MOLT-RELATED SENSITIVITY OF DAPHNIA-PULEX IN TOXICITY TESTING

Continuous and short-term exposures of Daphnia pulex to 0.56 mg Cr/l showed that these animals undergo cyclic changes in susceptibility. Short-term exposure (2 h) followed by transfer to clean water for 22 h resulted in significantly higher mortality for individuals that molted during exposure. Continuous exposure to chromate resulted in significantly shorter mean survival times for animals that had molted <4 h before being placed in chromate solution. Because laboratory cultures can be synchronized with respect to molting, the effects of cyclic sensitivity can bias bioassay results. By examining the animals at the beginning of the test, the degree of synchrony can be determined and appropriate steps taken.