Chronic life-cycle tests using the freshwater cladoceran Daphnia magna aim to measure the effect of toxic chemicals on adult reproduction, in terms of the number of viable offspring produced. A tacit assumption is made that the primary effect of toxicants in these tests is a sublethal effect on the parent, reducing egg production and hence fecundity. The authors' observations on two chemicals, sodium bromide and 3,4-dichloroaniline, demonstrate that this assumption is at least partly false. In these experiments, the primary toxic effect of these compounds was to kill developing eggs in the brood chamber. Total egg output was unaffected, whereas output of viable eggs was severely impaired-an acute lethal effect. A further experiment demonstrated that the effect was limited to those eggs developing in the brood chamber, with eggs in the ovary remaining unaffected. It was suggested that these observations may provide the basis for the development of short, sensitive early life-stage test with this species.
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We thank M. Bradley for helpful discussions and S. Ansell, able technical support. Portions of this work were supported 86000 160, NERC Grant TFS/86/AnE/4, and a publication S. Matraves, N. Homer, and Y. Waddell for by NATO Grant 3/A/87/PO, EC Grant B/ grant from JNICT Portugal.