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, approximately 10.0; alkalinity, approximately 380 mmol L(-1); Na(+), approximately 350 mmol L(-1); Cl(-), approximately 110 mmol L(-1); osmolality, approximately 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 CO(2) did not change with environmental dilution. Routine oxygen consumption (Mo(2)) was extremely high in 100% lake water but decreased by 40%-68% after acclimation to dilute lake water. At every fixed swimming speed, Mo(2) 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.
Wood
Chris M
CM
Department of Biology, McMaster University, Hamilton, Ontario L8S 4K1, Canada. woodcm@mcmail.cis.mcmaster.ca
Wilson
Paul
P
Bergman
Harold L
HL
Bergman
Annie N
AN
Laurent
Pierre
P
Otiang'a-Owiti
George
G
Walsh
Patrick J
PJ
D000222:Adaptation, Physiological; D000641:Ammonia / Q000378:metabolism; D000818:Animals; D001774:Blood Chemical Analysis; D002712:Chlorides / Q000097:blood; D004734:Energy Metabolism; D005618:Fresh Water / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007630:Kenya; D009994:Osmolar Concentration; D010101:Oxygen Consumption; D012964:Sodium / Q000097:blood; D013550:Swimming; D017210:Tilapia / Q000097:blood / Q000378:metabolism / Q000502:physiology*; D013997:Time Factors; D014508:Urea / Q000378:metabolism*
D002712:Chlorides; D000641:Ammonia; D014508:Urea; D012964:Sodium