The sharks electrical sense

A long time ago in 1678 Stefano  Lorenzini an Italian anatomist discovered small pores around the mouth and nose of sharks and rays.  By  peeling of the skin, he also noticed that these black openings were connected with transparent tubes fillled with a sort of cystalline gel.  

But it took almost  three centuries to fully understand the anatomy and function of these small pores and tubes.  That they transmitted electrical information via ampullae (small bulbar pouches filled with jelly) and nerve fibers to the medulla, a sensory station and  then to  higher  centers  in the sharks brain.  Unlike the lateral line, were the receptors are small hair cells sensitive to the movement of water.  In 1960  Murray  from the University of Birmingham in the UK made an important step forward. He used  new electrophysiological  techniques  to measure the activity of nerve cells and found out  that cells in the ampullae  were sensitive to  slight variations in salinity and electrical fields,  in the order of   one millionth of a  volt across a  square centimeter of seawater. In  1971  Adrianus Kalmijn  from the University of Utrecht in the Netherlands (who later moved to  the Scripps Institution of Oceanography), discovered with very sensitive amplifiers that sea animals also  produced bio-electric fields in seawater. Kalmijn also found out  that sharks in the aquarium were able  to detect small electrodes buried in the sand that  emitted very weak  electrical fields.  Then, the link between the two findings became clear:  electoreception could help the shark with locating  its prey. Its possible that in some shark species  electreception also serves  as a compass during migration, by using the earth magnetic fied as a reference.

The next (never say final!) step was to find out  how  sharks  actually  used their  remarkable  electrical sensory sensitivity  to detect and attack  their  prey.  In 1981 Kalmijn, Douglas Fieldsa and Gail Heyer described how electroreception could work in combination with  the other five senses during normal feeding. Smell and hearing would be most useful for locating prey from great distances. Vision, lateral line senses and taste would become more important at  closer ranges. Only during  the terminal phase of an attack, so  within a meter from the prey, electroreception becomes important to precisely locate the prey and correctly orient its jaws. For example the Hammerhead then will know where to locate and how to grab its favorite prey the stingray buried under the sand.

Sources:

The Sharks  Electrical sense. R. Douglas Fields. August 2007, Scientific American, Inc

The Electric Sense of Shark and Rays. A. J. Kalmijn in Journalof Experimental Biology, Vol. 55,pages 371–383; 1971.

Electroreception. T. H. Bullock, C. D. Hopkins, A. N. Popper andR. R. Fay. Springer Press, 2005.