Rapid cold hardening in Antarctic microarthropods

1. Rapid cold hardening was examined in three common Antarcticmicroarthropods using differential scanning calorimetry over timescales between 3 and 30 h, under field and controlled laboratory conditions. 2. In fresh field samples and cultures of the springtail, Cryptopygus antarcticus (Willem), and cultures of the mites, Alaskozetes antarcticus (Michael) and Halozetes belgicae (Michael), maintained under summer field-simulating conditions, supercooling point (SCP) distributions tracked microhabitat temperature variation over the observation period. 3. Controlled acclimation of samples of summer-acclimatized C. antarcticus caused significant cold hardening after 12 h at temperatures around 0 °C (+3 to -2 °C). No response was obtained at higher or lower temperatures, or in field-fresh winter-acclimatized animals. The latter did not lose cold hardiness when held at positive temperatures for 12 h. 4. Gradual cooling of C. antarcticus over 20 h from +5 to -5 °C caused a considerable increase in cold tolerance. Rewarming partially but non-significantly reversed this effect. The greatest response occurred between +3 and + 1 °C. Maximum faecal pellet production also occurred in this interval, but gut clearance alone was not sufficient to explain observed cold hardening. 5. It is hypothesized that these species possess a hitherto unrecognized capacity to alter cold hardiness in summer in response to environmental temperature cues over a shorter timescale than previously thought, by a mechanism that relies on neither gut clearance nor concentration of body fluids via water loss. This ability may reduce the developmental costs of premature entry into an inactive, cold-hardy state.