The tardigrade, also known as a waterbear, is a microscopic invertebrate found all over the world in ecosystems ranging from freshwater to terrestrial. Tardigrades often inhabit places that experience extreme conditions–such as deserts, high mountains, and polar regions–where many other life forms find it impossible to survive. Terrestrial waterbears are typically active only when surrounded by a small film of water. So how is it that this tiny creature can survive in extreme conditions, even in places that lack a steady supply of water?
Under stressed conditions such as extreme dryness or temperature, the waterbear practices several forms of cryptobiosis, a state in which metabolic activity is slowed or halted. The most studied of these is anhydrobiosis. The waterbear enters anhydrobiosis by contracting its body into something called a tun, whereby it loses more than 95% of its free and stored water; essentially, it dehydrates itself. In this state, the waterbear creates different proteins and sugars that help protect its cells. Once these cell protectants are synthesized, the waterbear reduces, and at times suspends, its metabolism. When conditions improve within the environment, the waterbear activates its metabolism once again, aided by hydration from water intake.
This summary was contributed by Ashley Meyers.
On the left, the tardigrade is starting to dehydrate with a sugar solution, trehalose, thickening as water is lost. This protects the tardigrade’s cells from damage until moisture conditions improve. Artist: Emily Harrington. Copyright: All rights reserved. See gallery for details.
Read more about tardigrades’ cryptobiosis (and the innovations it inspires) in Adelheid Fischer’s “The Science of Seeing Small Worlds in a Big Space” on page 36 of Zygote Quarterly:
“Most incredible of all, however, is the virtually indestructible nature of tardigrades while they remain in cryptobiosis. In laboratory experiments, cryptobiotic specimens have been chilled in liquid helium to -457°F (-272°C), which is only marginally above absolute zero. They have also been heated to temperatures exceeding 300°F (149°C), exposed to radiation doses far in excess of the lethal dose for humans, immersed in vats of liquid nitrogen, concentrated carbolic acid, hydrogen sulphide, brine, and pure alcohol, and even bombarded by deadly streams of electrons inside an electron microscope. Yet when removed from all of these incredibly hostile environments – which would have proven fatal for any other form of animal life – and moistened with water, these astounding creatures recovered.
They simply emerge from their cryptobiotic state, rehydrate themselves, and amble away on their four pairs of stubby claw-tipped legs, completely unharmed. Even today, the physiological mysteries behind the tardigrades’ unparalleled powers of endurance during cryptobiosis remain unsolved.” (Shuker 2001:113)
“…Tardigrades are known to enter cryptobiosis at any stage of their life cycle, from egg to adult. Cryptobiosis has to be considered a form of quiescence, being directly induced and maintained by the occurrence of adverse conditions for an active life, and promptly broken once the adverse conditions are removed…In tardigrades, there are several forms of cryptobiosis: anhydrobiosis, cryobiosis, anoxybiosis and osmobiosis. Anhydrobiosis is the most studied. Entering anhydrobiosis, tardigrades contract their body into a so-called tun, loosing most of their free and bound water (>95%), synthesizing cell protectants (e.g., trehalose, glycerol, heat shock proteins…and strongly reducing or suspending their metabolism…” (Bertolani et al. 2004:16)