516 ELECTRICAL SENSES 



one night they succeeded. When the current was switched to the other pair 

 of electrodes, the animals would let go, circle about for a while, and strike 

 again— this time at the electrodes on the other side of the odor source. Over 

 the summer, we recorded several hundred attacks, many of them in 

 double-blind fashion, leaving no doubt as to the validity of our observations. 

 At lower current levels the sharks kept responding, though from increasingly 

 shorter distances. 



These new field data clearly demonstrate that odor-motivated sharks can 

 detect and take prey by the exclusive use of their keen electric sense, not 

 only under favorable laboratory conditions, but also in their electrically 

 more intricate oceanic milieu. This, of course, does not imply that other 

 sense organs, such as the acustico-lateralis system, may not also play an 

 important part in nocturnal predation (cf. Myrberg 1978). Although in our 

 studies odor cues attracted the sharks from a distance, at short range the 

 bioelectric fields proved much more compelling to the animals. When 

 following an odor trail, the sharks obviously search for more precise infor- 

 mation to accurately locate their prey and, in the eventual attack, home in 

 on their victim's bioelectric field to seize it unerringly with one quick move. 



Behaviorally, elasmobranchs respond most readily to d.c. fields. Electro- 

 physiologically, however, the ampullae of Lorenzini are not true d.c. 

 receptors, though they do detect frequencies as low as 0.1 Hz (Murray 

 1962). That is, to sense the prey's d.c. field, elasmobranchs must move with 

 respect to their prey. Alternatively, they may detect the low-frequency 

 components that accompany the prey's ventilatory, fin, and body move- 

 ments. In fact, aroused by odor, sharks and skates zero in on d.c. as well as 

 low-frequency dipole fields of biological strength (Kalmijn 1971). The 

 behavioral frequency range corresponds well with the array of biological 

 stimuli, the physiological properties of the sense organs, and the physical 

 characteristics of the accessory structures (Murray 1962, Waltman 1966, 

 Kalmijn 1974). Furthermore, independent of the angle of approach, the 

 elasmobranchs aim directly at their prey, evidently deriving its location from 

 the spatial configuration of the animal's bioelectric field. 



Besides sharks, other fish, among them the American eel Anguilla rostrata, 

 visited the test site. The eels often nibbled at the opening of the chumming 

 tube, but they paid no attention to the current-passing electrodes. Their 

 behavior was particularly noteworthy, since the American eel had been 

 reported to exhibit transient cardiac decelerations when subjected to weak 

 electric fields (Rommel and McCleave 1972, 1973) similar to those observed 

 in elasmobranch fishes (Kalmijn 1966). The eels' responses, however, have 

 not been independently confirmed (Enger et al. 1976), nor are these fish 

 known to have specific electroreceptors (Leonard and Summers 1976). In 

 predation, they evidently rely more on chemical cues. On the other hand, 

 the catfish Ictalurus nebulosus again attacked the prey-simulating electrodes 

 when tested in a local freshwater pond (at dipole moments of 0.5 to 4 /jlA X 

 1 cm at 19 k£l • cm). 



For the 1977 season, we have outfitted a modified fiberglass 21-ft Boston 

 Whaler for behavioral studies on the electrical sensory performances of not 



