When a waterbear is in an unfavorable environment it can go into cryptobiosis. This is basically dying and then when conditions return to normal they come back to life. To do this they expel all the water from their bodies, pull in all their appendages, and bring their metabolism down to undetectable levels. This protective form is called a tun and it allows them to survive absolute zero, 500 degrees kelvin, in a vacuum much like space, and a nuclear blast. This is why they are often referred to as the most extreme survivor.
~Ella
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