The three species of blue mussels, Mytilus trossulus Gould 1850, M. edulis Linnaeus 1758 and M. galloprovincialis Lamarck 1819, have distinct global distribution patterns that are hypothesized to reflect differences in their tolerances of temperature and salinity. We examined effects on heart rate (beats min(-1)) of acute exposure and acclimation to different combinations of temperature and salinity to test this hypothesis and, in the context of the invasive success of M. galloprovincialis, to gain insights into the factors that may explain the replacement of the temperate Pacific native, M. trossulus, by this Mediterranean Sea invader along much of the California coast. Heart rate of M. trossulus was significantly higher than that of M. galloprovincialis, consistent with evolutionary adaptation to a lower habitat temperature (temperature compensation) in the former species. Heart rates of M. trossulus/M. galloprovincialis hybrids were intermediate between those of the parental species. Following acclimation to 14 degrees C and 21 degrees C, heart rates of all species exhibited partial compensation to temperature. Heart rate increased with rising temperature until a high temperature was reached at which point activity fell sharply, the high critical temperature (H-crit). H-crit increased with increasing acclimation temperature and differed among species in a pattern that reflected their probable evolutionary adaptation temperatures: M. galloprovincialis is more heat tolerant than the other two congeners. Ability to sustain heart function in the cold also reflected evolutionary history: M. trossulus is more cold tolerant than M. galloprovincialis. Heart rates for all three congeners decreased gradually in response to acute reductions in salinity until a low salinity (S-crit) was reached at which heart rate dropped precipitously. S-crit decreased with decreasing salinity of acclimation and was generally lowest for M. galloprovincialis. Mortality during acclimation under common garden conditions was greatest in M. trossulus and was highest at high acclimation temperatures and salinities. These intrinsic differences in basal heart rate, thermal and salinity responses, acclimatory capacity, and survivorship are discussed in the contexts of the species' biogeographic patterning and, for the invasive species M. galloprovincialis, the potential for further range expansion along the Pacific coast of North America.
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