Chemo-orientation of echinostome cercariae towards their snail hosts: the stimulating structure of amino acids and other attractants.

The cercariae of Pseudechinoparyphium echinatum and Echinostoma revolutum locate their host snails by turning back when swimming in decreasing gradients of the small molecular weight fraction (< 500) of snail conditioned water. Fractionation and chemical modifications of snail conditioned water from Lymnaea stagnalis showed that amino acids are necessary for the stimulating activity of snail conditioned water. A complete mixture of amino acids in concentrations determined from snail conditioned water had a high attraction. However, differently composed mixtures of amino acids and even single amino acids also had the same attraction as this complete mixture when used in concentrations corresponding to the total concentration of amino acids in snail conditioned water. Experiments with analogues and derivatives of amino acids showed that the primary alpha-amino group and the alpha-carboxyl group are necessary for the full effectiveness of amino acids. The highest effect was elicited by L-amino acids with a primary alpha-amino group, whereas the amino acid type and the chain length seemed to be unimportant. However, the full attraction of snail conditioned water was not achieved by amino acids alone. Chemical modifications of snail conditioned water suggested that the additional stimuli were neither inorganic ions nor organic acids or lipids. As the full attraction of snail conditioned water was obtained when the amino acid mixture of snail conditioned water was combined with its content of urea and ammonia, we conclude that the cercariae use only these excretory products as additional signals for their chemo-orientation. Chemo-orientation to amino acids, urea and ammonia seems to reflect a strategy to locate a broad spectrum of aquatic hosts.