COMPARATIVE TOXICANT SENSITIVITY OF SEXUAL AND ASEXUAL REPRODUCTION IN THE ROTIFER BRACHIONUS-CALYCIFLORUS

Cyclically parthenogenetic zooplankters like rotifers are important to ols for assessing toxicity in aquatic environments. Sexual reproductio n is an essential component of rotifer life cycles, but current toxici ty tests utilize only asexual reproduction. We compared the effects of four toxicants on a sexual and sexual reproduction of the rotifer Bra chionus calyciflorus. Toxicants has a differential effect on sexual an d asexual reproduction, with sexual reproduction consistently the most sensitive. Concentrations of 0.2 mu g/ml PCP (sodium pentachlorophena te) had no effect on the asexual reproductive rate, but significantly reduced sexual reproduction. Likewise, chlorpyrifos concentrations of 0.3 mu g/ml had no significant effect on asexual reproduction, but sex ual reproduction was significantly reduced. There was no difference in NOECs, LOECs, and chronic values for asexual and sexual reproduction for cadmium and naphthol tests. However, comparison of toxicant effect levels revealed that sexual reproduction was more strongly reduced at each toxicant concentration. The four toxicants tested inhibited sexu al reproduction 2 to 68 times more than asexual reproduction at the lo west observed effect concentrations. Toxicants inhibited sexual reprod uction in its initial step: sexual female production. Because sexual r eproduction is more sensitive, toxicity tests based exclusively on ase xual reproduction may not be protective of rotifer life cycles.