520 ELECTRICAL SENSES 



for the ventral and dorsal sense organs, respectively. Since the ampullae of 

 Lorenzini act as high-ohmic voltmeters (Waltman 1966; Kalmijn 1974), 

 there is practically no flow of current through their highly conductive 

 canals, and the emfs induced between the skin pores and the blind ampullary 

 endings develop without appreciable ohmic loss. Hence, across the sensory 

 epithelia forming the bases of the ampullae proper, potential differences close 

 to 



pamp 1 

 pore 1 



(v XB h ) - ds 1 



and 



J pore 2 

 (vXBfc) -ds 2 



appear. When the fish turns north or south, the potentials vanish; when the 

 fish turns west (instead of east), potentials of opposite polarity are induced 

 (Figure 7B). As these potentials are detectable to the elasmobranchs at 

 calculated swimming speeds as low as 2 cm/s, they prove the feasibility of 

 the proposed electromagnetic compass sense. Note that through interaction 

 with the vertical component of the earth's magnetic field the fish also induce 

 motional electric fields parallel to their transverse body axes. By additionally 

 sensing these fields with the laterally oriented ampullae of Lorenzini, the 

 elasmobranchs may determine the magnetic latitude of their position on the 

 globe as well. 



The first and simplest magnetic tests were performed on the leopard shark 

 (Triakis semifasciata) in outdoor fiberglass pools at the Scripps Institution of 

 Oceanography, La Jolla, California, in collaboration with Dr. Theodore H. 

 Bullock. With the fish steadily swimming along the circumference of their 

 circular habitat, we introduced a local magnetic field into the water by 

 passing an electrical current through a small induction coil (20 cm in 

 diameter) held outside the tank. The field was turned on when the animals 

 were at the far side of the pool. At that distance, the magnetic strength was 

 too low for the sharks to respond, and they quietly continued their lap. 

 Seconds later, however, on swimming into the region of the coil, the sharks 

 suddenly turned away from the imposed field and veered off to the center of 

 the tank, even though the coil did not distort the earth's ambient magnetic 

 field by more than 25 percent (Kalmijn 1973). 



Next, I noticed each morning the leopard sharks rest at a particular 

 location along the circumference of the tank in a sector a little off magnetic 

 north. To eliminate the possibility of visual orientation, we covered their 

 7-m pool with a large sheet of black plastic. We also took all experimental 

 structures out of the water, rotated the whole setup, and even moved it (for 



