Comparisons of thermal tolerance limits of organisms living across environmental temperature gradients represent a valuable means by which to study physiological adaptation. Intertidal organisms are subject to variation in levels of thermal stress both across latitude and shore height. However, our current understanding of how this variation relates to thermal adaptation is largely based on comparisons between species, and we lack a clear picture of the degree to which different populations from the same species display local adaptation along these temperature gradients. In this thesis, I investigate the extent and nature of intraspecific local adaptation of thermal tolerance across shore height and latitude in the widely distributed, direct-developing intertidal periwinkle Littorina saxatilis. First, I characterise thermal tolerance differences in two ecotypes of L. saxatilis vertically segregated along an intertidal gradient to determine whether they exhibit local thermal adaptation in line with temperature differences across shore height. Second, I assess thermal tolerance and performance in four populations of L. saxatilis across a latitudinal gradient to study the extent of local thermal adaptation across latitude. Finally, I compare methodological approaches for quantifying thermal tolerance limits, to address controversy regarding how the use of different approaches may affect our ability to assess intraspecific differences across latitude. This thesis demonstrates that: 1) vertically segregated ecotypes show divergence in thermal tolerance in line with their shore position, and these differences have a developmental basis; 2) upper thermal limits of cardiac activity are greatest in populations from higher latitudes, which appears to be a maladaptive consequence of metabolic cold adaptation; and 3) the use of different methodological approaches can substantially affect both measured upper tolerance limits and our ability to interpret trends across latitude. Overall, this thesis demonstrates that local adaptation of thermal tolerance limits in L. saxatilis is substantial across shore height, but conversely is less pronounced across latitude. This suggests more broadly that thermal selection across a few metres of shore height may have as great an effect on intraspecific thermal adaptation as large-scale climatic variation across latitude. At the same time, compensation of metabolic rate appears to be significant component of local adaptation across latitude in this species. This thesis also demonstrates the importance of utilizing methodological approaches which can assess changes in temperature sensitivity in response to different thermal conditions in intertidal organisms.

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