Antoine Grenier-Journe

Climate change is increasing the frequency and severity of heatwaves with potential consequences for host–parasite dynamics. While the acute heat tolerance of free-living organisms has been intensively studied, much less is known about it for parasites and their life cycle stages. We investigated critical thermal maxima (CT_max) in two closely related marine invasive parasitic copepods (Mytilicola orientalis and Mytilicola intestinalis) that infect bivalves in the European Wadden Sea, using temperature ramping assays across life cycle stages (free-living larvae and parasitic adult females) that mimicked rapid warming periods during heatwaves occurring in the spring and summer. CT_max values of both life cycle stages of the two parasite species were relatively high (36°C–46°C), considering local environmental temperatures and typical CT_max values of free-living copepods. The values differed by species and life cycle stage, with ontogenic differences partially depending on the acclimation temperature (spring 14°C or summer 20°C). Overall, M. orientalis exhibited significantly higher CT_max than M. intestinalis, both for the adult and the larval stages (6.6°C–6.8°C and 3.9°C–4.2°C higher, respectively) at both acclimation temperatures. Within M. orientalis, larvae had 2.6°C–4.8°C lower CT_max than adults at both acclimation temperatures, while M. intestinalis showed a stage-specific variation of 2.2°C only when acclimated to 14°C. Larval stages of both species had 2.8°C–3.1°C higher CT_max values when acclimated to 20°C compared to 14°C, whereas no effect of acclimation temperature was seen among adults of either species. Our results suggest that parasitic copepods have a high acute heat tolerance that depends on the species, life cycle stage and seasonal temperature acclimation, with implications for heatwave tolerance. While current summer heatwaves do not exceed the CT_max of parasites acclimated to summer temperatures, the lower CT_max values when acclimated to spring temperatures may indicate negative effects of spring heatwaves on larval stages. Beyond our study, the CT_max approach may be a practical tool to investigate the acute heat tolerance of parasites. Read the free Plain Language Summary for this article on the Journal blog.