Mammalian metabolic rates in the hottest fish on earth

The Magadi tilapia, Alcolapia grahami, a small cichlid fish of Lake Magadi, Kenya lives in one of themost challenging aquatic environments on earth, characterized by very high alkalinity, unusual water chemistry, and extreme O-2, ROS, and temperature regimes. In contrast to most fishes which live at temperatures substantially lower than the 36-40 degrees C of mammals and birds, an isolated population (South West Hot Springs, SWHS) of Magadi tilapia thrives in fast-flowing hotsprings with daytime highs of 43 degrees C and night-time lows of 32 degrees C. Another population (Fish Springs Lagoon, FSL) lives in a lagoon with fairly stable daily temperatures (33-36 degrees C). The upper critical temperatures (Ct(max)) of both populations are very high; moreover the SWHS tilapia exhibit the highest Ctmax (45.6 degrees C) ever recorded for a fish. Routine rates of O-2 consumption (MO2) measured on site, together with MO2 and swimming performance at 25, 32, and 39 degrees C in the laboratory, showed that the SWHS tilapia exhibited the greatest metabolic performance ever recorded in a fish. These rates were in the basal range of a small mammal of comparable size, and were all far higher than in the FSL fish. The SWHS tilapia represents a bellwether organism for global warming.