The electric sense of sharks and rays.

In 1917 Parker and van Heusen published their historic paper on the behavioural responses of the catfish, Amiurus nebulosus, to metallic and non-metallic rods. They found a blindfolded Amiurus to be remarkably sensitive to metallic rods, regularly responding to them even at a distance of some centimetres, whereas a glass rod did not elicit a reaction until it actually touched the skin of the animal. When much of the metal was exposed to the water, the fish swam away from it; when little was exposed, the fish turned towards the rod and often nibbled it. In a series of simple but convincing experiments Parker and van Heusen demonstrated that these responses were due to the galvanic currents generated at the interface between metal and aquarium water. The same responses could be obtained by sending a weak direct current into the water via two electrodes, kept 2 cm apart. In this case approach and nibbling were called forth by a current of a little less than 1 fiA and the avoidance reactions by currents of 1 fiA or more. Parker and van Heusen did not realize the significance of their results, but in fact they were studying the electrosensitivity of fishes that have as we now know distinct electroreceptors. Curiously enough, the electroreceptors of Amiurus were not identified experimentally and investigated electrophysiologically until very recently (Dijkgraaf, 1968; Roth, 1968, 1969). Even now, we can only speculate about the biological meaning of the electrosensitivity of the catfish. Long after the work of Parker and van Heusen, in the year 1951, a new impetus to the study of the electrosensitivity was given by Lissmann. He examined the electric discharges that Gymnarchus niloticus and other weakly electric fish almost continuously emit, and proposed the theory that they play an essential role in an electrosensory mechanism for object location. These fish would be able to detect a nearby object of an electrical conductivity different from that of the surrounding water by appreciating the distortion it causes to their electric field. Later on, Lissmann proved Gymnarchus to be indeed very sensitive to purely electrical stimuli as implied by his theory, and also showed the fish to be able to discriminate between objects of only slightly different conductivity (Lissmann, 1958; Lissmann & Machin, 1958). Lissmann tentatively regarded the weakly electric fish as evolved from a pre-electric fish without electric organs but already sensitive to electric fields. He suggested that at this early stage the electrosensitivity might have been used to detect muscular potentials of prey, predators, members of the same species, and of the animal itself. Today,