Tardigrades

Tardigrades, often called water bears, are tiny animals that are usually less than a millimeter long. Under a microscope, they look like plump, eight-legged bears slowly climbing through moss. They are found almost everywhere on Earth, from mountaintops to the deep sea, but they are best known for their extraordinary ability to survive conditions that would kill most other living things.

When their surroundings dry out, many tardigrades enter what is called the tun state. [A] They pull in their legs, curl into a ball, and lose almost all of the water in their bodies. [B] Their metabolism slows to less than 0.01 percent of its normal rate. [C] In this condition, they can survive for years. In one study, tardigrades that had been frozen in Antarctic moss for thirty years were brought back to life. [D]

In the tun state, tardigrades have survived extreme cold and heat, high levels of radiation, and even outer space. In a 2007 experiment, dried tardigrades were carried into orbit on the outside of a spacecraft, and some of them recovered after they were returned to Earth and given water. Scientists are now studying the proteins that protect tardigrade cells during drying, because similar methods might one day help preserve medicines that normally require refrigeration.