Effects of salinity on multiplication and transmission of an intertidal trematode parasite

Salinity levels vary spatially in coastal areas, depending on proximity to freshwater sources, and may also be slowly decreasing as a result of anthropogenic climatic changes. The impact of salinity on host–parasite interactions is potentially a key regulator of transmission processes in intertidal areas, where trematodes are extremely common parasites of invertebrates and vertebrates. We investigated experimentally the effects of long-term exposure to decreased salinity levels on output of infective stages (cercariae) and their transmission success in the trematode Philophthalmus sp. This parasite uses the snail Zeacumantus subcarinatus as intermediate host, in which it asexually produces cercariae. After leaving the snail, cercariae encyst externally on hard substrates to await accidental ingestion by shorebirds, which serve as definitive hosts. We found that at reduced salinities (25 or 30 psu), the cercarial output of the parasite was lower, the time taken by cercariae to encyst was longer, fewer cercariae successfully encysted and encysted parasites had lower long-term survival than at normal seawater salinity (35 psu). The strong effect of salinity on the replication and transmission of this parasite suggests that there may be sources and sinks of transmission to birds along coastal areas, depending on local salinity conditions. Also, unless it evolves to adapt to changing conditions, the predicted reduction in salinity as a consequence of climate change may have negative impact on the parasite’s abundance. Introduction Intertidal areas are subject to rapid and sharp environmental changes due to the daily exposure to air and water caused by tides. Salinity levels, though roughly constant in the open ocean, are highly variable in intertidal habitats, in both space and time. They can range from 2 to 3 psu due to freshwater input from precipitation, rivers and surface runoff, up to 60 psu following evaporation from shallow pools at low tide (Adam 1993; Berger and Kharazova 1997). On longer time scales, predicted climate change may lead to altered precipitation regimes and rising sea levels following the thawing of freshwater ice stores at the poles, both of which are likely to affect the salinity levels experienced by intertidal organisms (IPCC 2007). Not surprisingly, salinity is a key abiotic factor influencing smalland large-scale biotic interactions in intertidal ecosystems (Berger and Kharazova 1997; Ingole and Parulekar 1998). It determines the distribution (Crain et al. 2004), physiological performance (Hylleberg 1975; Pequeux 1995; Shock et al. 2009) and reproductive success (Deschaseaux et al. 2010) of a wide range of organisms living on mudflats or rocky shores. Because of its direct physiological impact on intertidal organisms, salinity may also affect interspecific interactions, such as predation or parasitism. Parasites are increasingly recognized as important players in intertidal communities (Sousa 1991; Mouritsen and Poulin 2002; Kuris et al. 2008). They, too, can affect the physiology, activity, survival and reproduction of their hosts (Mouritsen and Poulin 2002). The wider consequences of infection include the regulation of host populations (Lafferty 1993; Fredensborg et al. 2005) together with altered trophic relationships resulting in changes in community structure and food web properties (Mouritsen Communicated by S. D. Connell. F. Lei R. Poulin (&) Department of Zoology, University of Otago, PO Box 56, Dunedin, New Zealand e-mail: [email protected] 123 Mar Biol (2011) 158:995–1003 DOI 10.1007/s00227-011-1625-7