Heat tolerance and physiological plasticity in the 1

Polar amplification of global warming has led to an average 2C rise in air temperatures in parts of the 14 polar regions in the last 50 years. Poikilothermic ectotherms that are found in these regions, such as 15 Collembola and mites, may therefore be put under pressure by changing environmental conditions. 16 However, it has also been suggested that the thermal sensitivity of invertebrates declines with higher 17 latitudes and, therefore, that polar ectotherms may not be at risk. In the current study, the heat 18 tolerance and physiological plasticity to heat stress of two well-studied Antarctic invertebrates, the 19 collembolan, Cryptopygus antarcticus, and the mite, Alaskozetes antarcticus, were investigated. Both 20 species showed considerable heat tolerance, with each having an Upper Lethal Temperature (ULT) 21 above 35C (1 hour exposure). These species were also able to survive for over 43 d at 10C and for 22 periods of 5-20 min at 40C. Across all experimental procedures, A. antarcticus possessed a 23 somewhat greater level of heat tolerance than C. antarcticus. Water loss during short duration 24 exposures did not differ between the two species at 30, 35 and 40C, suggesting that the greater 25 tolerance of A. antarcticus over this timescale was not due to higher desiccation resistance. 26 Physiological plasticity was investigated by testing for Rapid Heat Hardening (RHH) and long-term 27 acclimation. RHH was observed to a small degree in both species at a warming rate of 0.5C min, 28 and also 0.2C min in A. antarcticus alone. Longer-term acclimation (1 week at 10°C) did not 29 enhance the heat tolerance of either species. Even with this limited physiological plasticity, the results 30 of this study indicate that C. antarcticus and A. antarcticus have capacity in their heat tolerance to 31 cope with current and future environmental extremes of high temperature. 32