat. Lastly, it tells us that planets such as Mars, which we long thought could not support life, might contain living organisms, after all, giving us hope that there could be more life out there to find.