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

Wood, C. M.
Wilson, P.
Bergman, Harold
Bergman, A. N.
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.
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