510 ELECTRICAL SENSES 



1974). In fish, for example, the mucous membranes lining the mouth and 

 the gill epithelia in the pharynx give rise to steady d.c. fields, usually 

 modulated by ventilatory movements. Externally, the bioelectric fields are 

 of a distributed-dipole configuration and accordingly fall off steeply with 

 increasing distance. Yet, the voltage gradients in the vicinity of small fish and 

 wounded crabs often measured over 0.01 juV/cm at distances up to 25 cm. 

 As these fields exceeded the elasmobranch threshold sensitivity, they 

 strongly suggested the possibility of electrical prey detection. 



To substantiate the electrical aspects of predation, I decided to analyze 

 the feeding responses of the shark Scyliorhinus canicula and the skate Raja 

 clavata to small specimens of the flounder Pleuronectes platessa (Kalmijn 

 1971). The prey fish was carefully introduced into the seawater habitat, and 

 after it had hidden itself in the sand a few drops of whiting extract were 

 spread diffusely throughout the water. Aroused by the odor, the sharks and 

 skates began within seconds to haphazardly search the bottom of the pool. 

 When they came within 10 to 15 cm of the flounder, they made well-aimed 

 dives at the prey, uncovered it from under the sand, and devoured it 

 voraciously (Figure 2A). Next, I enclosed the flounder in a flat agar chamber 

 to conceal the prey visually, chemically, and mechanically without impeding 

 its bioelectric field. (Dissolved in seawater, 3- to 4-percent agar makes a rigid 

 structure that is virtually transparent to electrical current.) The agar chamber 

 was placed on the bottom of the pool, just under the surface of the sand. To 

 keep the flounder alive, the chamber was ventilated with a steady flow of 

 seawater. Despite these changes, the sharks and skates again made well-aimed 

 attacks from the same distance and in the same frenzied manner as when 

 no agar was screening the prey (Figure 2B). These results were in full accord 

 with the assumption that sharks and skates can locate prey bioelectrically. 



To prove that the 1-cm-thick roof of the agar chamber presented an 

 adequate barrier to odor stimuli, I substituted cut pieces of whiting for the 

 live flounder. The whiting formed the regular diet of the sharks and skates, 

 and was kept frozen for several days before use. This time, though strongly 

 motivated by the odor of the seawater flow ventilating the chamber, the 

 predators did not respond at all to the food when swimming over the agar 

 structure (Figure 2C). Then, I put the flounder back in the chamber and 

 covered the whole structure with a very thin, electrically insulating film of 

 polyethylene. Under these circumstances, the sharks and skates did not 

 respond to the live prey either, though they eagerly searched and often 

 passed right over it (Figure 2D). This dramatic effect could not conceivably 



Figure 2 In feeding, the shark Scyliorhinus canicula cues in on the bioelectric field of 

 its prey. (A) Motivated by the diffuse odor of fish extract, Scyliorhinus shows well-aimed 

 attacks at a small flounder (Pleuronectes platessa) hiding in the sand (solid arrow). 

 (B) With the flounder enclosed in an agar chamber to conceal the fish visually, chem- 

 ically, and mechanically without impeding its bioelectic field, Scyliorhinus continues 

 to dive at its prey (which is kept alive by ventilating the chamber with a flow of sea- 

 water: dashed arrows). (C) However, with cut pieces of fish instead of the live 



