Sharks are literally wired for hunting. The finned predators of the high seas are equipped with a special sense called electroreception that allows them to home in on prey with deadly accuracy. Other members of the elasmobranch fish family -- rays and skates -- also share this trait, but sharks' electroreception abilities are the most finely tuned.
Electroreception simply means the ability to detect electrical currents. What does electricity have to do with sharks' underwater habitat? Any muscular movement or twitches in living animals and fish create small electrical currents. At hospitals, electrocardiogram machines track the electricity resulting from our heart beating.
Open air does not conduct this electricity away from our bodies, but thankfully for sharks, salt water does. Salt in salt water contains sodium and chlorine ions. Ions are particles that have an electrical charge because they have lost or gained an electron. In water, these sodium and chlorine ions in salt separate and move freely, transporting electricity.
You can compare this to how batteries work. It's set up like an electrochemical cell that separates the negatively and positively charged ions. When connected by a wire, those opposite charges attract, meaning the positive and negative particles flow toward each other to pick up or drop off electrons to become stable again.
A similar thing happens in the interaction of living cells and salt water. Because fish cells have a charge different from the saltwater solution in which they swim, the contact creates a weak voltage in the same way as a battery. Sharks can sense the tiniest changes in this electrical current, down to one-billionth of a volt [source: Fields]. If two AA batteries were connected 1,000 miles (1,600 kilometers) apart, a shark could detect if one ran out [source: Viegas].
How can sharks do that? Read about the part of sharks' bodies that regulate this unique internal homing device on the next page.
Electroreception for Hunting and Navigating
The source of sharks' electroreception lies around their snouts and lower jaws. If you look closely at a shark's face, you'll see tiny dots around its mouth that look like large blackheads. These vary in number depending on each species' hunting activity. Active sharks will have 1,500 or more, while the more sedentary ones have a few hundred [source: Parker].
The dots are open pores collectively called ampullae de Lorenzini. Filled with an electrically conductive jelly, the bottoms of the ampullae are lined with hairlike cells called cilia. Electrical currents travel through the jelly to the cilia. In humans, cilia inside of our ears alert our brains to noise when moved by sound waves. In sharks, the cilia respond to changes in nearby electrical currents transported by the jelly. The cilia trigger the release of neurotransmitters in sharks' brains, which tells them something alive looms close by.
The ampullae de Lorenzini compose part of sharks' lateral line. The lateral line is a sensory organ in many fish and amphibians that stretches down their sides from gills to tail. The long, hollow tube opens out into the skin at perforated scales. This system allows sharks to sense water displacement, pressure and direction.
The lateral line and electroreception, along with sharks' other senses combine to make them incredibly keen hunters. Since two-thirds of a shark's brain is devoted to smell, its olfactory sense can get the shark hot on the trail of its next meal even in dark waters [source: Parker]. It's only when the shark gets about 3 feet (1 meter) away from its target that electroreception kicks in to orient its jaws for an accurate, final attack [source: PBS]. For that last few feet of the attack, great white sharks actually roll their eyes back into their heads for protection and let electroreception take over navigation [source: Dingerkus].
In experiments testing sharks' electroreception skills, scientists have confirmed that the fish will indeed make last-minute feeding decisions based on electrical impulses. For example, when given the option between dead fish and an electrically charged rod, a shark will initially head for the fish, then alter its course toward the metal rod at the last minute [source: Fields]. Through this type of research, scientists are hoping to develop a shark deterrent that tricks its electroreception sense.
The power of electroreception also explains why sharks will continue attacking human victims even while being rescued by another person. Instead of going for the fresh meat of the rescuer, sharks will be repeatedly drawn to their previous victims because of the salt released from blood in the water [source: Viegas]. The higher salt concentration increases the intensity of the electrical field around the victim.
Since sharks can track electrical changes so well, scientists also are investigating whether electroreception plays a role in their navigation skills. Some theorize that the Earth's magnetic fields may interact with salt water to form electrical currents that sharks follow during migration [source: Parker].
Want to learn more about sharks? Visit the links on the next page.
Related HowStuffWorks Articles
More Great Links
- Carwardine, Mark. "Shark." Firefly Books. 2004. (May 13, 2008)http://books.google.com/books?id=Qh44RNa5yh0C
- Dingerkus, Guido. "The Shark Watcher's Guide." Wanderer Books. 1985.
- Fields, Douglas R. "The Shark's Electric Sense." Scientific American. August 2007. (May 13, 2008)http://www.sciam.com/article.cfm?id=the-sharks-electric-sense
- Helfman, Gene S.; Collete, Bruce B.; Facey, Douglas E. "The Diversity of Fishes." Blackwell Publishing. 1997. (May 13, 2008)http://books.google.com/books?id=dPG_-2in8kIC
- NOVA Online. "Electroreception." Shark Attack. PBS. Updated May 2002. (May 13, 2008)http://www.pbs.org/wgbh/nova/sharkattack/hotsciencesharks/senseelectro.html
- Parker, Jane and Parker, Steve. "The Encyclopedia of Sharks." Firefly Books. 2002.
- ReefQuest Centre for Shark Research. "Electroreception." Biology of Sharks and Rays. (May 13, 2008)http://www.elasmo-research.org/education/white_shark/electroreception.htm
- Viegas, Jennifer. "Shark Snot Key to Following Bloody Trail." Discovery News. Nov. 6, 2007. (May 13, 2008)http://dsc.discovery.com/news/2007/11/06/shark-gel-blood.html