Mar Biol (2009) 156:2303–2311 DOI 10.1007/s00227-009-1258-2 123 ORIGINAL PAPER Non-host organisms aVect transmission processes in two common trematode parasites of rocky shores Katrin Prinz · Thomas C. Kelly · Ruth M. O’Riordan · Sarah C. Culloty Received: 20 May 2009 / Accepted: 6 July 2009 / Published online: 28 July 2009  Springer-Verlag 2009 Abstract The transmission of free-living trematode stages is mediated by various environmental factors, of which the presence of ambient organisms within the host space is a potential major determinant. In two laboratory mesocosm experiments, we investigated the inXuence of four intertidal rocky shore species on transmission success of cercariae of the digenean trematodes Echinostephilla patellae (encysting in the tissue of blue mussels Mytilus edulis) and Parorchis acanthus (encysting on mussel shells). Encystment success of both parasite species was signiWcantly lower in the presence of test organisms when compared to controls. Observations revealed that barnacles Austrominius modestus actively Wltered cercariae, whereas the larvae were obstructed by the seaweeds Corallina oYcinalis and Fucus serratus. Anemones Actinia equina both physically disturbed and consumed cercariae. In a fur- ther laboratory experiment, grazing gastropods (Littorina littorea, Patella vulgata, and Gibbula umbilicalis) were found to signiWcantly reduce the numbers of P. acanthus metacercariae in artiWcially prepared dishes by ingestion of cysts. Our results suggest that non-host organisms may play a key role in regulating the transmission of free-living trematode stages in rocky shore ecosystems, which is espe- cially important with regard to the relative diversity and density of species in these habitats. The Wndings also emphasize the need to include parasites into marine food webs, since cercariae seem to be consumed by certain organisms to a considerable extent and could possibly rep- resent an important energy source. Introduction Parasites form a vital part of biotic communities within marine intertidal ecosystems (Thomas et al. 1997; Mouritsen and Poulin 2002) and may account for a substantial pro- portion of the overall biomass (Kuris et al. 2008). They have to be considered as an important determinant in eco- system functioning by directly aVecting speciWc physiolog- ical host functions (Huxham et al. 1993; Mouritsen and Jensen 1994; Gorbushin 1997), thus altering host popula- tion dynamics (Desclaux et al. 2004; Fredensborg et al. 2005; Thieltges 2006b) and inXuencing community struc- ture (Sousa 1991; Mouritsen and Poulin 2005). Digenean trematodes are the dominant metazoan parasites of macro- invertebrates inhabiting the littoral zone (Mouritsen and Poulin 2002). The life cycles of these endoparasites gener- ally include the exploitation of diVerent host organisms, which requires the development of free-living infective stages to ensure transmission to the next host. In the Wnal or deWnitive host, adult stages of the parasite sexually repro- duce and shed eggs with the host’s faeces into the water. From the eggs, free-living larval stages, ciliated miracidia, hatch and penetrate the Wrst intermediate host, which is usually a gastropod. Within the Wrst intermediate host, cercariae are produced in great numbers via asexual reproduction. On release, the short-lived larvae actively seek and penetrate a second intermediate host, in which they encyst and develop into metacercariae. In some trema- tode species, the cercariae encyst in the open on a substrate Communicated by F. Bulleri. K. Prinz (&) · T. C. Kelly · R. M. O’Riordan · S. C. Culloty Department of Zoology, Ecology and Plant Science, University College Cork, Distillery Fields, North Mall, Cork, Ireland e-mail: k.prinz@mars.ucc.ie