512 ELECTRICAL SENSES 



be due to the mechanical properties of the polyethylene film, as even the 

 stiff agar roof to which the film was added failed to appreciably weaken the 

 feeding response. Hence, the sharks and skates had not detected the agar- 

 screened prey merely by visual, chemical, or mechanical cues. 



By the same token, the all-or-none effect of the polyethylene film was 

 fully in line with the hypothesis of electrical prey detection. The film 

 offered an extremely high electrical resistance, whereas the agar layer did not 

 distort the flounder's bioelectric field to any extent. To provide direct 

 evidence, however, I simulated the presence of the flounder by passing an 

 electrical current of biological strength between two salt-bridge electrodes 

 hidden in the sand. After odor motivation, the sharks and skates, as ex- 

 pected, displayed the same characteristic feeding behavior with respect to 

 the electrodes (whether or not covered with agar) as they did to the actual 

 prey (Figure 2E). They dug tenaciously at the source of the field, responding 

 again and again when coming across the electrodes. Similar results were 

 obtained in the leopard shark (Triakis semifasciata), the lemon shark 

 (Negaprion breuirostris), the smooth dogfish (Mustelus canis), the thornback 

 (Platyrhinoidis triseriata), the small skate (Raja erinacea), the round stingray 

 (Urolophus halleri), and the freshwater stingray (Po tamo try gon circularis) 

 (Kalmijn 1974, in Szabo et al. 1972, and unpublished). Thus, these 

 experiments not only established the elasmobranch's electric sense, but also 

 signified its great biological importance in the animals' daily life. 



FIELD OBSERVATIONS ON CAPE COD SHARKS 



The earlier experiments were all performed on captive sharks, skates, and 

 rays in polyvinyl and fiberglass pools under well-controlled laboratory 

 conditions. In the ocean, the situation is more complex, as the electric fields 

 of prey mingle with those of physical and chemical origin (Kalmijn 1974). 

 Therefore, the author (1977a, b) has sought to verify the laboratory results in 

 tests on wild specimens roaming freely in their natural habitat. It appeared 

 very difficult, however, to approach the animals without introducing into 

 the environment galvanic fields or other perturbations likely to interfere 

 with the fish's normal behavior. After all, the electrical sensitivity of sharks 

 and catfish had first manifested itself in the animals' unusual responses to 

 metallic objects in contact with the water. 



During the summer of 1976, 1 learned from longline fishing off Cape Cod, 

 Massachusetts, that the smooth dogfish (Mustelus canis) regularly frequents 

 the shallow, inshore waters of Vineyard Sound on its nightly feeding 

 excursions. This predatory shark is a warm-season visitor arriving at Woods 

 Hole in May and leaving for the South again in late October or shortly there- 

 after. It is an active bottom hunter, preying on small fish as well as crustaceans 

 and other invertebrate animals. The females reach an average length of 1.15 

 m; the males are somewhat smaller. The smooth dogfish is truly viviparous; 

 the newborn measure 29 to 37 cm (Bigelow and Schroeder 1953). 



