Obligatory Urea Production and the Cost of Living in the Magadi Tilapia Revealed by Acclimation to Reduced Salinity and Alkalinity

Alcolapia grahami is a unique ureotelic tilapia that lives in the highly alkaline, saline Lake Magadi, Kenya (pH, ~10.0; alkalinity, ~380 mmol L-1; Na+, ~350 mmol L-1; Cl-, ~110 mmol L-1; osmolality, ~580 mosm kg-1). The fish survived well upon gradual exposure to dilute lake water (down to 1%, essentially freshwater). Urea excretion continued, and there was no ammonia excretion despite favorable conditions, indicating that ureotelism is obligatory. Levels of most ornithine-urea cycle enzymes in the liver were unchanged relative to controls kept for the same period in 100% lake water. The fish exhibited good abilities for hypo- and hyperregulation, maintaining plasma Na+, Cl-, and osmolality at levels typical of marine and freshwater teleosts in 100% and 1% lake water, respectively. Plasma total CO2 did not change with environmental dilution. Routine oxygen consumption (Mo2) was extremely high in 100% lake water but decreased by 40%-68% after acclimation to dilute lake water. At every fixed swimming speed, Mo2 was significantly reduced (by 50% at high speeds), and critical swimming speed was elevated in fish in 10% lake water relative to 100% lake water. Osmotic and Cl- concentration gradients from water to plasma were actually increased, and osmotic and Na+ gradients were reversed, in 10% and 1% dilutions relative to 100% lake water, whereas acid-base gradients were greatly reduced. We suggest that approximately 50% of the animal's high metabolic demand originates from the cost of acid-base regulation in the highly alkaline Lake Magadi. When this load is reduced by environmental dilution, the energy saved can be diverted to enhanced swimming performance.