Non-host organisms affect transmission processes in two common trematode parasites of rocky shores

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 influence of four intertidal rocky shore species on transmission success of cercariae of the digenean trematodes Echinostephillapatellae (encysting in the tissue of blue mussels Mytilus edulis) and Parorchisacanthus (encysting on mussel shells). Encystment success of both parasite species was significantly lower in the presence of test organisms when compared to controls. Observations revealed that barnacles Austrominiusmodestus actively filtered cercariae, whereas the larvae were obstructed by the seaweeds Corallinaofficinalis and Fucusserratus. Anemones Actiniaequina both physically disturbed and consumed cercariae. In a further laboratory experiment, grazing gastropods (Littorinalittorea, Patellavulgata, and Gibbulaumbilicalis) were found to significantly reduce the numbers of P. acanthus metacercariae in artificially 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 especially important with regard to the relative diversity and density of species in these habitats. The findings 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 represent an important energy source.