Calcium uptake in aquatic insects: Influences of phylogeny and metals (Cd and Zn)

Abstract

Summary Calcium sequestration in the hypo-osmotic freshwater environment is imperative in maintaining calcium homeostasis in freshwater aquatic organisms. This uptake process is reported to have the unintended consequence of potentially toxic heavy metal (Cd, Zn) uptake in a variety of aquatic species. However, calcium uptake remains poorly understood in aquatic insects, the dominant invertebrate faunal group on most freshwater ecosystems. Here we examined Ca uptake and interactions with heavy metals (Cd, Zn) at low ambient Ca levels (12.5 μmol l-1) in 12 aquatic insect species within Ephemerellidae (mayfly) and Hydropsychidae (caddis fly), two families differentially responsive to trace metal pollution. We found Ca uptake varied 70-fold across the 12 species studied. Body weight and clade (family) were found to significantly influence both Ca uptake and adsorption (p < 0.05). Zn and Cd uptake rate constants (kus) exhibited a strong correlation (r = 0.96, p < 0.0001), suggesting a shared transport system. Ca uptake failed to significantly correlate with either Zn or Cd kus. Further, neither Zn nor Cd exhibited inhibitory effects toward Ca uptake. In fact we saw evidence of modest stimulation of Ca uptake rates in some metal treatments. This work suggests that insects generally differ from other freshwater taxa in that aqueous Ca uptake does not appear to be compromised by Cd or Zn exposure. It is important to understand the trace metal and major ion physiology of aquatic insects due to their ecological importance and widespread use as ecological indicators.