The Arctic copepod <i>Calanus hyperboreus</i> is more tolerant to marine heatwaves than temperate copepods in the Oslofjord

<i>Calanus hyperboreus</i> plays a key role in the functioning of Arctic ecosystems. It is considered highly vulnerable to ocean warming and marine heatwaves, which would reduce its range, expected to shift northward. Yet, <i>C. hyperboreus</i> is reported as far south as the Skagerrak, where it is considered non-native and transported by ocean currents. We argue that this may be an isolated population adapted to warmer temperatures. To test this hypothesis, we exposed <i>C. hyperboreus</i> from the Oslofjord to temperatures from 0°C to 24°C, for 5 days. We recorded survival to identify the upper threshold of thermal tolerance and DNA damage to detect sublethal effects. The thermal response of <i>C. hyperboreus</i> was compared with that of the dominant copepod species in the Oslofjord, <i>Calanus finmarchicus</i> and <i>Metridia longa</i>. We found that the survival of <i>C. hyperboreus</i> did not decrease before reaching 16°C-20°C which was much higher than 13°C-16°C and 4°C-8°C for <i>C. finmarchicus</i> and <i>M. longa</i>, respectively. <i>Calanus hyperboreus</i> showed the least DNA damage, highlighting the adaptation of its physiology to the Oslofjord. Our results suggest the existence of local adaptations to warming in <i>C. hyperboreus</i> that could determine its fate under climate change.