Electric Organs and Their Control

It has been known for more than 2000 years that some fish can induce painful, numbing sensations when handled. In the late 1600s, it was discovered that this sensation resulted from the production of strong electrical currents produced by the fish (Kellaway 1946). In On the Origin of Species by Means of Natural Selection (1859), Darwin paid special attention to difficulties with his theory, devoting an entire chapter to the subject. The evolution of electrogenesis was one such area of concern. He noted the difference in structure, innervation, and electrical power of different electric organs (EOs), and the absence of geological evidence for a possible common ancestor between the different taxa exhibiting EOs. He concluded that rather than being homologous structures, the EOs of different fishes should be considered analogous structures that evolved independently. He also raised questions about potential functions for weak EOs. Although electric emissions from weakly electric fish had not yet been discovered, Darwin believed that a functional, weak EO was a necessary step in the evolution of strong EOs. About a century later, the rigorous study of a fish that was found to produce weak electric fields provided answers to such questions (Lissman 1951). Behavioral experiments clearly demonstrated that weak EOs function in electrosensory imaging of the environment (active electrolocation) and communication (Lissman 1958; Lissman and Machin 1958). Since these initial discoveries, there has been a wealth of research into the biology of active electrosensory and electromotor systems.