The Effects of Reducing Water pH and Total CO2 on a Teleost Fish Adapted to an Extremely Alkaline Environment

Abstract

ABSTRACT The teleost fish Oreochromis alcalicus grahami is well adapted to an extremely alkaline environment (pH 10, total CO2 184 mmol I−1) in Lake Magadi, Kenya. O. a. grahami excretes all nitrogenous wastes as urea, and exposure to neutral water (pH7, total CO2<1mmol l−1) results in a complete inhibition of urea excretion. In the present study, we further characterized the physiological effects of transferring O. a. grahami from alkaline to neutral water. Exposure to neutral water resulted in a significant decrease in blood pH, a large reduction in plasma HCO3− levels, a severe impairment of swimming ability, an increase in Na+ influx, but no change in O2 consumption. A closely related species of tilapia, O. nilotica, living in neutral waters of the Sagana River, was relatively unaffected by acute exposure to tapwater at pH 10, but showed a marked alkalosis in Lake Magadi water prior to death. These results demonstrate that O. a. grahami thrive in an environment of high pH and bicarbonate/carbonate concentration, conditions that are fatal to other species adapted to neutrality. O. a. grahami, however, is extremely unusual amongst aquatic animals in its adverse response to neutral water.