When the water in rock pools is comfortable for us to roll up our jeans and paddle (between 20 and 25 °C), M. novaezealandensis thrives. At present this happens during low tide on hot summer days and the warmth sparks the release of multitudes of cercariae (free-swimming trematode larvae) into the water from the bodies of their snail hosts, ready to drill their way into their next host, the amphipod. In such temperature, the cercariae survive for relatively long periods, are at their infective peak and develop well inside the amphipods. These conditions are expected to occur more often and for longer periods with global warming - not particularly good news for the host snails and amphipods of M. novaezealandensis bombarded by increased numbers of this parasite and suffering death and destruction (particularly the amphipods) as a result.
But the heat gets all too much for M. novaezealandensis at temperatures greater than 30 °C when there are still many cercariae but they infect amphipods at lower rates and their lifespans are shortened. The amphipods also die at such heat, making it harder for the parasites to find their hosts and live in them long enough to develop. At present these extremes are rare, but the increase in high-temperature days as predicted would disrupt the parasite’s life cycle further and decrease the population of amphipods. As amphipods are an important food source for other animals, as well as the decomposers of the intertidal world, their demise can have widespread consequences.
Who knows what changes global warming will be bringing to the wider ecosystem; lab experiments, such as the one providing these results in this study, can only offer an indication. Further research into the effects of climate change on host-parasite systems will be important given the pivotal role of parasites and the complexity of the ecosystems that they are part of. Perhaps the behaviour of the snail, amphipod and gull hosts will also be affected by temperature changes, sea level rise or alterations in habitats and such selection pressure over generations of hosts and parasites will turn up the heat on evolution, resulting in offspring that may be quite differently to those that are alive today.
Studer, A., Thieltges, D. W., & Poulin, R. (2010). Parasites and global warming: Net effects of temperature on an intertidal host-parasite system. Marine Ecology Progress Series 415: 11-22.
This post was written by Sally Thorsteinsson