Temperature effects on the impact of two invasive parasitic copepods on the survival, growth, condition, and reproduction of native mussels

An increase in temperature due to climate change may affect the geographic ranges of invasive parasites and alter their impact on native hosts. Our goal was to determine if the effects of infection by two species of invasive endoparasitic copepods on native blue mussel hosts (Mytilus edulis) change with increasing temperatures. We investigated this with a laboratory experiment using temperatures that represent annual mean and mean summer water temperatures of past observations and future predictions for the study area, the European Wadden Sea (10–26 °C). Over a period of 8–20 weeks, infection with Mytilicola intestinalis lowered mussel condition and infection with Mytilicola orientalis decreased mussel shell growth. High temperatures decreased mussel growth and condition in general, but only at low temperatures (10–14 °C) the parasite-induced loss of condition was evident compared to uninfected mussels. Mussel mortality and reproductive activity were not affected by parasite infection, although both were impacted by temperature: the highest temperature (26 °C) increased mussel mortality, and gamete ripening only occurred at lower temperatures (10–18 °C). Taken together, these results suggest that both infection and high temperatures have independent negative effects. However, an increase in temperature does not worsen the effect of infection on individual mussel hosts, and neither does infection decrease host tolerance for long-term exposure to high temperatures. These findings add to our understanding of the interplay between increasing temperature and the interaction between invasive parasites and native hosts, and help predicting host and parasite dynamics in systems affected by species invasions and climate change.