| Summary: | Different physiological thresholds of species across biogeographic boundaries can show a long-term scope for adaptation. Characterised by cold-stenothermal conditions, polar and deep-sea environments are constrained by thermal stability across wide regions, and polar invertebrates are broadly considered to be highly sensitive to only subtle changes in thermal regime. We examine the respiratory response of two distinct biogeographic populations of the widespread bivalve Lissarca notorcadensis from deep waters in the Southern Ocean to acute thermal changes, and present distinct respiratory responses for each population. Populations from the Weddell Sea living in cooler water temperatures (<0 °C) show a lower tolerance to temperature increases, identified by an increase in oxygen consumption at temperatures from ?1 to 4 °C and mortality at 5 °C. In contrast, populations from the Scotia Sea, experiencing a thermally more variable cold-stenothermal environment driven by seasonality, show no significant increase in oxygen consumption up to 6 °C before peaking at 8 °C, and mortality during acclimation to 9 °C. Our results are discussed in relation to the hypothesis that long-term adaptation to thermal envelopes, over multiple generations, likely determines the degree of thermal resilience to warming and at population-specific levels. This contrasts to previous laboratory-based (short-term acclimation or acute) thermal response studies, which have shown high sensitivities and low acclimation capacities to temperature increases. We highlight the need for long-term acclimation studies and pose questions as to how selection for population-specific thermal tolerances may take place in a global warming scenario and within a macroecological context.
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