The ability to withstand short-term heat shocks and long-term elevated ambient temperatures suggests different sensitivity to future climatic changes for two sympatric Mediterranean land snail species, Theba pisana and Xeropicta derbentina (Helicoide

In the 1940s, Xeropicta derbentina was introduced to the Provence region in south-eastern France. Since then, it has coexisted sympatrically with the native species Theba pisana. X. derbentina's successful establishment in Provence is commonly attributed to the hot and dry climate there. In the context of continually rising temperatures and increasing aridity resulting from global change, we conducted open-top chamber (OTC) experiments with both species to simulate extreme heat conditions to be expected in the context of climate change. After exposure in the OTCs, the temperatures on the shell surface did not differ between the two species. Nevertheless, differences in the survival of the species following heat shocks, which were induced by transferring the snails to the hot soil, were striking. An initial heat shock at the start of the OTC exposure was survived significantly better by T. pisana than by X. derbentina. However, the prevailing conditions in the OTC apparently weakened T. pisana to such an extent that only few individuals survived another heat shock a week later. In contrast, X. derbentina exhibited heat hardening induced by the OTC conditions, meaning that a heat shock after a week of adaptation to the OTCs resulted in much lower mortality than in individuals of the same species that had not adapted to OTC conditions before the heat shock. After a week of exposure to OTCs, the survival rate following heat shock was also significantly higher in X. derbentina than in T. pisana. These results suggest that X. derbentina will have a selective advantage over T. pisana as their environment continues to warm up unless more heat-tolerant phenotypes of the latter species evolve or immigrate.