Effects of salinity on an intertidal host–parasite system: Is the parasite more sensitive than its host?

Intertidal habitats are characterised by highly fluctuating environmental conditions including varying salinity regimes. Changes in salinity may be gradual or abrupt; for example, heavy rainfall or evaporation during warm periods can either decrease or increase salinity. Trematodes are the most common parasites in intertidal ecosystems and their transmission is known to be highly influenced by environmental conditions. However, effects of salinity on the transmission of intertidal trematodes are not well studied. Here, we investigated the effects of long-term (i.e. several weeks) exposure to different salinities (25, 30, 35 and 40 psu) on the transmission of Maritrema novaezealandensis from its first intermediate snail host ( Zeacumantus subcarinatus) to a second intermediate amphipod host ( Paracalliope novizealandiae), in order to evaluate overall net effects. The following steps were assessed: output of parasite transmission stages (cercariae) from infected snail hosts, survival and infectivity of cercariae, susceptibility of amphipod hosts to infection and survival of amphipod hosts including parasite development within amphipod hosts. Output and survival of cercariae increased with increasing salinity whilst infectivity of cercariae and susceptibility of amphipods to infection were not clearly affected. Survival of amphipods was significantly longer at lower salinities and parasite development in infected amphipods was concomitantly more advanced. Overall, the results suggest that the parasite and the amphipods are differentially affected, and that under normal to increased salinities conditions are more favourable for the parasite than for the amphipod host. ► Salinity emerges as a modulator of parasite transmission in the intertidal. ► Salinity differentially affects the parasite and its host. ► The trematode is positively affected at normal to increased salinities. ► Output and survival of trematode cercariae increase with increasing salinity. ► The survival of the amphipod host, however, is longest at lower salinities.