Tardigrades: The World's Toughest Critters

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Extreme Survival

Water bears are such unique creatures that scientists have given them their very own phylum, Tardigrada. When threatened, their metabolism slows to 0.01% of its normal rate and they enter a death-like state called "cryptobiosis". In this near invincible state they resemble tiny barrels and are known as "tuns" (top left).

If the surrounding environment remains uncomfortable they can stay in tun form for over 100 years. Once it becomes more habitable (ideally nice and moist), they wake from their cryptobiosis and change from tuns back into little water bears. One scientist reportedly managed to revive a water bear which had been in cryptobiosis for 120 years, but sadly it died soon after.

Water bears transform into tuns by replacing nearly all the water in their bodies with a specialized sugar called trehelose which protects their cell membranes from damage. There are four different types of cryptobiosis that they can undergo in response to a threatening environment:

1. Osmobiosis
   Water bears undergo osmobiosis when there is a dramatic change in ion concentration in their surrounding environment. For example, scientists have found that they transform into tuns if they are dropped into very salty water.
2. Cryobiosis
   Cryobiosis enables water bears to survive freezing and thawing, allowing them to live in polar regions and in high mountains which often experience sudden changes in temperature. Water bears have survived being heated for a few minutes at 151°C, and being chilled for days at -200°C. Incredibly, they even managed to stay alive for a few minutes at near absolute zero, at a temperature of -272.8°C - which is colder than outer space. At this temperature there is no free molecular vibration, so no metabolism can exist; the water bears are "dead".
3. Anoxybiosis
   Anoxybiosis is the form of cryptobiosis that a water bear undergoes when there is a severe lack of oxygen. In the wild they might use anoxybiosis if they find themselves swimming in water with a low oxygen content, but scientists have also used it to keep them alive for 10 days in the vacuum of space!
4. Anhydrobiosis
   This is the most common form of the cryptobiotic state and occurs when a water bear can not get enough water for its normal bodily functions. It is not surprising that this is the most common reason water bears turn into tuns; when they are living in their local mossy patch they are much more likely to experience a drought then have to deal with strange ion concentrations, extreme temperatures or a lack of air.

Essentially cryptobiosis is the process of dying, then coming back to life. The implications of applying this biological magic trick to other organisms have begun to be explored. Scientists have discovered how to use cryptobiosis to preserve sperm, seeds, blood and food. Recently Japanese doctors used the water bear's sugar trehelose to keep a rat's heart in a fridge at 4°C for 10 days and then revive it; normally doctors can only keep a human heart alive for up to four hours before a transplant. The futuristic implications of human cryptobiosis may even include slowing the metabolism of passengers on long space journeys to achieve a Star Trek style "suspended animation".

Tardigrades can even survive in space. Read on to find out more....