What is a Water Bear?
A water bear, known in the scientific community as a Tardigrade, is a unique organism within the kingdom Animalia. The name Tardigrade comes from the latin word Tardigradus, which means “slow moving”. They amble like a bear, hence their nickname. As far as their habitat and diet, these creatures have a wide variety of strategies. For the most part, they live in mostly aquatic environments but they can live anywhere and everywhere – even on your face! Most species are herbivorous and eat moss or lichen, though others are carnivorous and will eat other small organisms such as rotifers and nematodes. Some even are cannibalistic and will eat each other!
How do they…?
Tardigrades are quite unique in terms of how they move, get nutrition, defend themselves, and reproduce. Although they have an extra pair of legs, they move very similar to that of a bear. Unlike a bear however, water bears obtain their food through stylets. The styletes allow them to pierce their food while a pharyngeal bulb sucks out the internal contents of the food. Water bears have a complete digestive system, very similar to that of large mammals. To help sense the food and environment around them, they have long hairs sounding their body, functioning like the whiskers of a cat.
In terms of reproduction, tardigrades can reproduce both sexually and asexually. Female tardigrades lay eggs, up to 30 eggs at a time, typically inside of her newly molted skin. Then soon after, a male comes around and fertilizes them. These eggs however, do not require fertilization in order to develop because in some species of tardigrades, males don’t have reproductive organs. Once fertilized or layed, tardigrades are born fully developed for they exhibit no larval stages.
Even though tardigrades are extremely tiny and seem as though they would be quite vulnerable, they have very few predators in the wild. The only predators they have are nematodes and other tardigrades. Even with so few predators, tardigrades have special intrinsically disordered proteins (TDPs) which help them survive even in the harshest of conditions. The TDPs in water bears are proteins that lack a 3D structure which form a glass like structure in their cells. This “glass” helps protect them from dying. When their cells activate the TDPs, tardigrades place their bodies in a state of cryptobiosis, where they slow their metabolic rate down so low that it is undetectable. This also helps protect water bears and helps them survive in extreme environments. With all of these adaptations, tardigrades have been described as indestructible and it is extremely difficult to kill them.
Why are they Special?
As mentioned, Tardigrades have pretty extraordinary survival skills and are thought to be near impossible to kill. So why can they survive such extreme conditions, like starvation, radiation, and being in a complete vacuum? These tiny organisms are capable of entering cryptobiosis! Cryptobiosis is a type of dormancy where Tardigrades expel most of the water in their bodies and shrive up into a “tun” form. When in a cryptobiotic state, their metabolism is reduced to a point where it is undetectable and they are able to remain in this state for decades at a time. This ability helps them to survive even the harshest of environments and makes them extremely fascinating for scientists to study! In addition to starvation, radiation, and utter vacuum, Tardigrades can survive temperatures as high as 300 degrees Fahrenheit and as low negative 458 degrees Fahrenheit, six times more pressure than at the deepest point in the ocean, and dehydration.
Are they aliens?
Although tardigrades have survival techniques uncommon and not well understood in science, there is not enough evidence to conclude that they are extraterrestrial beings. However, some scientists believe in the Panspermia Hypothesis. Panspermia is the hypothesis that life exists throughout the entire Universe and is distributed via space dust, meteors, asteroids, comets, or other interstellar vehicles.
Some theorize that tardigrades traveled to Earth millions of years ago via meteorite. Traces of biotic material have been found on 4.1 billion year old rocks in Western Australia. Some think that they arrived here randomly and it has even been theorized that intelligent extraterrestrial beings deposited tardigrades on Earth to spawn life.
Though some aspects of Panspermia have been proven, such as that certain animals (tardigrades) can survive space travel, there is not enough evidence to conclude that tardigrades originated somewhere else in the Universe.
How can they survive extreme conditions?
Tardigrades are thought to be able to survive in such extremes primarily because of two special features. The first of these is their ability to repair their DNA., Tardigrades were found to have a unique DNA-associating protein called DSUP that suppresses the occurrence of DNA breaks due to radiation. In a 2017 study, scientists cultured human cells and engineered them to express the DSUP protein. These cells were then exposed to X-Ray radiation and it was found that the engineered cells had approximately 50% less DNA fragmentation compared to cells without the DSUP protein. Therefore, it is easy to see how the DSUP protein contributes to the Tardigrades’ elevated tolerance of radiation and DNA damage.
The other feature that aids Tardigrades in their extreme survival skills is Horizontal Gene Transfer (HGT). HGT is the transfer of genes between species and Tardigrades owe one-sixth of their genome to this phenomenon – nearly double the amount in the most extreme known cases. These acquired genes come primarily from bacteria, but also from plants and fungi, and they have impacted the Tardigrade genome, particularly the areas related to stress tolerance. Additionally, their ability to repair their DNA likely predisposes these organisms to incorporate foreign material into their genome.
Applications of Tardigrades in Research
Scientists, in their studies for life elsewhere, often lack one key thing for studying space: samples of life. Instead, they look at current “extremophile creatures” because they can survive in the extreme conditions found in space.
Tardigrades may be used in experiments to evaluate the survival in the atmospheres of other planets in our solar system, as well as continued studies of their radiation tolerance. There is also interest in the mechanism by which tardigrades survive extreme stress and radiation and whether that ability can be extrapolated to vertebrate organisms.
Research is also being done on tardigrade genomes to determine how they repair and protect their DNA. Understanding mechanisms of preserving genetic integrity in other organisms will help provide insights on molecular solutions that have evolved as a result of cellular “challenges”. Their antioxidative responses and cell protection are particularly of interest in cancer research as well. The cell protection and repairing systems used in tardigrades can also be a huge help in improving our treatment of traumatic injuries and heart attacks.
Boothby, T.C., Tenlen, J.R., et al. (2015). Evidence for Extensive Horizontal Gene Transfer from the Draft Genome of a Tardigrade. Proceedings of the National Academy of Sciences. 112 (52), 15976-15981; doi: 10.1073/pnas.1510461112
Bordenstein, S. (2017, June 7). Tardigrades. Retrieved from https://serc.carleton.edu/microbelife/topics/tardigrade/index.html
Hashimoto, T., & Kunieda, T. (2017). DNA Protection Protein, a Novel Mechanism of Radiation Tolerance: Lessons from Tardigrades. Life (Basel, Switzerland), 7(2), 26. https://doi.org/10.3390/life7020026
Jönsson, K. I. (2019). Radiation Tolerance in Tardigrades: Current Knowledge and Potential Applications in Medicine. Cancers, 11(9), 1333. doi: 10.3390/cancers11091333
Panspermia. (2020, January 27). Retrieved from https://en.wikipedia.org/wiki/Panspermia#Hoaxes
Pesheva, E. (2019, January 25). Could an extremophile hold the secret to treatment of devastating injuries? Retrieved from https://news.harvard.edu/gazette/story/2019/01/could-an-extremophile-hold-the-secret-to-treatment-of-devastating-injuries/
Pullen, L. (2008, October 16). ‘Alien’ Water Bears Amaze Scientists. Retrieved from https://www.space.com/5974-alien-water-bears-amaze-scientists.html
Tardigrades. (2018, February 2). Retrieved from https://www.americanscientist.org/article/tardigrades
Tardigrades use unique protein to protect themselves from desiccation. (2017, March 16). Retrieved from https://phys.org/news/2017-03-tardigrades-unique-protein-desiccation.html
Weronika, E., & Łukasz, K. (2017). Tardigrades in Space Research – Past and Future. Origins of life and evolution of the biosphere : the journal of the International Society for the Study of the Origin of Life, 47(4), 545–553. https://doi.org/10.1007/s11084-016-9522-1