When the body fluids of an invertebrate are frozen,

the invertebrate is no longer considered capable of movement and the SCP is seen as the absolute limit of mobility. In many temperate and tropical species, the lower lethal thresholds, and thus also the CTmin and chill coma, are well Ion Channel Ligand Library in vivo above the SCP (Bale, 2002). However, in the current study, prior to acclimation, the chill coma temperature of all three species, and the CTmin of the two Collembola, were within 2–3 °C of the SCP (Fig 1; Table 1). Likewise, the continental Antarctic collembolan, Isotoma klovstadi, was observed to be capable of walking at all temperatures down to its SCP, with an average chill coma onset temperature of −11.9 to −13.3 °C over the summer season ( Sinclair et al., 2006). These organisms are consequently Ku 0059436 able to search for more preferable habitats as the temperature falls, and possibly perform beneficial activities, such as foraging, very near to their SCP. Climate

warming has resulted in a significant rise in polar temperatures, and will undoubtedly lead to future increases (Arctic Council, 2005, Convey et al., 2009 and Turner et al., 2009). An advantage may therefore be gained by being able to acclimate to higher temperatures. However, the species examined here showed no acclimation ability allowing an increase in their upper activity thresholds following a

two week period at 9 °C, and even showed a decline in both their CTmax and heat coma (Fig. 2). Everatt et al. (2013) and Slabber et al. (2007) also found that acclimation to higher temperatures (9 and 15 °C, respectively) either resulted in no change in, or impaired, survival at temperatures above 30 °C in both Collembola and Acari. Further, a number of studies have shown little plasticity in upper thermal tolerance traits in non-polar species, including these in the cricket, A. domesticus, the fruit fly, D. melanogaster, dung beetles, and the tsetse fly, Glossina pallidipes ( Gaston and Chown, 1999, Goto et al., 2000, Hoffmann et al., 2005, Lachenicht et al., 2010 and Terblanche et al., 2011). There is now a general consensus that thermal tolerance shows less phenotypic plasticity at higher temperatures than at lower temperatures in invertebrates, and that this may be due to each involving a distinct suite of physiological and molecular mechanisms ( Bowler and Terblanche, 2008). Even though the polar species of this study show a limited ability to acclimate their upper thermal thresholds to higher temperatures, the upper thermal tolerance they already possess (see Section 4.2.) gives these invertebrates sufficient capacity to cope with future climate warming. Intriguingly, a subtle difference may exist between the locomotion speeds of the mite and the Collembola. In A.