Tardigrades (sometimes called "water bears") are small (0.1 - 1.5 mm), often microscopic arthropod-like organisms that superficially resemble a cross between a bear and a bug. Tardigrades can be found in large numbers on lichens and mosses, though they are found everywhere. A small clump of moss may have a few thousand tardigrades. Scientists often find tardigrades by soaking a piece of moss in fresh water.
Tardigrades are most famous for their extreme hardiness. They are found on practically every square meter of the Earth's surface, from the sea floor, where the ambient pressure is as much as 1000 times the surface, to the top of the Himalayan mountains, where there is a third less oxygen than at sea level. Tardigrades have been found beneath layers of ice meters thick and in hot springs with a temperature above the boiling point of water.
Tardigrades are among the few animals, including rotifers, nematodes, and brine shrimp, which are capable of going into a state of suspended animation known as cryptobiosis, where metabolism lowers to less than 0.01% of normal, and water content can drop to 1% of normal. It is prompted when environmental conditions become too harsh for normal survival. In this state, the tardigrade ceases all movement, and can essentially remain this way indefinitely until environmental conditions improve. The cryptobiosis is made possible by a non-reducing sugar called trehalose, which protects their membranes until revival.
Tardigrades in the cryptobiotic state are capable of surviving extremes of pressure, temperature, radiation, and dessication. Scientists have shown tardigrades can survive the vacuum of space, or pressure of 6,000 atmospheres, six times greater than the pressure at the bottom of the Marianas Trench, the lowest point in the sea. Tardigrades can be heated to 151 °C (304 °F) for a few minutes, chilled for days at -200 °C (-329 °F), even survive a cooldown to a single degree above absolute zero for a few minutes.
Tardigrades can withstand 5,700 grays of radiation, about 500 times more than is fatal for a human. The ability to withstand such radiation is especially unusual, as there are no natural areas on Earth with such high radiation levels, so evolution has had no particular reason to select for animals with such incredible tolerance to it.
Tardigrades continue to be a popular research subject for scientists studying microscopic animals.