514 



ELECTRICAL SENSES 



To present the sharks with electrically simulated prey, two pairs of 

 seawater salt-bridge electrodes were tied to the polypropylene line and 

 positioned on the sand, one on either side of the odor source and 25 cm 

 from it (Figure 4). Mecca underwater plugs with stainless steel pins and 

 integral cables connected the seawater-agar content of the 30- to 90-cm-long 

 Silastic salt-bridge tubes (3.2 mm I.D.) to the electrical stimulator setup in 

 the rubber raft. The use of a constant-current source and salt-bridge 

 electrodes practically eliminated the adverse effects of polarization at the 

 stainless-steel/seawater-agar interfaces. From the raft, we could conveniently 

 vary the strength of the field and select the pair of electrodes to be energized 

 while the other pair functioned as the control. The applied d.c. dipole 

 moments ranged from 1 to 8 fiA X 5 cm (dipole current X distance between 

 electrodes), approximately corresponding to the bioelectric fields of small 

 prey at a seawater resistivity of 20.0-21.5 12* cm and a temperature of 

 20-22°C (Kalmijn 1971). 



After entering the test area, the dogfish began randomly searching the 

 sand, evidently trying to locate the odor source. Both young and mature 

 sharks were observed, sometimes alone, sometimes in groups of two to five. 

 Neither the raft nor the underwater light appeared to disturb them. Most 

 interestingly, when nearing the underwater setup, the animals did not bite at 

 the opening of the chumming tube, but turned sharply toward the current 



CONTROL 



25 CM 



25 CM 



(B) 



swa 



Figure 4 (A) Polypropylene line with linear array of chumming the tube and salt-bridge 

 electrodes. Plastic markers indicate the positions of the odor source (os) and electrode 

 openings (el' and ell"). (B) Two of the underwater plugs connecting the salt-bridge tubes 

 to the electrical cables. The stainless-steel pins (ssp) are in direct contact with the seawater- 

 agar content (swa) of the salt-bridge tubes. 



