Transmission of parasites may be mediated by their habitat, consisting of abiotic and biotic components. I investigated the effect of 2 important habitat components in intertidal ecosystems, tidal level (abiotic) and density of upstream hosts (biotic), on the transmission of trematode cercariae to cockle (Cerastoderma edule) hosts. A field survey showed no general trend in metacercarial loads of cockles regarding tidal level but species-dependent reactions. Parasites originating from Littorina littorea (Himasthla elongata, Renicola roscovita) showed highest infection levels in the low intertidal while parasites originating from Hydrobia ulvae (H. continua, H. interrupta) showed highest infection levels in the mid-intertidal. This reflected the density of upstream hosts at both tidal levels and positive relationships between the density of upstream hosts and metacercarial load in cockles suggested the biotic habitat component to be the dominant factor in transmission. This was confirmed by a field experiment, manipulating tidal level and the density of infected upstream snail hosts. While tidal level had no significant effect on the number of metacercariae of H. elongata acquired by cockles, the effect of upstream host density was strong. In conclusion, although tidal level usually is a very important abiotic habitat component in intertidal ecosystems leading to conspicuous zonation patterns in free-living organisms, it seems of minor importance for trematode transmission. In contrast, the biotic component upstream host density is suggested to be the dominant predictor for trematode transmission to second intermediate hosts. Assessing the relative importance of abiotic and biotic habitat components in transmission is vital for the understanding of transmission processes in the field.
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