Desiccation in general leads to severe damage of cellular structures, which commonly results in the death of cells and the organism. However, a number of so-called anhydrobiotic organisms have developed remarkable mechanisms, allowing them to minimize or avoid such damage and survive extreme dehydration in a cryptobiotic state [1][2][3][4][5][6]. Several species of invertebrate taxa have this ability, including the embryonic cysts of crustaceans, rotifers, insect larvae, nematodes and tardigrades [2,3,[7][8][9][10][11][12]. Additionally, many procaryotes, such as bacteria and cyanobacteria [13,14], and even plant seeds [15][16][17][18][19] and adult plants, for example the resurrection lycopode Selaginella lepidophylla [5,20], demonstrate dehydration tolerance. Although Antonin van Leuwenhoek described anhydrobiosis over 300 years ago [21], the underlying mechanisms are still not fully understood. However, over the last three decades, researchers have come to recognize the important role of polyhydroxy compounds such as the non-reducing disaccharide trehalose [22][23][24]. This sugar is found in high concentrations in a wide variety of anhydrobiotic organisms, including nematodes, embryonic cysts of crustaceans, and yeast. Trehalose concentrations as high as 13-18% of the dry weights have been reported for anhydrobiotic cysts of the crustacean Artemia franciscana [25][26][27] whereas the nematode Aphelenchus avenae can accumulate 10-15% of its dry weight as trehalose during anhydrobiosis [8,9]. Studies on the anhydrobiotic insect larvae Polypedilum vanderplanki report up to 18% trehalose in the dry body mass [11]. Significantly increased trehalose levels also have been found in the Arctic collembolan Onychiurus arcticus during partial desiccation, induced by sub-zero temperatures [28]. The disaccharide sucrose fulfils a similar role in plants and accumulates in desiccation-tolerant plant seeds and resurrection To withstand desiccation, many invertebrates such as rotifers, nematodes and tardigrades enter a state known as anhydrobiosis, which is thought to require accumulation of compatible osmolytes, such as the non-reducing disaccharide trehalose to protect against dehydration damage. The trehalose levels of eight tardigrade species comprising Heterotardigrada and Eutardigrada were observed in five different states of hydration and dehydration. Although many species accumulate trehalose during dehydration, the data revealed significant differences between the species. Although trehalose accumulation was found in species of the order Parachela (Eutardigrada), it was not possible to detect any trehalose in the species Milnesium tardigradum and no change in the trehalose level has been observed in any species of Heterotardigrada so far investigated. These results expand our current understanding of anhydrobiosis in tardigrades and, for the first time, demonstrate the accumulation of trehalose in developing tardigrade embryos, which have been shown to have a high level of desiccation tolerance.Abbreviations HPAEC, h...

Survival in microhabitats that experience extreme fluctuations in water availability and temperature requires special adaptations. To withstand such environmental conditions, tardigrades, as well as some nematodes and rotifers, enter a completely desiccated state known as anhydrobiosis. We examined the effects of high temperatures on fully desiccated (anhydrobiotic) tardigrades. Nine species from the classes Heterotardigrada and Eutardigrada were exposed to temperatures of up to 110 degrees C for 1 h. Exposure to temperatures of up to 80 degrees C resulted in a moderate decrease in survival. Exposure to temperatures above this resulted in a sharp decrease in survival, with no animals of the families Macrobiotidae and Echiniscidae surviving 100 degrees C. However, Milnesium tardigradum (Milnesidae) showed survival of >90% after exposure to 100 degrees C; temperatures above this resulted in a steep decrease in survival. Vitrification is assumed to play a major role in the survival of anhydrobiotic organisms during exposure to extreme temperatures, and consequently, the glass-transition temperature (T(g)) is critical to high-temperature tolerance. In this study, we provide the first evidence of the presence of a glass transition during heating in an anhydrobiotic tardigrade through the use of differential scanning calorimetry.

Living in harsh and variable environments that are prone to periodic desiccation, tardigrades exhibit remarkable tolerance against physical extremes through a state known as anhydrobiosis. To study the effect of this state on the longevity and hence the lifecycle in the taxon Tardigrada for the first time, we exposed a tardigrade species, Milnesium tardigradum, to alternating periods of drying and active feeding periods in a hydrated state. Compared with a hydrated control, the periodically dried animals showed a similar longevity, indicating that the time spent in anhydrobiosis was ignored by the internal clock. Thus, desiccation can produce a time shift in the age of tardigrades similar to the model described for rotifers that has been termed 'Sleeping Beauty'.

SUMMARYMany limno-terrestrial tardigrades live in unstable habitats where they experience extreme environmental conditions such as drought, heat and subzero temperatures. Although their stress tolerance is often related only to the anhydrobiotic state, tardigrades can also be exposed to great daily temperature fluctuations without dehydration. Survival of subzero temperatures in an active state requires either the ability to tolerate the freezing of body water or mechanisms to decrease the freezing point. Considering freeze tolerance in tardigrades as a general feature, we studied the survival rate of nine tardigrade species originating from polar, temperate and tropical regions by cooling them at rates of 9, 7, 5, 3 and 1°C h -1 down to -30°C then returning them to room temperature at 10°C h -1 . The resulting moderate survival after fast and slow cooling rates and low survival after intermediate cooling rates may indicate the influence of a physical effect during fast cooling and the possibility that they are able to synthesize cryoprotectants during slow cooling. Differential scanning calorimetry of starved, fed and cold acclimatized individuals showed no intraspecific significant differences in supercooling points and ice formation. Although this might suggest that metabolic and biochemical preparation are non-essential prior to subzero temperature exposure, the increased survival rate with slower cooling rates gives evidence that tardigrades still use some kind of mechanism to protect their cellular structure from freezing injury without influencing the freezing temperature. These results expand our current understanding of freeze tolerance in tardigrades and will lead to a better understanding of their ability to survive subzero temperature conditions.