Sodium Balance of Amphibian Larvae Exposed to Low Environmental pH

Abstract

Sodium influx and efflux and body ionic contents of amphibian larvae were measured during acute and chronic exposures to acidic and neutral waters. Acute exposure to low pH's (2.5-4.0) depressed sodium influx and markedly accelerated sodium efflux. The resulting net loss of 50% of body sodium was fatal. Increasing the external calcium concentration extended survival time by slowing the loss of sodium. Chronic exposure to low pH caused a 21%-62% reduction in body sodium content; potassium content did not change. It also caused sodium influx to increase and efflux to decrease in comparison with fluxes measured during acute exposure at the same pH. Interspecific sensitivity differences were related to the relative magnitude of disruption in sodium balance. The three species of tadpoles studied differed in body sodium content under control conditions. Initial body sodium content was inversely correlated with acid tolerance and may represent a previously unknown mechanism for coping with physiologically stressful low-pH environments.