Spatial heterogeneity in recruitment of larval trematodes to snail intermediate hosts

Spatial variation in parasitism is commonly observed in intermediate host p opulations. However, the factors that determine the causes of this variatio n remain unclear. Increasing evidence has suggested that spatial heterogene ity in parasitism among intermediate hosts may result from variation in rec ruitment processes initiated by definitive hosts. I studied the perching an d habitat use patterns of wading birds, the definitive hosts in this system , and its consequences for the recruitment of parasites in snail intermedia te hosts. Populations of the mangrove snail, Cerithidea scalariformis, coll ected from mangrove swamps on the east coast of central Florida are parasit ized by a diverse community of trematode parasites. These parasites are tra nsmitted from wading birds, which frequently perch on dead mangrove trees. I tested the hypothesis that mangrove perches act as transmission foci for trematode infections of C. scalariformis and that the spatial variation of parasitism frequently observed in this system is likely to emanate from the distribution of wading birds. On this fine spatial scale, definitive host behaviors, responding to a habitat variable, influenced the distribution, a bundance and species composition of parasite recruitment to snails. This ca usal chain of events is supported by regressions between perch density, bir d abundance, bird dropping density and ultimately parasite prevalence in sn ails. Variation between prevalence of parasites in free-ranging snails vers us caged snails shows that while avian definitive hosts initiate spatial pa tterns of parasitism in snails through their perching behaviors, these patt erns may be modified by the movement of snail hosts. Snail movement could d isperse their associated parasite populations within the marsh, which may p otentially homogenize or further increase parasite patchiness initiated by definitive hosts.