Physiological and morphological correlates of extreme acid tolerance in larvae of the acidophilic amphibian Litoria cooloolensis

Abstract

In the coastal sandy lowlands of east Australia, several anuran species including the Cooloola Sedgefrog Litoria cooloolensis, show remarkable tolerance to dilute highly acidic waters as low as pH 3.5. To investigate the physiological and morphological underpinnings of acid tolerance in L. cooloolensis larvae, we compared Na+ balance, uptake and efflux rates, and gill and skin morphology in larvae reared in circum-neutral (pH 6.5) and pH 3.5 water. We hypothesised that acute exposure to pH 3.5 water would cause an initial loss of ionic homeostasis in larvae, but with chronic exposure larvae would restore Na+ balance. Net Na+ flux rates were not significantly different from zero in larvae reared at pH 3.5 and in acid-na&iumlve animals maintained in pH 6.5 water. Animals reared at pH 6.5 and acutely exposed to pH 3.5 exhibited a net loss of Na+, due to a significant inhibition of Na+ uptake. In contrast, L. cooloolensis larvae reared at pH 3.5 maintained Na+ balance at pH 3.5 and did not exhibit inhibition of Na+ uptake at this pH. Investigation of Na+ transport kinetics and the morphology of the gills and integument suggests tolerance of L. cooloolensis larvae to low pH may be attributed to a high capacity for branchial Na+ uptake, increased tight junction length and elevated mucus production in the gills and integument. These factors confer resistance to acid damage and disruption of ionic homeostasis which would otherwise result in the death of larvae exposed to waters of pH 4.0 and less.