Comparative Trace Metal Physiology in Aquatic Insects

Despite their dominance in freshwater systems and use in biomonitoring and bioassessment programs worldwide, little is known about the ion/metal physiology of aquatic insects. Even less is known about the variability of trace metal physiologies across aquatic insect species. Here, we measured dissolved metal bioaccumulation dynamics using radiotracers in order to 1) gain an understanding of the uptake and interactions of Ca, Cd and Zn at the apical surface of aquatic insects and 2) comparatively analyze metal bioaccumulation dynamics in closely-related aquatic insect species. Dissolved metal uptake and efflux rate constants were calculated for 19 species. We utilized species from families Hydropsychidae (order Trichoptera) and Ephemerellidae (order Ephemeroptera) because they are particularly species-rich and because they are differentially sensitive to metals in the field -- Hydropsychidae are relatively tolerant and Ephemerellidae are relatively sensitive. In uptake experiments with Hydropsyche sparna (Hydropsychidae), we found evidence of two shared transport systems for Cd and Zn -- a low capacity-high affinity transporter below 0.8 muM, and a second high capacity-low affinity transporter operating at higher concentrations. Cd outcompeted Zn at concentrations above 0.6 muM, suggesting a higher affinity of Cd for a shared transporter at those concentrations. While Cd and Zn uptake strongly co-varied across 12 species (r = 0.96, p < 0.0001), neither Cd nor Zn uptake significantly co-varied with Ca uptake in these species. Further, Ca only modestly inhibited Cd and Zn uptake, while neither Cd nor Zn inhibited Ca uptake at concentrations up to concentrations of 89 nM Cd and 1.53 muM Zn. Ca, Cd and Zn bioaccumulation parameters all varied across orders of magnitude within the two families examined. Familial differences were striking across uptake rate constants and bioconcentration factors, however not across efflux rate constants. Body size was an important driver of uptake rate constants (and consequentially, bioconcentration factors). While the variation in Cd uptake and efflux rate constants was previously shown to be heavily influenced by phylogenetic position in species from orders Ephemeroptera, Plecoptera and Trichoptera, the tremendous variability displayed by these two families effectively erased the phylogentic signal. In analyses of all available Cd uptake and efflux rate constants for aquatic insects from this study and from the literature, we discovered clade (as genus) to explain significant amounts of variation across metal bioaccumulation parameters. Analyses across more taxonomically-divergent aquatic organisms identified phylogenetic signal in efflux rate constants across four aquatic phyla. Further, the strong covariation of Zn and Cd efflux rate constants in hydropsychids and ephemerellids led to successful predictions of Zn uptake rate constants from known Cd values in five of six aquatic insect species. This work has led to two overall conclusions which should be taken into consideration when using aquatic insect data in regulatory toxicology. First, aquatic insects appear to have trace metal physiologies which differ from other aquatic organisms (e.g., fish, Daphnia) regarding the trafficking of dissolved metal. Hypocalcemia does not appear to be the mechanism of toxicity in dissolved Cd/Zn as evidenced by the lack on interactions between Ca and Cd/Zn. This potentially contributes to their acute metal tolerance in the laboratory (as opposed to their observed metal sensitivity in the field). Second, aquatic insects, particularly within families Hydropsychidae and Ephemerellidae, have highly variable physiologies. This has important implications in that we need to better understand how well "surrogate species" represent their fellow congeners and account for the variation in aquatic risk assessments aimed at protecting insect diversity.