Different temperature tolerance of northern and southern European populations of a freshwater Isopod Crustacean species (Asellus aquaticus L.)

Temperature is a major factor for ectothermic organisms, directly affecting biochemical and physiological processes. The increase in temperature and its effects have prompted many climate change studies, many of which focus on latitudinal gradients as they readily provide contrasting thermal regimes. In this study we evaluated the response in terms of growth, survival, and reproduction rates of two latitudinally separate populations of Asellus aquaticus (L.) to high temperature (30 °C). The comparisons revealed i) differences between the two populations, with higher tolerance found in the southern group, as well as between genders and body size classes. Only individuals from the southern population were able to complete the biological cycle at 30 °C, and no individuals from the northern population survived after 60 days; ii) differences between life stages: the survival rate of juveniles was greater than that of adults and the southern population had the highest proportion of temperature-resistant juveniles. The results confirm the importance of body size in the response to rising temperature and suggest that life stage should be taken into account in thermal adaptation studies. Analyses of intraspecific variability across different thermal regimes associated with the different latitudes illustrate how rising temperature can affect biological traits.