Electrolocation and jamming avoidance in the electric fishGymnarchus niloticus (Gymnarchidae, Mormyriformes)

Abstract

The African electric fish, Mormyriformes, evolved electric organs and electroreceptors independently from the South American electric fish, Gymnotoidei. Like other electric fish,Gymnarchus is able to electrolocate objects which differ in conductivity from the surrounding water. Its performance in electrolocation can be measured by different behavioral methods (Figs. 1, 2 and 3). Similarly to the gymnotoid fishEigenmannia, Gymnarchus fires its electric organ in a nearly sinusoidal manner at a fairly stable frequency near 300 to 400 Hz, with considerable interindividual variation. Electrolocation deteriorates when sinusoidal signals of sufficient intensity approach the frequency of the animal's electric organ discharges (EODs) (Figs. 4 and 5).Gymnarchus improves its performance by shifting its EOD frequency away from stimulus frequencies and thereby maintaining differences of at least 4 Hz. Thus, in another example of convergent evolution,Gymnarchus andEigenmannia have developed the same behavioral strategy to protect their electrolocation ability from jamming signals.