The earth emits weak electrical and magnetic forces (which compasses detect to tell us which way we are heading). While the electrosensory systems of most fish cannot detect these forces, the systems of elasmobranchs are sensitive enough to detect these electrical fields. It is hypothesized that sharks use the earth’s magnetic fields to navigate in the water. (As a note, this has not been scientifically proven yet as is purely a hypothesis. There is much debate surrounding the proposed compass sense). The earth’s magnetic pulls, ocean currents, and other factors all contribute to the electrical fields that are found in the oceans. It is believed that sharks have an ‘internal compass’ and can determine their heading by the forces surrounding them. In 1990, Carey and Sharold tracked migrating blue sharks off the Atlantic coast of the U.S. and found that they follow very straight courses for hundreds of kilometers. In 1993, Kimley tracked hammerhead sharks and found that the sharks follow a route that follows the magnetic pattern of the sea floor. Evidence like this supports the idea that sharks navigate using the earth’s electrical forces (Paulin, 1995).
Researchers have proposed that sharks have two different compass senses, an active and a passive sense. When sharks are determining their direction based on the flow of water, they are using their passive sense. When the sharks are using the earth’s magnetic fields, this is considered the active sense. It is considered active because they use the electrical forces emitted by their swimming motions and compare them to the earth’s magnetic forces to determine their heading. Sharks use both the active and passive sense simultaneously, thus enabling them to determine their drift with the ocean currents as well as the magnetic ‘compass headings’ (Kalmijn, 2000).
|STRUCTURE OF THE AMPULLAE|
|THE FUNCTION OF THE GEL IN THE AMPULLAE|
|THE COMPASS SENSE|
|NEURO-ECOLOGY AND PERIPHERAL MORPHOLOGY|
|ELECTRICAL SHARK DETERRENTS|