Toxicity of cadmium and zinc to encystment and in vitro excystment of Parorchis acanthus (Digenea: Philophthalmidae)

Abstract

The toxicity of cadmium, zinc and cadmium}zinc mixtures at concentrations ranging from 1000 to 50000 lg}l were investigated against cercariae and metacercariae of Parorchis acanthus obtained from the dog whelk Nucella lapillus. Cercarial encystment at concentrations of 25000 lg}l or higher was signi®cantly impaired by all test metals; however, at lower concentrations only zinc demonstrated toxicity. Mixtures of cadmium and zinc had a synergistic effect compared with single metal toxicity but only at 50000 lg}l. Excystment in vitro was only signi®cantly affected by cercariae exposed to cadmium}zinc mixtures whilst encysting. Twenty-four h exposures of fully formed cysts had no effect on excystment in vitro. Effects on in vitro excystment rates over a 2 h period demonstrated widespread effects for cercariae-exposed P. acanthus. No effects were evident on excystment rates of cyst-exposed parasites.