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|77.4 - Summer 2004|
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> Summer 2004 > Articles
The Sixth Sense
By Lucy Wang
(Credit: Institute of Marine Sciences)
It is a well-known fact that sharks can detect the scent of blood from miles away. Pop culture has often attributed the shark’s keen predatory skills to its sharp sense of smell and its sharper set of teeth. Surprisingly, however, the shark’s jaw strength is not chart-topping in the animal kingdom. Furthermore, smell may not even be its most useful sense in hunting down prey.
Sharks, skates, rays, and sawfishes are collectively known as elasmobranches, a unique class of fishes that has developed a “sixth sense” whereby it can detect electric fields. How is this electroreceptivity useful? All living organisms generate electric fields through normal cellular activity. These fields, however, tend to be extremely weak. By evolving a close-range sensory system that is able to detect weak electric fields, sharks can pinpoint the location of an otherwise hidden prey.
In the laboratory, the shark’s electroreceptivity is tested by inducing weak electric currents within an artificial environment. Under such conditions, sharks orient themselves with the resulting electric field and bite as if responding to a natural prey. Such experiments have shown that some sharks can detect electric fields less than one nanovolt per square centimeter, the equivalent of an electric field generated by a flashlight battery connected to electrodes 10,000 miles apart in the ocean!
The shark’s amazing electroreceptivity is due to special electrically sensitive cells, called ampullae of Lorenzini, which are located within hundreds of pores distributed in discrete patterns along the shark’s body. Each pore is filled with an electrically sensitive gel that surrounds a bundle of modified hair cells. These hair cells are receptive to local changes in electrical polarity. Their signals trigger the release of neurotransmitters, which inform the brain of the electric fields present in their surrounding environment.
The ability to accurately detect electric fields is useful to sharks not only in detecting prey but also in the attack and kill. For example, when the prey is wounded, electrolytes leak into the environment, creating an electric field up to three times larger than when the prey is unwounded. A shark’s hypersensitivity to this electric field allows it to keep a clear sense of where the prey is at all times during the bloody struggle of a predatory attack. Some sharks, such as the Great White, also roll their eyes into their sockets upon striking to protect them from injury. Consequently, sharks are temporarily blind at the moment of attack and therefore must rely heavily on their electroreceptivity to successfully execute the attack.
Beyond being a prey-locator, the shark’s sixth sense also acts as a type of global navigational system. The earth itself generates its own unique magnetic fields. As a shark swims through the ocean, these magnetic fields induce electric currents within the shark’s own electrolyte-filled body. Being sensitive to these electric fields gives sharks the ability to navigate along the “magnetic highways” on the seafloor.
Finally, the electroreceptive sense also helps sharks locate each other, a skill that is particularly helpful in the search for mates. Thus, the shark’s sixth sense significantly heightens its ability to steer, feed, and mate. With such an efficient design, it is no wonder that sharks are one of the oldest and most successful species on the planet.About the Author
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