Desiccation stress at sub-zero temperatures in polar terrestrial arthropods
Cold tolerant polar terrestrial arthropods have evolved a range of survival strategies which enable them to survive the most extreme environmental conditions (cold and drought) they are likely to encounter. Some species are classified as being freeze tolerant but the majority of those found in the Antarctic survive sub-zero temperatures by avoiding freezing by supercooling. For many arthropods, not just polar species, survival of desiccating conditions is equally important to survival of low temperatures. At sub-zero temperatures freeze avoiding arthropods are susceptible to desiccation and may lose water due to a vapour diffusion gradient between their supercooled body fluids and ice in their surroundings. This process ceases once the body fluids are frozen and so is not a problem for freeze tolerant species. This paper compares five polar arthropods, which have evolved different low temperature survival strategies, and the effects of exposure to sub-zero temperatures on their supercooling points (SCP) and water contents. The Antarctic oribatid mite (Alaskozetes antarcticus) reduced its supercooling point temperature from -6 to -30 degreesC, when exposed to decreasing sub-zero temperatures (cooled from 5 to -10 degreesC over 42 days) with little loss of body water during that period. However, Cryptopygus antarcticus, a springtail which occupies similar habitats in the Antarctic, showed a decrease in both water content and supercooling ability when exposed to the same experimental protocol. Both these Antarctic arthropods have evolved a freeze avoiding survival strategy. The Arctic springtail (Onychiurus arcticus), which is also freeze avoiding, dehydrated (from 2.4 to 0.7 g water g(-1) dry weight) at sub-zero temperatures and its SCP was lowered from c. -3 to below - 15 degreesC in direct response to temperature (5 to -5.5 degreesC). In contrast, the freeze tolerant larvae of an Arctic fly (Heleomyza borealis) froze at c. -7 degreesC with little change in water content or SCP during further cold exposure and survived frozen to -60 degreesC. The partially freeze tolerant sub-Antarctic beetle Hydromedion sparsutum froze at c. -2 degreesC and is known to survive frozen to -8 degreesC. During the subzero temperature treatment, its water content reduced until it froze and then remained constant. The survival strategies of such freeze tolerant and freeze avoiding arthropods are discussed in relation to desiccation at sub-zero temperatures and the evolution of strategies of cold tolerance.