Determining acute thermal limits of trematode cercariae using the CT_max approach

Short-term thermal stress during heatwaves can potentially affect the transmission of parasites by increasing mortality of infective stages. Using the critical thermal maximum approach (CT_max), commonly used in free-living species, we exposed trematode cercariae of three marine species (Cryptocotyle lingua, Himasthla elongata, Renicola parvicaudatus) to a temperature ramping rate similar to realistic heatwave scenarios in the field and determined their acute temperature limits. The median CT_max values of all cercariae were much higher than the typical temperature extremes observed during heatwaves in the field. CT_max values slightly differed among the three trematode species and were higher than the thermal limit of their first intermediate host, the periwinkle Littorina littorea. This suggests that heatwave effects on trematode cercariae are likely not direct but more indirect, resulting from effects on the survival of their first intermediate host. Given its straightforward methodology and the potential for standardized comparisons of thermal limits among species and ecosystems, the CT_max approach holds promise for ecological research on the thermal biology of parasites.