The rapid cold hardening response of Collembola is influenced by thermal variability of the habitat

1. It has been argued that species living under unpredictable thermal conditions need to have more flexible physiological capabilities to meet with thermal stress than species living in thermally stable environments. Here we investigate if the ability to rapidly cold-harden in Collembola is influenced by thermal conditions of the habitat. 2. Collembola exploit diverse habitats and are therefore exposed to different thermal environments: soil dwelling (euedaphic) species occupy relatively stable environments, whereas surface dwelling (epedaphic) species can be exposed to more fluctuating thermal environments, but a single species can also be found in diverse thermal habitats within its geographic distribution. 3. We compared the inherent cold shock tolerance and ability to rapidly cold-harden in three epedaphic, two near surface dwelling (hemiedaphic) and four euedaphic species of Collembola using a similar experimental approach for all species. Additionally we compared three populations of the epedaphic species, Orchesella cincta , sampled along a climatic gradient (Norway, Denmark, Italy). 4. Inherent cold shock tolerance was estimated as LT 50 by assaying cold shock survival following a 2 h exposure to a range of temperatures from 1 ° C to –12 ° C. Rapid cold-hardening (RCH) was induced by cooling individuals from 20 ° C to a temperature 7 ° C above the LT 50 during 80 min, followed by 1 h at the specific cold shock temperature, which was close to the LT 50 of the particular species. 5. There was large variation in cold shock survival among species. The capacity to rapidly coldharden was found in all three ecotypes. 6. Genetic difference in the ability to rapidly cold-harden was seen in O. cincta from different climatic regions, consistent with the predictability of the thermal environment of their habitat. Population differences matched the daily fluctuations in temperature (CV) recorded at the site of collection as well as the day-to-day predictability (autocorrelation). The role of phylogenetic inertia was tested using sequence data from the cytochromec oxidase I (COI) gene and no signal of phylogeny was detected that could explain these population differences. 7. Our results show that genetic differences in RCH ability exist, consistent with latitudinal gradients in thermal fluctuations and predictability; thus comparative studies can provide important insight when exploring the role of acclimation in the geographical distribution of species.