Metamorphosis enhances the effects of metal exposure on the mayfly, Centroptilum triangulifer

Background/Question/Methods

The response of larval aquatic insects to stressors such as metals is used to assess the ecological condition of streams worldwide. However, nearly all larval insects metamorphose from aquatic larvae to winged adults, and recent surveys indicate that adults may be a more sensitive indicator of stream metal toxicity than larvae. One hypothesis to explain this pattern is that insects exposed to elevated metal in their larval stages have a reduced ability to successfully complete metamorphosis. To test this hypothesis we exposed late-instar larvae of the mayfly, Centroptilum triangulifer, to an aqueous Zn gradient (32-476 μg/L) in the laboratory.

Results/Conclusions

Before metamorphosis began, larval survival was unaffected by Zn. When metamorphosis began, Zinc reduced wingpad development at concentrations above 139 μg/L, but not below. In contrast, emergence of subimagos and imagos tended to decline with any increase in Zn. For example, at Zn concentrations below 105 μg/L (the hardness-adjusted aquatic life criterion), the rate of survival between the wingpad and subimago stages declined 5-fold across the Zn gradient. These results support the hypothesis that metamorphosis may act as a survival bottleneck, enhancing the effects of stream metal exposure beyond the aquatic-terrestrial boundary.