

508 ELECTRICAL SENSES 



As sharks, skates, and rays base their actions on the physical stimuli they 

 receive from the oceanic environment, a thorough knowledge of their 

 sensory abilities is necessary to a full appreciation of the behavior of these 

 ancient fishes. Certainly, underwater light, sound, and odor fields are quite 

 different from those on land. Moreover, elasmobranchs respond also to weak 

 electrical voltage gradients, which they detect with the ampullae of Lorenzini. 

 Thus, electroreception adds another, unique dimension to the sensory world 

 of the elasmobranch fishes. 



The biological significance of the animals' electrical sensitivity first 

 became evident in behavioral studies in which sharks, skates, and rays were 

 observed to cue in on bioelectric fields emanating from prey. Elasmobranchs 

 may also sense the electric fields of ocean currents flowing through the 

 earth's magnetic field, and use this faculty to follow the currents during 

 migration, as a daily means of orientation, and to compensate for passive 

 drift (passive electro-orientation). Eventually, by swimming through the 

 earth's magnetic field, the animals induce electric fields that may enable 

 them to establish their compass headings (active electro-orientation). Marine 

 stingrays have shown their ability to orient with respect to the earth's 

 magnetic field in recent training experiments. 



After a short review of the studies that led to the discovery of the 

 elasmobranchs' electric sense, this article describes new field observations on 

 the electrical aspects of predation and experiments on captive sharks and 

 rays involving geomagnetic orientation. For the morphology, physiology, 

 and physics of the receptor system, the reader is referred to Bennett and 

 Clusin (1978), Murray (1974), and Kalmijn (1974). 



THE DISCOVERY OF AN UNCONVENTIONAL 

 SENSORY MODALITY 



Studying the sensory behavior of the North American catfish Ictalurus 

 nebulosus, Parker and Van Heusen (1917) observed blindfolded specimens 

 react to the approach of metallic rods at distances of several centimeters, 

 whereas a glass rod did not elicit a noticeable response until it actually 

 touched the animals' skin. Through successive elimination of the physical 

 stimuli emanating from the metallic rods, the investigators convincingly 

 demonstrated that the distant responses were due to galvanic currents 

 generated at the interface between metal and aquarium water. On the verge 

 of revealing the electric sense of freshwater catfish, they nevertheless failed 

 to realize the biological implications of their novel finding. 



In 1934, Dijkgraaf also noticed a great sensitivity to metallic objects in the 

 dogfish Scyliorhinus canicula, a small, bottom-dwelling shark of the 

 Mediterranean and coastal Eurpoean waters. A quarter of a century later, 

 Kalmijn confirmed the electrical nature of the shark's response and 

 undertook to investigate its biological significance. (The results of both 

 studies were reported in Dijkgraaf and Kalmijn's joint paper of 1962.) 

 Previously, Lissmann had discovered that the African and South American 



