Well it is Shark Week on Discovery Channel and this made me think of an interesting discussion with my students in my sensory physiology course this past winter.   They were interested in electroreception, specifically how it works and also why we don’t have this sensory system in our repertoire.  Electroreception is the biological ability to perceive electrical stimuli in the environment. This ability has only thus far been found in aquatic or amphibious vertebrates.  Water is a much better conductor of electricity than air, which might be part of the explanation as to why such a sensory system evolved in only aquatic animals.

Electroreceptive animals use this sense to locate electrically conductive objects near them.  This is important in environments where the animal cannot depend on vision such as in caves, deep or murky water, and at night.  Some animals also use these electric fields to detect buried prey via the electrical field given off by muscle activity (i.e. cardiac activity).  They can detect electric fields as weak as 100th million of a volt.

Sharks and rays rely heavily on electroreception in the final stages of their attacks, as can be demonstrated by the robust feeding response elicited by electric fields similar to those of their prey.  In fact, the metal used in shark cages gives off an electric field that attracts sharks, which is helpful when studying sharks.  But sometimes electric fields aren’t so helpful, for example a problem arose with the early submarine telegraph cables that were damaged by sharks that sensed the electric fields produced by these cables. Sharks thus far are the most electrically sensitive animals observed.

The electric field sensors of sharks are called the ampullae of Lorenzini (discovered by Stefano Lorenzini who happened to be a physician and marine biologist).  They consist of electroreceptor cells (they are modified mechanoreceptors) connected to the seawater by pores on their snouts and other zones of the head.  Each ampulla is a bundle of sensory cells that are innervated by several nerve fibers.  These fibers are enclosed in a gel-filled tubule that has a direct opening to the surface through a pore.  The gel (a glyco-protein based substance) has electrical properties similar to a semiconductor, that also allows for temperature changes to be translated into electrical information that the shark can use to help detect temperature gradients.  Recent research suggests that the ampullae may allow sharks to detect changes in water temperature which can influence migration patterns and depth movements.  It has also been demonstrated that strong magnets may in fact deter sharks from biting.  It has been proposed to be a possible way to avoid by-catch of sharks in fishing nets and may even  serve as a protective measure for surfers.  Further testing is needed, but interesting results thus far.

Enjoy Shark Week!