Biotic interference in parasite transmission: Can the feeding of anemones counteract an increased risk of parasitism in amphipods at higher temperature?

• We assess if anemones can reduce parasite transmission to amphipod hosts.
• The microcosm experiment included 3 anemone densities and 2 temperatures.
• Density of anemones, predatory non-host organisms, did not have strong effects.
• Temperature increased infection levels in amphipod hosts and reduced their survival.
• Combining abiotic and biotic factors is crucial for improving ecological realism.

The transmission of parasites is embedded in the complexity of natural systems and is influenced not only by prevailing abiotic conditions, but also by the composition of the ambient community. In particular, temperature affects the number of transmission stages (e.g. cercariae of trematodes) released into the environment as well as their survival and infectivity. Temperature, however, also influences the metabolic rate and feeding activity of non-host organisms. We tested the hypothesis that at higher temperatures, the predatory anemone Anthopleura aureoradiata, known to ingest cercariae, may interfere with the transmission of the intertidal trematode parasite Maritrema novaezealandensis hence reducing the number of cercarial transmission stages successfully infecting Paracalliope novizealandiae amphipod hosts. In a microcosm experiment with two temperatures (15 and 22 °C) and three densities of anemones (0, 5 and 10 per microcosm), however, only a significant effect of temperature was found, with more parasites infecting surviving amphipods at the higher temperature. The effect of anemone density was not significant, although there was a trend towards fewer parasites infecting amphipods when anemones were present at 22 °C. Our results show that the effect of non-host species on trematode transmission might not be strong enough to mitigate increased parasite transmission at higher temperatures. However, mortality of amphipods kept at 22 °C was highest with low and medium anemone density; possibly reflecting an increased transmission pressure and parasite-induced mortality, therefore masking the actual outcome of this experiment based on surviving amphipods. Despite this, all findings point towards a higher risk of parasite-induced mortality of small crustaceans with increasing temperature, which is of particular concern in the context of global warming.