Comparative Sodium Transport Patterns Provide Clues for Understanding Salinity and Metal Responses in Aquatic Insects

Scheibener, S; Richardi, V; Buchwalter, David

ASTRACT: The importance of insects in freshwater ecosystems has led to their extensive use in ecological monitoring programs. As freshwater systems are increasingly challenged by salinization and metal contamination, it is important to understand fundamental aspects of aquatic insect physiology (e.g., osmoregulatory processes) that contribute to insect responses to these stressors. Here we compared the uptake dynamics of Na as NaCl, NaHCO3 and Na2SO4 in the caddisfly Hydropsyche betteni across a range of Na concentrations (0.06–15.22 mM) encompassing the vast majority of North American freshwater ecosystems. Sulfate as the major anion resulted in decreased Na uptake rates relative to the chloride and bicarbonate salts. A comparison of Na (as NaHCO3) turnover rates in the caddisfly Hydropsyche sparna and the mayfly Maccaffertium sp. revealed different patterns in the 2 species. Both species appeared to tightly regulate their whole body sodium concentrations (at ∼47 ± 1.8 umol/g wet wt) across a range of Na concentrations (0.06–15.22 mM) over 7 days. However, at the highest Na concentration (15.22 mM), Na uptake rates in H. sparna (419.1 M Na g−1 hr−1 wet wt) appeared close to saturation while Na uptake rates in Maccaffertium sp. were considerably faster (715 g M Na g−1 hr−1 wet wt) and appeared to not be close to saturation. Na efflux studies in H. sparna revealed that loss rates are commensurate with uptake rates and are responsive to changes in water Na concentrations. A comparison of Na uptake rates (at 0.57 mM Na) across 9 species representing 4 major orders (Ephemeroptera, Plecoptera, Trichoptera and Diptera) demonstrated profound physiological differences across species after accounting for the influence of body weight. Faster Na uptake rates werive to salinization in field studies. The metals silver (Ag) and copper (Cu), known to be antagonistic to Na uptake in other aquNa uptake at extremely high concentrations, while Cu generally stimulated Na uptake in aquatic insects, rather than suppress it.sures in traditional toxicity testing and highlight the need to better understand fundamental physiological processes in this ecBe associated with species described as being sensitatic taxa did not generally exhibit this effect in aquatic insects. Ag only reduced These results help explain the lack of insect responses to dissolved metal expoologically important faunal group.