Variations in physiological responses to thermal stress in congeneric limpets in the Mediterranean Sea

Cardiac activity (Arrhenius breakpoint temperatures and Q10 relationships) and heat shock response (hsp70 expression) were measured in the congeneric limpets Patella rustica, P. caerulea and P. ulyssiponensis in order to test the relationship between their vertical distribution and physiological thermal tolerance. These species exhibit different vertical distributions along Mediterranean shores and despite the narrow tidal range in the Mediterranean, they experience different environmental conditions and consequently had specific thermal windows. Cardiac activity of the upper zoned P. rustica was maintained at higher temperatures than its mid- or low shore counterparts, P. caerulea and P. ulyssiponensis. P. rustica had the highest Arrhenius breakpoint temperature (37.9 ± 2.1 °C, mean ± SD), followed by P. caerulea (35.9 ± 2.6 °C), and finally the low-zoned P. ulyssiponensis (32.2 ± 2.3 °C). The same pattern was found for Q10 relationships. Expression of hsp70 increased at 34 °C and kept increasing with temperature in P. rustica. In P. caerulea, expression reached a maximum at 36 °C and decreased at 38 °C, suggesting that hsp70 expression in P. rustica provides a more efficient defence against thermal stress than in P. caerulea. As summer environmental temperatures in the Mediterranean regularly reach 35 to 38 °C, performances of these limpets suggest that they are already living at the edges of their thermal window, and further temperature changes may have large-scale consequences for these keystone species.