Evolutionary and acclimation-induced variation in the thermal limits of heart function in congeneric marine snails (genus Tegula): implications for vertical zonation.

We analyzed the thermal limits of heart function for congeneric species of the marine snail Tegula that have different patterns of vertical zonation. T. funebralis is found in the low to mid-intertidal zone, and T. brunnea and T. montereyi live in the low-intertidal or subtidally. As indices of thermal limits of heart function, we used the temperature at which heart rate initially decreased rapidly during heating (the Arrhenius break temperature, or ABT) and the temperature at which heart ceased to beat with either heating or cooling (the flatline temperature, or FLT(hot) or FLT(cold), respectively). These three indices provide an estimate of the thermal range within which Tegula heart function is maintained. For field-acclimatized specimens, the thermal range of the high-intertidal T. funebralis was greater than those of its two lower-occurring congeners (higher ABT, higher FLT(hot), lower FLT(cold)). We also demonstrated the effects of constant thermal acclimation on the heart rate response to heat stress. Acclimation to 14 degrees C and 22 degrees C resulted in increases in ABT and FLT(hot), with the largest changes in T. brunnea and T. montereyi. Although T. funebralis is more heat tolerant and eurythermal than its two lower-occurring congeners, it can encounter field body temperatures that exceed ABT, indicating that T. funebralis faces a larger threat from heat stress, in situ. These findings are consistent with recent studies on other taxa of marine invertebrates that have shown, somewhat paradoxically, that warm-adapted, eurythermal intertidal species may be more impacted by global warming than congeneric subtidal species that are less heat tolerant.