Abstract
Experiments were conducted to investigate the period of sex differentiation in small cladoceran (D. magna) neonates. Gravid females were exposed to a juvenile hormone mimicking insecticide, fenoxycarb, which had been found to induce daphnids to develop into male neonates in previous studies. The stage when eggs are in the ovary prior to their release into the brood chamber was estimated to be the period susceptible to the juvenile hormone (JH) analog. The effects of JH analog exposure on daphnids, both the change in the offspring sex ratio and in the number of neonates, disappeared immediately when exposure ceased. This suggests that the male neonates are induced by the JH analog through disruption to the endocrine system. Twenty-one-day reproduction experiments revealed that the development into males was not induced by bisphenol A, nonylphenol, or octylphenol, known disruptors of vertebrate endocrine systems, at concentrations that caused reduction in the reproduction rate. 20-Hydroxyecdysone, a molting hormone in invertebrates, induced a high mortality rate in test animals at high concentrations but neither reduced the reproduction rate nor increased the development into males. These results indicate that the development of D. magna into males is not caused by chemical stress but by endocrine disruption. Finally, the offspring sex ratio in D. magna can be a useful endpoint to detect endocrine disrupting effects by juvenile hormones analogs.
