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 the most 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 °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 °C and night-time lows of 32 °C. Another population (Fish Springs Lagoon, FSL) lives in a lagoon with fairly stable daily temperatures (33–36 °C). The upper critical temperatures (Ct max ) of both populations are very high; moreover the SWHS tilapia exhibit the highest Ct max (45.6 °C) ever recorded for a fish. Routine rates of O 2 consumption (MO 2 ) measured on site, together with MO 2 and swimming performance at 25, 32, and 39 °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.