Physiological costs of infection by the invasive parasitic copepod Mytilicola intestinalis accumulate across temporal scales in the blue mussel Mytilus edulis

Exploitation of host resources by parasites can have profound impacts on infected hosts. When prevalence is high parasite infection can even feed back on host population dynamics. Especially when parasites are invasive species, their new native hosts can suffer from exploitation due to a lack of co-evolutionary history. Nevertheless, energetic costs are often subtle at the level of the individual and hard to link to population level effects. Sublethal energetic costs accumulate over time and can in sum be traded off against fitness over longer time scales. To study temporal accumulation of infection costs, we used a series of controlled infection experiments to assess the physiological effects exerted by the invasive parasitic copepod Mytilicola intestinalis on its newly acquired native host, the blue mussel Mytilus edulis in the North Sea, where the parasite can reach prevalences > 70 %. To link short-term physiological responses to long term components of fitness, we combined several methodological approaches measuring changes of physiological traits that act on different time scales. Stable isotope analyses over different seasons and environments revealed a direct consumption of host tissue by M. intestinalis. Tissue repair increased energy demand that could directly be observed in accelerated heartbeat rates, and changes of amino acid metabolism measured by ¹H NMR spectroscopy. Although these effects were comparatively small in size, the resulting altered energy budget of the mussel host led to a lower body condition index under controlled laboratory conditions on medium time scales (months) and slower growth in the field over longer time scales (1 year). In combination, our experimental results show that small short-term physiological changes can translate to fitness relevant negative effects on life history traits when integrated across temporal scales.