Filtering activity of Daphnia in low concentrations of a pesticide

A concentration of lindane (γ‐hexachlorocyclohexane) of 0.05 mg liter−1, much lower than the 48‐h lethal concentration (LC50) of 3.8 mg liter−1, resulted in a 25% depression in the frequency of movements of filtering limbs and mandibles of Daphnia pulex in a food‐free medium. Different instars (from 2nd to 8th) were affected to the same extent. Animals 6–12 h after molting were more sensitive than those 30–36 h after molting. Due to the greater number of molts before reproduction in larger Daphnia species, the effect of low pesticide concentrations may be size‐selective and, therefore, responsible in part for the disappearance of large cladocerans from eutrophic lakes enriched from agriculture.