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 revolu tum locate their host snails by turning back when swimming in decreasi ng gradients of the small molecular weight fraction (<500) of snail co nditioned water. Fractionation and chemical modifications of snail con ditioned water from Lymnaea stagnalis showed that amino acids are nece ssary for the stimulating activity of snail conditioned water. A compl ete mixture of amino acids in concentrations determined from snail con ditioned water had a high attraction. However, differently composed mi xtures of amino acids and even single amino acids also had the same at traction as this complete mixture when used in concentrations correspo nding to the total concentration of amino acids in snail conditioned w ater. Experiments with analogues and derivatives of amino acids showed that the primary cc-amino group and the a-carboxyl group are necessar y for the full effectiveness of amino acids. The highest effect was el icited 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 a mino acids alone. Chemical modifications of snail conditioned water su ggested that the additional stimuli were neither inorganic ions nor or ganic acids or lipids. As the full attraction of snail conditioned wat er was obtained when the amino acid mixture of snail conditioned water was combined with its content of urea and ammonia, we conclude that t he cercariae use only these excretory products as additional signals f or their chemo-orientation. Chemo-orientation to amino acids, urea and ammonia seems to reflect a strategy to locate a broad spectrum of aqu atic hosts. (C) 1998 Australian Society for Parasitology. Published by Elsevier Science Ltd.