Sharks have tiny pores on their snout which are used to detect electric fields. These pores, called ampullae of Lorenzini, go down into a canal filled with mucus and is full of glucopolysaccharides which helps to direct signals to a particular receptor cell at the base of the canal. Electroreceptor cells are similar to that of the lateral line system, used by many fishes to detect current flow, in that it is inervated at cranial nerve 7.
These pores were once thought to detect salinity, since when salt was poured onto them, they caused sporadic movement in the sharks. Stefano Lorenzini later discovered that rather than salinity, these were detecting electrc fields. Later tests by Ad Kalmijin later confirmed this discovery. The actual sensitivity of this electroreception sense is very important. A common idea is that the shark uses this to sense muscle contractions. While this is true, a shark can sense muscle contractions as the cells depolarize and contract, a shark can sense an even more sensitive mechanism. Ampullae of Lorenzini are actually fine tuned to detect the ions passing through the gills of other fish. Thus, when a shark is up close to a fish and unable to see it due to the position of its eyes, the shark can still "sense" the fish and eat it.
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