ABSTRACT
Electric organs have evolved eight times in different taxa of both cartilagenous (Torpediniformes, Rajiformes) and bony fishes (Mormyroidea, Gymnotiformes, Siluriformes, Perciformes). Due to the convergent evolution there general traits which characterize the electric organs, but on the contrary there are specific traits, e.g. position of the organs, embryological origin etc., which are only found in single taxa. Based on the amplitude of the electric discharge produced, strong (ca. 30–800 V) or weak electric fish (mainly ca. 1 V) are distinguished. The strong electric fish use their electric discharge for prey capture and defense; the weak electric fish use the discharge in the context of object detection (orientation) and electric communication. All electric organs are derived from muscle tissue; however, in one case, in the Apteronotid family of the Gymnotiformes, the electric organ is derived from modified axons of electromotor neurones, located in the spinal cord. In some taxa (Gymnotiformes, Mormyridae) larval electric organs exist, which degenerate later during ontogeny.
