How Tardigrades Survive Almost Everything

Johann August Ephraim Goeze, a German pastor with a microscope, found the first tardigrade in 1773 and called it kleiner Wasserbär — "little water bear." The name stuck. Adults are about half a millimeter long, walk on eight stubby legs, and live everywhere there is a film of water: moss, lichens, deep-sea sediment, the underside of a leaf in your yard.

What makes them famous is what happens when the water leaves. A tardigrade in a drying puddle pulls in its legs, contracts into a barrel-shaped "tun," and replaces the water in its cells with a sugar called trehalose and a family of intrinsically disordered proteins that researchers at the University of Tokyo named CAHS. The proteins gel into a glass-like matrix that holds membranes and DNA in place. Metabolism stops. The animal is no longer doing anything that biology would call living.

In that state it survives temperatures from a few degrees above absolute zero to about 150 °C, pressures six times what you find at the bottom of the Mariana Trench, and ionizing radiation a thousand times the lethal dose for a human. In 2007, a European Space Agency experiment exposed live tardigrades to the vacuum and unfiltered solar UV of low Earth orbit for ten days. Most of the ones shielded from UV came home and reproduced.

Tun-state survival is not immortality, though. Ordinary, hydrated tardigrades die from the same things everything else does — starvation, predators, a hot day. The trick is only that they get to skip the parts of life where conditions are bad.