Influence of sexual dimorphism in Chironomus riparius meigen on toxic effects of cadmium

Second instars of Chironomus riparius Meigen were exposed to different concentrations of Cd-spiked sediment to determine effects on males and females. The wet weights of the larvae were determined after 10 d of exposure. The animals were then returned to the sediment and allowed to emerge. Time to emergence, sex, and adult survival time were recorded. Larval weights declined with increasing Cd concentrations, but the concentration-weight responses of males and females were similar. Hence, the impact of sexual dimorphism on growth measurements in sediment toxicity tests is likely to be small. The cumulative emergence of male and female chironomids did not differ significantly between sexes, except at Cd at 3,200 mg/L (measured sediment concentration 317 mug/kg). Adults emerged from controls between days 9 and 10 and from treatments up to and including Cd at 3,200 mg/L treatment within 18 d. Only one adult emerged from the treatment with Cd at 12,800 mg/L (measured sediment concentration 446 mug/kg), and time to emergence was delayed until day 30. This study has confirmed that Cd in sediments can reduce the survival, growth, development, and emergence of C. riparius, and that emergence patterns may differ between the sexes at higher concentrations of Cd. However, the difference between sexes in effects on larval growth will not greatly influence the results of toxicity tests if test treatments contain different proportions of males and females. Whether survivorship of the sexes differs when larval stages are exposed to Cd remains unclear.