Effects of endocrine-active chemicals on the development of sex characteristics of Daphnia magna

Standard reproductive assays with daphnids involve parthenogenetically reproducing females and exclude the assessment of effects on sexual reproduction. The goals of this study were to characterize sexual differentiation of male and female daphnids (Daphnia magna) and to evaluate whether exposure to putative endocrine-disrupting chemicals may perturb the development of sex characteristics. Anatomical sex differences that developed during maturation in males included elongated first antennae and morphologic alterations in the head capsule and carapace edge. Reproductive maturation in females was associated with the development of a brood chamber and abdominal process. Alterations in the growth rates of the first antennae of males and the abdominal process of females were used to evaluate the effects of chemical exposure on the development of these sex characteristics during maturation. Exposure of female daphnids to the nonsteroidal vertebrate estrogen diethylstilbesterol and the insect juvenile hormone analog methoprene at concentrations as low as 3.0 and 0.080 mu M, respectively, stimulated development of the abdominal process. Exposure of males to the steroidal vertebrate androgen androstenedione (greater than or equal to 6.0 mu M) stimulated development of the first antennae. These results demonstrate that the development of secondary sex characteristics in daphnids can be altered by chemical exposure.