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Hopkins Marine Station, Department of Biological Sciences, Stanford University, Pacific Grove, California 93950-3094
* To whom correspondence should be addressed, at Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039. E-mail: caren{at}mbari.org
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 FLThot or FLTcold, 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 FLThot, lower FLTcold). We also demonstrated the effects of constant thermal acclimation on the heart rate response to heat stress. Acclimation to 14 °C and 22 °C resulted in increases in ABT and FLThot, 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.
Abbreviations: ABT, Arrhenius break temperature, or the temperature at which heart rate decreases rapidly as temperature rises • FLT, flatline temperature, or the temperature at which heart beat ceases, at either high or low temperatures
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