Ionoregulation in Daphnia Magna; Mechanisms of Major Ion Toxicity in Adults and Physiology of Ionoregulation in Juveniles

Abstract

Elevations in major ions (sodium (Na+), potassium (K+), calcium (Ca2+) and magnesium (Mg2+) cations paired with chloride (Cl-), sulphate (SO42-) and (bi)carbonate (HCO3-/CO32- ) anions) in freshwater environments through anthropogenic activities cause physiological disturbances in freshwater animals. Because these animals are adapted for active ion uptake to combat passive ion loss to the external environment, increases in ambient major ion concentrations can alter ionoregulation. There has been a concerted research effort into toxicity of major ions and the development of predictive models, which require extensive physiological data, to support the development of comprehensive regulations surrounding pollution by major ions. The branchiopod crustacean, Daphnia magna has been the focus of many of these toxicological studies. The physiological effects of elevated ambient major ion concentrations in adults were investigated. Transepithelial potential (TEP) and hemolymph ion concentrations were altered in animals exposed to elevated ion concentrations approaching previously described LC50 values. These changes in TEP and hemolymph ion concentrations are indicative of physiological disturbance and may be indicators of toxicity. Diffusional gradients and active ion pumps were found to contribute to TEP in D. magna, unlike in freshwater fish. Notable differences between adult and juvenile D. magna in sensitivity to ionic composition of the water and ionoregulation have been described. These differences suggest that both juveniles and adults should be considered in studies focused on monitoring major ion pollution. Ion transport through the nuchal organ in embryo and neonate D. magna was directly measured. Influx of Na+ and efflux of NH4+, H+ and Cl- was observed. K+ flux is dependent on developmental stage. The results from the evaluation of the physiological effects of increased ambient major ions in adults and mechanisms of ion transport in juveniles will aid in establishing environmental regulations for major ions in aquatic ecosystems.