Evolutionary Patterns in Trace Metal (Cd and Zn) Efflux Capacity in Aquatic Organisms

The ability to eliminate (efflux) metals is a physiological trait that acts as a major driver of bioaccumulation differences among species. This species-specific trait plays a large role in determining the metal loads that species will need to detoxify to persist in chronically contaminated environments and, therefore, contributes significantly to differences in environmental sensitivity among species. To develop a better understanding of how efflux varies within and among taxonomic groupings, we compared Cd and Zn efflux rate constants (<i>k</i>_e values) among members of two species-rich aquatic insect families, Ephemerellidae and Hydropsychidae, and discovered that <i>k</i>_e values strongly covaried across species. This relationship allowed us to successfully predict Zn efflux from Cd data gathered from aquatic species belonging to other insect orders and families. We then performed a broader, comparative analysis of Cd and Zn <i>k</i>_e values from existing data for arthropods, mollusks, annelids, and chordates (77 species total) and found significant phylogenetic patterns. Taxonomic groups exhibited marked variability in <i>k</i>_e magnitudes and ranges, suggesting that some groups are more constrained than others in their abilities to eliminate metals. Understanding broader patterns of variability can lead to more rational extrapolations across species and improved protectiveness in water-quality criteria and ecological assessment