518 



ELECTRICAL SENSES 



through the earth's magnetic field, sharks, skates, and rays induce electric 

 fields that depend on the directions in which they are heading. Because the 

 induced voltage gradients are well within the dynamic range of the 

 elasmobranchs' highly sensitive electroreceptor system, they may constitute 

 the physical basis of an electromagnetic compass sense (Figure 6). Since in 

 this instance the fish themselves generate the fields they detect, this is a form 

 of active electro-orientation. 



I have elaborated on the physical principles of electromagnetic orientation 

 in previous papers (Kalmijn 1973, 1974). Here, I will only briefly review the 

 special case of active electro-orientation (Figure 7A). When the fish swims to 

 the east and thus crosses the horizontal component of the earth's magnetic 

 field, it generates— according to Faraday's law— an internal, dorsoventral emf 

 of induction 



f 



(v X B h ) • ds, 



with v the swimming velocity, B h the horizontal component of the earth's 

 magnetic induction, and s any internal path from the ventral (V) to the 

 dorsal (D) surface of the animal. The induced emf gives rise to electrical 

 currents that flow ventrodorsally through the moving fish and loop back 

 through the stationary environment (with respect to which the motion takes 

 place). 





MAGNETIC 

 NORTH 



N 



INDUCED ELECTRICAL 

 CURRENTS 



HORIZONTAL COMPONENT 

 OF EARTH'S MAGNETIC FIELD 



VELOCITY OF 

 EASTBOUND SHARK 



Figure 6 A shark swimming through the earth's magnetic field induces electric fields 

 that provide the animal with the physical basis of an electromagnetic compass sense. 

 (From Kalmijn 1974.) 



