A warmer climate with less extensive ice cover will lead to higher total primary production in the Arctic, which has the potential to increase the overall secondary production. Altered climate conditions will also affect timing of primary production, with consequences for the grazers of marine ecosystems. Depending on the grazers' ability to adapt to these new conditions, some organisms will be favored more than others, resulting in ecological winners and losers. We will focus on secondary marine producer s, more specifically zooplankton and the copepods Calanus spp. which comprise up to 90% of the zooplankton biomass in Arctic seas and thus constitute the key link between primary producers and higher trophic levels. Our study will target the differences b etween two key Arctic and sub-Arctic secondary producers, Calanus glacialis, endemic to the Arctic, and C. finmarchicus, endemic to the North-Atlantic. Both species are central in the work of both Norwegian and Russian marine biologists, but with limited collaboration until now. We will combine the use of historical data, new field collections and experimental work with a new Norwegian-Russian collaboration and exchange. Our main research questions are on the differences in the reproductive strategies (pa rticularly timing) of the two copepod species and on their potential for hybridization, meaning gene flow between what is considered separate species. We are particularly interested in how their responses to environmental conditions and their changes may vary, including response to temperature change and changes in timing, quantity and quality of their main food. We need a better understanding of these biological processes if we are to understand their dynamics and potential responses to climate change.