Environmental Adaptations: Cryobiosis

Abstract

Tardigrades are well known to withstand very low temperatures in the anhydrobiotic state. However, they even tolerate such low temperatures like −196 °C in the fully hydrated state which is then described with the term cryobiosis. Although this extreme subzero temperature tolerance got quite a lot of attention, there is little knowledge regarding their physiological and biochemical adaptations connected to ecological representative subzero temperatures. General studies on cold tolerance have highlighted some strategies including freeze avoidance, rapid cold hardening and freeze tolerance. Although studies on survival rates, cooling rates and ice formation in tardigrades show high interspecific variations in subzero temperature survival, the water bears seem to tolerate ice formation within their bodies and therefore belong to freeze-tolerant organisms. Calorimetric studies also provide evidence for homogenous ice nucleation, indicating that ice formation is not largely affected by ice-nucleating agents. Ability to tolerate low temperatures and freezing even in embryonic developmental stages further increases the adaptive benefit of tardigrades to cope with low-temperature events.