by Laura Van Remortel
BIOL/WATER 361, Fall 2013
There is much controversy on the capabilities of the phylum Tardigrada. There is some research that claims that Tardigrada can withstand extreme temperatures for extended periods of time, that they can survive in outer space and return to Earth, that they can withstand radiation that would kill a human, and that they have the capability of living for over a hundred years. What is really occurring is researchers are looking for the oddities among the water bears, and in most cases exaggerating the claims made by the original authors. None of these traits have been proven in more than a handful of Tardigrada species, if at all.
Tardigrades are known for their ability to enter into a latent stage where the organism goes completely dormant. This process is known as cryptobiosis, and many researchers over the years have tried to explain how this process works and how it affects tardigrades. Up to 95% of their metabolism can shut down during cryptobiosis, allowing growth and reproduction to slow or stop completely (Nelson & Marley 2000). There are many different environmental signals that can cause a tardigrade to go into a latent state, such as extreme temperatures, lack of a water source, and an increase in radiation. Some of these stages can last for years and according to some research, for more than a century.
So far, five different types of cryptobiosis have been found in the phylum Tardigrada: encystment, anoxybiosis, cryobiosis, osmobiosis, and anhydrobiosis (Nelson & Marley 2000). The process of encystment causes the tardigrade to form a cyst around its body in order to store food resources and water. This allows the organism to feed and stay alive in harsh conditions. Cysts have been known to withstand various types of environmental stressors but extreme temperature is not one of them due to the high content of water in the cyst. Some cysts can last well over a year (Nelson & Marley 2000). Anoxybiosis is one of the shortest lived cryptobiotic states. The organism can only last for a few hours to a few days at a time. This process occurs when there is a depletion of oxygen in the water the organism is living in (Nelson & Marley 2000). This state can occur in extreme temperatures because it allows the organism to go into a latent stage when temperatures drop or increase suddenly, such as in the Antarctic. Osmobiosis occurs when there is an increase in osmotic pressure. If salinity levels rise in freshwater environments, tardigrades have the ability to form a tun which makes the cuticle less porous to allow for a lower rate of transpiration which allows for a cryptobiotic state to form (Jonsson & Bertolani 2001). Anhydrobiosis is one of the more controversial and most prominent types of cryptobiosis. When there is a complete loss of water the tardigrade is known to enter a state of anhydrobiosis. The controversy surrounds the idea of how much desiccation an organism can take and what level is it considered to be in anhydrobiosis.
The entire phylum of Tardigrada consists of microscopic organisms that range in size of 0.25-0.50mm, and all of the organisms contain four pairs of legs with a claw at the ends. The common name of this phylum is the water bears. There are three classes of Tardigrada. The first is the Eutardigrada which is made up of mostly fresh water species with some limno-terrestrial species. The terrestrial species are more prominently used as a laboratory specimen due to easy access and their ability to perform cryptobiosis. The strictly freshwater species may lack the ability of anhydrobiosis, but there is not enough research to prove this (Nelson & Marley 2000). The Heterotardigrada are on the opposite side of the spectrum as most of this group is marine. A third class, Mesotarigrada, has been proposed, but due to an earthquake the habitat of the only species was destroyed before anyone could confirm it. To separate the two major classes there are four distinct features to analyze; the shape of the claws, the cuticle, the buccal-pharyngeal apparatus, and the eggs (Nelson & Marley 2000).
In 1948, a paper was published by Tina Franceschi called, “Anabiosi nei Tardigradi” that stated she had revived a eutardigrade that was in a desiccated state for 120 years. The Tardigrada samples that she had received belonged to the Botanical Institute of Turin. The specimens had been in an enclosed area in a state of complete dryness since 1828. She noted that there were tremors in different parts of the body, mainly in the front pair of legs. This statement attracted many scientists, and many were astounded by her findings. Just from that statement alone some people have made the assumption that all tardigrades can live to over a hundred years as long as they are in a cryptobiotic state. This is an unrealistic idea and has never been replicated. In fact, some scientific literature exaggerates this claim (Jonsson & Bertolani 2001). For example, Copley (1999) compared tardigrades to superheroes and claimed that Tardigrada were the toughest animals on Earth which may lead one to believe it is an over exaggerated claim.
A meta-analysis done by Jonsson & Bertolani (2001) showed that the paper written in 1948 is the only account ever recorded about a tardigrade living more than a century, once again pointing out that there were only tremors to the feet and the organism did not fully revive. Instead, they found two other studies that found the average tardigrade lived 7-8 years (Jonsson & Bertolani 2001). One of which was a study done in Antarctica, that had three different species that were already cold tolerant and were kept in a lab type setting. Only one of the species lived more than 8 years in an anhydrobioic state (Somme & Meier 1995).
Persson et al. (2011) conducted a study on extreme stress tolerance. He and his research team sent tardigrades and rotifers into low earth orbit to see how galactic cosmic radiation would affect the water bears in a microgravity environment. Three species of Tardigrada were used, and when compared to previous studies their results of survival were very low, many of the organisms were not even found after returning to Earth (Persson et al. 2011). This showed that only a few of the tardigrades were capable of withstanding the radiation, and they were not part of the majority &em; proves that not all of Tardigrada can withstand vast amounts of radiation including the time spent in cryptobiosis. The amount of radiation that was used was variable but much more than a human could withstand. However, the research team also took embryos as part of the experiment and exposed them to the same conditions. All of the eggs from the eutardigrade Milnesium tardigradum were able to hatch (Persson et al. 2011). This shows that the embryonic state may be much more resistant than the adult life stage for this species and adults struggled to survive the radiation.
Persson et al. (2011) also did a laboratory experiment involving extreme cold temperatures. They took a group of specimens and froze them at -196 degrees Celsius. However, the animals that survived were only exposed to the extreme cold for 120 minutes and only lived for 142 days afterwards. Also, no animal survived being frozen for more than 20 hours (Persson et al. 2011). This shows that tardigrades do have limitations, and they once again do not live as long as people have claimed and cannot endure extreme temperatures for long derations.
Another argument that has arisen is whether or not to consider tardigrades are “alive” during a cryptobiotic state. Since their metabolism is completely shut down and there are no signs of life, the animal, by definition is not considered alive. Hengherr et al. (2008) did an experiment to test the longevity of Tardigrada. They separated groups of M. tardigradum, one that stayed hydrated and one that was forced into an anhydrobiotic state every seven days to see who would live longer. Without counting the time of anhydrobiosis, both of the groups lived an average of 81.2 days. With the time taken into account the experimental group lived on average 133.2 days (Hengherr et al. 2008). When tardigrades are forced into an anhydrobiotic state and then rehydrated multiple times their life span seems to be shorter.
More data still needs to be found on exactly how cryptobiosis functions and the absolute extremes that tardigrades can withstand. If the exaggerations of the tardigrades are removed people can learn accurate details and still be amazed by their capabilities. Researchers need to take a step back and take the time to reanalyze the original published data of the Tardigrada. Only then, will research be able to go forward in discovering more about this amazing group of microorganisms.
Even though Tardigrada can live in many different extreme habitats many of the claims made by researchers have been exaggerated. There have been many studies that produce results of an average of less than ten years for the life span of a tardigrade with few leading to more extreme results. Not all species of Tardigrada are capable of staying in a dormancy stage that lasts for years. Depending on the type of cryptobiosis the organism may not even live for more than a few hours. As far as exposure to radiation, the results are very inconclusive as to what species can survive since only the eggs of one species were able to hatch after being exposed. Tardigrades have been known to live in many different climates from extreme heat to extreme cold and their dormancy stage is one of the toughest in the animal kingdom. However, depending on how long the process of anhydrobiosis takes and how many times a tardigrade goes through anhydrobiosis affects the life span of the organism. Time spent in a state of anhydrobiosis does not count toward the actual life span of Tardigrada as it is preforming no function of a living organism. Overall, many studies are incorrect at what tardigrades are capable of doing, and many more studies need to be conducted before any of these claims can be proven.
- Copley, J., 1999. Indestructible. New Scientist 164: 45-46.
- Franceschi, T., 1948. Anabiosis in tardigrades. Istituto Di Zoologia 22: 138.
- Hengherr, S., Brummer, F. & Schill, R.O. 2008. Anhydrobiosis in tardigrades and its effects on longevity traits. Journal of Zoology 275: 216-220.
- Jonsson, K. I. & Bertolani, R. 2000. Facts and fiction about long-term survival in tardigrades. Journal of Zoology London 255: 121-123.
- Nelson, D. & Marley, N. J. 2000. The biology and ecology of the lotic Tardigrades. Freshwater Biology 44: 93-108.
- Persson, D., Halberg, K. A., Jorgensen, A., Ricci, C., Mobjerg, N., & Kristensen, R. M. 2011. Extreme stress tolerance in tardigrades: surviving space conditions in low earth orbit. Journal of Zoological Systematics Evolutionary Research 49: 90-97.