SWITCH · Host-switching under climate change: the potential of the sub-Arctic intertidal

All living organisms have parasites. Many parasites are host-specific and only infect a few, or even a single, suitable host species. Yet parasites retain a hidden capacity to infect broader host ranges under certain conditions, which may be activated by climate change. Coastal zones are especially vulnerable to climate-driven disturbances but it remains unknown how these changes will influence parasite specificity in intertidal ecosystems. This knowledge gap must be urgently addressed, considering the vital ecosystem services coastal zones provide. The ideal model system for this task is trematodes—parasites with complex life cycles that are exceptionally specific to their first intermediate hosts, yet have experienced multiple host-switching events in their evolution.The goal of SWITCH is to assess the potential of trematode parasites to colonize new first intermediate hosts in the warming sub-Arctic intertidal. I will start by estimating the current richness of trematode communities in the intermediate (gastropods) and definitive (coastal birds) hosts along the Northern Norwegian coastline. To do so, I will use morphological and DNA identification techniques for trematodes from gastropods (objective 1), and non-invasive faecal sampling and DNA-metabarcoding for trematodes from birds (objective 2). With input from these two objectives, I will experimentally test (objective 3) how and to what extent temperature variation affects the ability of trematodes to establish in alternative first intermediate host species by quantifying infection outcomes under conditions simulating global warming.Together, this novel approach will provide the first comprehensive assessment of host specificity and host-switching potential in sub-Arctic coastal systems. The findings will advance fundamental understanding of host–parasite interactions under climate change and inform environmental monitoring, biodiversity management, and aquaculture policy in European coastal regions.