ELECTRIC AND MAGNETIC SENSES 



521 



other reasons) to another site. Surprisingly, this did not change the sharks' 

 early-morning preference. However, when we roughly neutralized the earth's 

 ambient magnetic field with two large coils mounted to the outside of the 

 tank, the animals apparently lost their sense of position and distributed 

 themselves randomly. 



IN BODY TISSUES: 



(yx§h-/o J)-ds-^0 



(SEAWATER SHORT- 

 CIRCUIT) 



A 



AMP2 IN AMPULLARY 

 AMP1 CANALS 



(vx|h)-d5 1 



P0RE1 



VENTRAL 

 i 



w 



Y *B h 



B 



Figure 7 (A) Cross section of a shark heading east with a velocity v in a magnetic field 

 of horizontal induction B ;] . In the shark, a ventro dorsal voltage gradient v X B h is 

 induced. This in turn causes an electrical current of density J to flow: ventro-dorsally 

 within the moving fish and back through the stationary environment. With p the 

 resistivity along its path, the current develops an ohmic voltage gradient -pJ opposing the 

 induced voltage gradient v X B h . Since the fish is virtually short-circuited by the highly 

 conductive seawater environment, the average -pJ of the body tissues effectively 

 counteracts the v X B h , and the dorso-ventral potential difference tends to zero. In the 

 blind ampullae of Lorenzini, however, -pJ is negligibly weak. That is, v X B ;i remains 

 uncompensated and, integrated along the ampullary canals, gives rise to the pertinent 

 electrical stimuli across the sensory epithelia that form the bases of the ampullae proper. 

 (B) Successive stages of a shark circling through the compass directions. (After Kalmijn 

 1974.) 



