CHEMO-ORIENTATION OF ECHINOSTOME CERCARIAE TOWARDS THEIR SNAIL HOSTS - AMINO-ACIDS SIGNAL A LOW HOST-SPECIFICITY

The cercariae of Pseudechinoparyphium echinatum and Echinostoma revolu tum approach their intermediate host snails by turning back when swimm ing 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 treat ed with 0.45 mu M-silver nitrate. As silver nitrate binds to the cilia ted papillae of the cercarial surface, papillar chemoreceptors may be involved. Snail conditioned water from different snail species stimula ted different intensities of chemo-orientation in both species. of dig eneans. However, when the samples of water conditioned with different snail species or even fish, tadpoles and leeches were diluted to the s ame total amino acid concentrations, the intensities of cercarial resp onses were similar. Therefore, some specificity of the chemo-orientati on seemed to be achieved only by the total concentration of amino acid s and not by particular mixtures of amino acids. In fact, amino acid s pectra released by freshwater snails varied not only among different s nail species, but also within the same species. (C) 1998 Australian So ciety for Parasitology. Published by Elsevier Science Ltd.