Cercarial emergence of Ichthyocotylurus erraticus (Rudolphi, 1809), I-variegatus (Creplin, 1825) and Apatemon gracilis (Rudolphi, 1819) (Digenea : Strigeidae): contrasting responses to light : dark cycling

Many digenean cercariae have been shown to emerge from their molluscan host s with distinct shedding patterns that have enabled the discrimination of m orphologically similar species, or even strains. In this study the cercaria l emission patterns of three strigeid species, Ichthyocotylurus erraticus, I. variegatus and Apatemon gracilis, from experimentally infected natural h osts were found to exhibit rhythms that correlated with the light:dark cycl e. Both Ichthyocotylurus spp. exhibited a diurnal pattern of release in whi ch cercariae emerged during the light period. Each demonstrated a latent pe riod before the liberation of large numbers of cercariae and yielded simila r numbers of cercariae daily. These rhythms offered no means for the discri mination of these two morphologically similar species. A. gracilis cercaria e demonstrated a very different circadian rhythm in which the majority emer ged at the onset of darkness with no latent period, whereas the cercarial n umbers released daily were far greater. Differences could be related to pis cine host behaviour.