The ampullae detect
electric fields in the
water, or more precisely the difference between the
voltage at the skin pore and the voltage at the base of the electroreceptor cells. A positive pore stimulus would decrease the rate of
nerve activity coming from the electroreceptor cells and a negative pore stimulus would increase the rate of nerve activity coming from the electroreceptor cells.
Sharks may be more sensitive to electric fields than any other animal, with a threshold of sensitivity as low as 5 nV/
cm. That is 5/1,000,000,000 of a volt measured in a centimeter-long ampulla. Since all living creatures produce an electrical field in
muscle contractions, it is easy to imagine the shark may pick up weak electrical stimuli from the muscle contractions of animals, particularly prey. On the other hand, the electrochemical fields generated by paralyzed prey were sufficient to elicit a feeding attack from sharks and rays in experimental tanks, therefore muscle contractions are not necessary to attract the animals. Shark and rays can locate prey buried in the sand, or
DC electric dipoles simulating the main feature of the electric field of a prey buried in the sand.
The electric fields produced by oceanic currents moving in the magnetic field of the earth are of the same order of magnitude as the electric fields that sharks and rays are capable of sensing. Therefore, sharks and rays may orient to the electric fields of oceanic currents, and use other sources of electric fields in the ocean for local orientation. Additionally, the electric field they induce in their bodies when swimming in the magnetic field of the
earth may enable them to sense their magnetic heading.