BRAIN AND BODY OXYGEN REQUIREMENTS OF GNATHONEMUS-PETERSII, A FISH WITH AN EXCEPTIONALLY LARGE BRAIN

Vertebrates have repeatedly been noted for having remarkably constant ratios of brain to body O-2 consumption, the brain using 2-8 % of rest ing body O-2 consumption, suggesting that evolution has put strict lim its on the energetic cost of brain function. Only man, with a value of 20 %, is an exception to this rule. However, the results presented he re suggest that, in the electric fish Gnathonemus petersii, the brain is responsible for approximately 60% of body O-2 consumption, a figure three times higher than that for any other vertebrate studied, includ ing man, The exceptionally high energetic cost of the G. petersii brai n appears to be a consequence both of the brain being very large and o f the fish being ectothermic. It was also found that G. petersii has a high ability to utilise O-2 at low levels. Thus, during falling [O-2] , this species was found to maintain both its O-2 uptake and its elect ric discharge rate down to an ambient O-2 level of 0.8 mgl(-1) (at 26 degrees C), although it was unable to tolerate an [O-2] below 0.3 mgl( -1). During severe hypoxia (<0.8 mgl(-1)), G. petersii attempted to gu lp air from the water surface. These results establish a new record fo r the energetic cost of a vertebrate brain and they show that the spec ies possessing such a brain has a high capacity for utilising O-2 at v ery low ambient concentrations.