Maximum thermal tolerance trades off with chronic tolerance of high temperature in contrasting thermal populations of Radix balthica

Thermal adaptation theory predicts that thermal specialists evolve in environments with low temporal and high spatial thermal variation, whereas thermal generalists are favored in environments with high temporal and low spatial variation. The thermal environment of many organisms is predicted to change with globally increasing temperatures and thermal specialists are presumably at higher risk than thermal generalists. Here we investigated critical thermal maximum (CT max) and preferred temperature (Tp) in populations of the common pond snail (Radix balthica) originating from a small-scale system of geothermal springs in northern Iceland, where stable cold (ca. 7°C) and warm (ca. 23°C) habitats are connected with habitats following the seasonal thermal variation. Irrespective of thermal origin, we found a common Tp for all populations, corresponding to the common temperature optimum (Topt) for fitness-related traits in these populations. Warm-origin snails had lowest CT max. As our previous studies have found higher chronic temperature tolerance in the warm populations, we suggest that there is a trade-off between high temperature tolerance and performance in other fitness components, including tolerance to chronic thermal stress. Tp and CT max were positively correlated in warm-origin snails, suggesting a need to maintain a minimum "warming tolerance" (difference in CT max and habitat temperature) in warm environments. Our results highlight the importance of high mean temperature in shaping thermal performance curves.