ELECTRIC AND MAGNETIC SENSES 



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mormyrid and gymnotid fishes produce weak electrical discharges to actively 

 probe their environments (Lissmann 1951, 1958). However, which fields the 

 nonelectric sharks and catfish might detect in their surroundings remained an 

 open question at the time. 



Subsequent research at the University of Utrecht, the Netherlands, soon 

 disclosed that marine elasmobranchs are indeed remarkably sensitive to weak 

 electric fields (Kalmijn 1966). The author established transient cardiac 

 decelerations upon application of uniform electric fields by recording the 

 heartbeat of free-swimming skates {Raja clavata) with permanently im- 

 planted electrodes. The skates showed their cardiac reflex down to voltage 

 gradients as low as 0.01 /iV/cm, thus exhibiting the highest electrical 

 sensitivity known in the animal kingdom. In later behavioral tests, sharks and 

 skates appeared most responsive to frequencies in the range from (direct 

 current) to about 8 Hz (Kalmijn 1971, 1974). The receptors detecting these 

 weak, low-frequency electric fields are the ampullae of Lorenzini, delicate 

 sensory structures in the elasmobranchs' protruding snouts (Murray 1962, 

 Dijkgraaf and Kalmijn 1963) (Figure 1). 



Measuring the electric fields in the laboratory habitat of the sharks and 

 skates, I found that aquatic animals produce direct-current (d.c.) and 

 low-frequency voltage gradients in the water, which mainly stem from 

 potential differences at their skin-water interfaces (Kalmijn 1966, 1969, 



ROSTRUM 



NARIS 



Figure 1 Lateral-line canals and ampullae of Lorenzini in the head of the shark 

 Scyliorhinus canicula. The lateral-line canals (in heavy black) contain the mechanorecep- 

 tive neuromasts. They connect to the outside through linearly arranged skin pores (open 

 circles). The openings of the Lorenzinian ampullae (solid dots) form a more dispersed 

 pore pattern. Each gives access to an often long, jelly-filled canal (broken lines) ending in 

 a blind sensory swelling. The ampullae proper are clustered together in a rostral (I), an 

 infraorbital (II), and a small mandibular (III) capsule. By the technique of selective 

 denervation, the ampullae of Lorenzini were shown to be the receptors responsible for 

 the shark's behavioral reactions to weak electric fields (Dijkgraaf and Kalmijn 1963). 



