Parasites and global warming: net effects of temperature on an intertidal host–parasite system

Climate changes, particularly global warming, are likely to impact host–parasite interactions. However, our understanding of the effects of environmental factors on marine host–parasite systems is limited. We conducted a series of laboratory experiments on the effects of temperature on all transmission steps of the intertidal trematode Maritrema novaezealandensis from its first intermediate snail host Zeacumantus subcarinatus to the second intermediate amphipod host Paracalliope novizealandiae. By measuring output of cercarial transmission stages from snails, cercarial survival and infectivity, susceptibility of amphipods to infections, amphipod survival and parasite development within amphipods, we evaluated overall net temperature effects. At low temperatures (<20°C), transmission was low and amphipod survival unaffected. At intermediate temperatures (20 to 25°C), output and infectivity of cercarial transmission stages was at an optimum, which may increase the risk of infection intensity-dependent mortality of amphipods. Also, temperature directly increased amphipod mortality, but accelerated parasite development within amphipods. At high temperatures (≥30°C), transmission of the parasite was reduced (few cercariae, low infectivity), but temperatureinduced mortality of amphipods was most pronounced. Our approach revealed that temperature strongly, but differentially, affects the various steps of the transmission process, pointing to the amphipod as the most vulnerable component. An increased impact of parasites on amphipod populations with global warming is predicted and the possible disruption of the host–parasite system seems realistic under unusual future circumstances such as prolonged heatwaves. We suggest that more holistic studies of host–parasite interactions are essential for a better understanding of potential responses of host–parasite systems to global changes.