The Arctic copepod Calanus hyperboreus is more tolerant to marine heatwaves than temperate copepods in the Oslofjord.

Calanus hyperboreus 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, C. hyperboreus 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 C. hyperboreus 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 C. hyperboreus was compared with that of the dominant copepod species in the Oslofjord, Calanus finmarchicus and Metridia longa. We found that the survival of C. hyperboreus did not decrease before reaching 16°C-20°C which was much higher than 13°C-16°C and 4°C-8°C for C. finmarchicus and M. longa, respectively. Calanus hyperboreus 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 C. hyperboreus that could determine its fate under climate change.