Effects of Estrogenic Xenobiotics on Molting of the Water Flea,Daphnia magna

The effects of five xenobiotics, 2,4,5-trichloribiphenyl (PCB29), the polychlorinated biphenyl (PCB) Aroclor 1242, diethyl phthalate, lindane, and 4-octylphenol, on molting ofDaphnia magnawere investigated. All except PCB29 are known to have unexpected estrogenicity in vertebrates. Daphnids exposed to PCB29, Aroclor 1242, and diethyl phthalate took significantly more time to complete four molts than did the controls. The inhibitory effects of theseortho-chlorinated PCBs suggest that certain structural features, most probably includingortho-chlorination, are related to the ability of a PCB to affect molting. Agents with multicyclic structures, such as PCBs, are more effective in inhibiting molting than are single-ringed xenobiotics, such as diethyl phthalate, which suggests that hydrophobicity may be a requirement for binding to the ecdysteroid receptor. These molt-inhibiting agents with multiple rings appear to bear more structural resemblance to the steroidal molting hormones of arthropods, the ecdysteroids, than do the single-ringed ones. While the possibility of alternative mechanisms, such as impairment of ecdysteroidogenesis exists, the results obtained herein support the hypothesis that some xenobiotics which disrupt endocrine processes in vertebrates can also interfere with the hormonally regulated molting process in arthropods through acting as antagonists of endogenous ecdysteroids by binding to and thereby blocking the ecdysteroid receptor.