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Abstract

Daphnia pulex respond to water-borne chemicals released by predatory phantom midge larvae (Chaoborus spp.) with adaptive life-history and morphological responses. These responses can be modified by some classes of pesticides. The aim of this study was to measure the effects of a cyclodiene pesticide, endosulfan, on the Chaoborus-induced responses of D. pulex. Chaoborus induced the development of neckteeth, a faster growth rate, larger size at maturity, and higher fecundity in D. pulex. There was no evidence of any costs associated with neckteeth production except a small increase in age at maturity. Endosulfan was lethal to free-swimming D. pulex only at 300 μg l⁻¹, but caused significant mortality to embryos at concentrations as low as 0.1 μg l⁻¹. It inhibited the development of neckteeth at 100 μg l⁻¹, and reduced the growth rate of the induced morph at concentrations greater than or equal to 0.1 μg l⁻¹. Endosulfan had a unimodal effect on the expression of neckteeth in maternally-exposed daphnids, with maximal inhibition at intermediate concentrations. Endosulfan increased the number of neckteeth only in the first instar of maternally-exposed neonates at 200 μg l⁻¹. More generally, the results of this study suggest that anthropogenic pollution may inhibit phenotypic plasticity, indirectly limiting the temporal and spatial range of affected species.