Toxic effects of Europium chloride on developing zebrafish (Danio rerio) embryos

Lanthanides, including Europium, show a broad spectrum of industrial applications and their release into environment has been significantly increased during the last decades. However, toxic effects of lanthanides on aquatic biota have not been consistently studied. The aim of the present study was to assess toxicity of Europium chloride hexahydrate (EuCl 3 . 6H 2 O) on zebrafish embryos under standardised experimental conditions. Embryos were exposed to a range of Europium concentrations (0.05-500 ppm) either for the first 10 or for 72 hrs after fertilisation. The embryonic development was assessed by several biological and physiological endpoints (e.g. mortality rate during the incubation period, hatching rate, standard length at hatch, development of pigmented eyes, body morphometrics, heart contractility as well as heart rate). Exposure of embryos to Europium affected all the parameters of embryonic development resulting in significant dose-dependent mortality, delay of hatching, decreased standard length and heart rate as well as in delayed heart formation. Malformations and significant differences in morphometrics, however, were not observed. The effects of Europium on heart formation and especially on the heart rate, which was manifested starting at concentration as low as 0.05 ppm, suggested that the heart muscle may constitute the target organ for the toxicity of the metal.