Host-finding in Echinostoma caproni: miracidia and cercariae use differentsignals to identify the same snail species

The snail host signals releasing host-finding responses in miracidia and ce rcariae of Echinostoma caproni were analysed by fractionation of snail-cond itioned water (SCW). Cercariae responded non-specifically to organic and hy drophilic, low molecular weight components of SCW showing their typical tur ning response. Hydrolysis of peptides in SCW had no effect on cercarial res ponses. An artificial mixture of amino acids in concentrations determined f rom SCW as well as glycine alone in a concentration corresponding to the to tal concentration of amino acids in SCW showed nearly the same efficacy as SCW itself. Miracidia responded to a high molecular weight glycoprotein fra ction, which could be isolated from SCW by ion-exchange and size-exclusion chromatography. In contrast to an Egyptian Schistosoma mansoni strain, the echinostome miracidia were not able to differentiate between different snai l species. The results show for the first time that miracidia and cercariae of the same species may use different signals to identify the same snail h ost species. This indicates an independent evolution of host-finding mechan isms in the two parasite stages.