Acute heat tolerance ( <scp> CT _max </scp> ) of two invasive parasitic copepods differs across species, life cycle stages and acclimation temperatures

Abstract 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.