Water-bears

Tardigrades (tardus "slow" + gradi to "step"), also well known as "water bears", are microscopic small metazoans. Most of the over 900 known tardigrade species are cosmopolitan species. The first tardigrades were discovered in 1773 after microscopes were invented. Around 800 limno-terrestrial species live in moist environments that facilitate the animal's gaseous exchange and avoid desiccation. However, such habitats frequently undergo seasonal changes that impact animals, but they are able to survive these periods of adverse conditions due to the ability to enter a cryptobiotic state.

Surviving without water

In adverse environments, terrestrial and freshwater tardigrades form the "Tönnchenform" or tun state. Gradient-dependent changes in integumental permeability retard water losses during preliminary desiccation and adaptive cellular processes reduce the metabolic activity of the cells. In this state, they are capable of surviving for very long periods. Although it has been assumed that tardigrades have a very long life span, little information is available about their exact longevity. In the cryptobiotic stage, tardigrades show extraordinary tolerance to physical extremes including high energy radiation, immersion in organic solvents, brief exposure to high temperatures and prolonged exposure to indefinitely low temperatures. When environmental conditions are adequate, tuns rehydrate and the animals resume metabolic activity.

Cryptobiosis

In 1702, the pioneering dutch microscopist Van Leeuwenhoek was the first to describe the phenomenon of anhydrobiosis in a species of bdelloid rotifer. 250 years later, in 1959, David Keilin defined the term cryptobiosis as "...the state of an organism when it shows no visible signs of life and when its metabolic activity becomes hardly measurable, or comes reversibly to a standstill." A cryptobiotic state could encompass processes induced by desiccation (anhydrobiosis), low temperature (cryobiosis), lack of oxygen (anoxybiosis) or combinations of these. This capability has been dramatically achieved by tardigrades. However, the mechanisms are poorly understood.