Sub-lethal and sub-specific temperature effects are better predictors of mussel distribution than thermal tolerance

Three classes of mussel, the invasive Mytilus galloprovincialis and 2 genetic lineages of the indigenous Perna perna, show partially overlapping distributions along a large-scale temperature gradient in South Africa. We asked whether their physiological tolerances explain their distributional limits, investigating the effects of acute temperature change on heart rate, oxygen consumption and anaerobic end-product accumulation in air and water in the laboratory. For all 3 classes, heart rate and oxygen consumption were significantly reduced in air and were not correlated with temperature. During immersion, temperature had powerful effects on metabolic rate, but the ranking of heart rates reversed between heat and cold stress. The eastern P. perna lineage had higher heart rates with rising temperatures, while M. galloprovincialis showed higher heart rates during cooling. Despite no difference in upper thermal limits among mussel classes, their Arrhenius activation energies differed significantly, in parallel with their warm to cool distributions in the order P. perna east > P. perna west > M. galloprovincialis. The results indicate that the eastern lineage of P. perna is better adapted to warm temperatures and M. galloprovincialis is better adapted to high shore conditions. Upper thermal limits gave only a crude approximation of the effects of temperature stress. Although the thermal limits of these populations were similar, their overall responses to stress differed markedly, reflecting their distributions and potentially affecting their competitive interactions. We suggest that thermal limits offer poor explanations for species distributions, and highlight the critical importance of both sub-lethal effects and sub-specific differences in physiology.