Variations in the heart rate of Mediterranean mussels Mytilus galloprovincialis and brown mussels Perna perna under thermal stress on rocky shores of South Africa

Intertidal mussels experience prolonged emersion during low tide, followed by rapid submergence cooling during high tide, causing temperature-induced stress responses. This study examined variations in heart rate to investigate the relationship between emerged and submerged thermal stress in Mediterranean mussels Mytilus galloprovincialis and brown mussels Perna perna from one site on a south coast warm-water coastline and two sites on a west coast cold-water coastline in South Africa. Gaping behaviour in air displayed by P. perna appears to contribute towards their survivability under warmer conditions, as their flatline temperature was higher as compared with M. galloprovincialis, which do not display gaping behaviour. Warm-water acclimatised mussels of both species appeared better able to survive higher temperatures than cold-water acclimatised mussels. However, both mussel species are possibly living at their upper thermal tolerance based on their sublethal Arrhenius breakpoint temperature (ABT). The values of ABT were higher while exposed to air than when submerged, revealing physiological mechanisms at work to cope with changes in the intertidal zone.