Chemo-orientation of echinostome cercariae towards their snail hosts: amino acids signal a low host-specificity.

The cercariae of Pseudechinoparyphium echinatum and Echinostoma revolutum approach their intermediate host snails by turning back when swimming in decreasing concentration gradients of snail conditioned water. Host stimuli are small molecular amino compounds, probably amino acids. This response was inhibited reversibly when the cercariae were treated with 0.45 microM-silver nitrate. As silver nitrate binds to the ciliated papillae of the cercarial surface, papillar chemoreceptors may be involved. Snail conditioned water from different snail species stimulated different intensities of chemo-orientation in both species of digeneans. However, when the samples of water conditioned with different snail species or even fish, tadpoles and leeches were diluted to the same total amino acid concentrations, the intensities of cercarial responses were similar. Therefore, some specificity of the chemo-orientation seemed to be achieved only by the total concentration of amino acids and not by particular mixtures of amino acids. In fact, amino acid spectra released by freshwater snails varied not only among different snail species, but also within the same species.