ELECTRIC AND MAGNETIC SENSES 



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To observe the sharks' feeding behavior, I went to sea with my student 

 collaborators in an inflatable rubber raft (Zodiac Mark II) free of any metal 

 under the waterline. On station in 2.5- to 3.0-m-deep water over a sand patch 

 devoid of weeds, we attracted the sharks by pumping small amounts of 

 liquefied herring through a long Tygon tube (7.9 mm I.D.) that ran from the 

 raft to the bottom of the sound (Figure 3). The chumming tube was 

 attached to a polypropylene line 7.9 mm in diameter, suspended from a 

 Styrofoam float and stretched over the ocean floor between two polyvinyl 

 pipes anchored in low-profile cinder blocks. Starting after dark, we 

 illuminated the area with a 100-W, battery-operated, underwater light. To 

 look from the surface down into the water, we used a glass-bottomed 

 viewing box secured behind the stern of the raft. 



Figure 3 Electrical prey detection in the shark Mustelus canis: field setup in Vineyard 

 Sound, Massachusetts. After dark, the sharks' feeding behavior is observed from an in- 

 flatable rubber raft with the aid of a glass-bottomed viewing box and a sealed-beam under- 

 water light. The sharks are attracted and motivated to search for food by the odor of fish 

 extract that exudes from a chumming tube running from the raft to the bottom of the 

 sea. A weak electrical current is passed between two electrodes — either the ones to the 

 left (el ) or the ones to the right (el ) of the odor source (os) — to mimic the bioelectric 

 field of small prey, the other pair functioning as the control. Instead of biting at the 

 opening of the chumming tube, the sharks turn sharply toward the current-carrying 

 electrodes (el ) to attack the electrically simulated prey. (From Kalmijn 19776.) 



