As stated earlier, the heads of sharks are covered with hundreds of pores. These pores are known as the Ampullae of Lorenzini. Each ampulla consists of a small chamber that is formed by alveoli. A canal that is about one millimeter wide and rage from three to twenty centimeters in length connects the ampulla to the surface of the skin. The walls of the canals are very thick and excellent conductors. When Lorenzini first discovered the ampullae in 1678, he noted that “the walls of the canals are much thicker than what is appropriate for a simple duct” (Brown et al, 2002). This canal is lined with sensory hair cells, similar to that of the human ear. The entire chamber is filled with a gel, which will be discussed more on the next page. The hair cells within each ampulla serve as voltage detectors and release neurotransmitters when stimulated. The specific neurotransmitter that is released is determined by the difference between their pore and internal potentials (Brown at al, 2002).
The lengths of the ampulla canals let sharks detect small local fields from individual fish as well as the large uniform electric fields in the water. Longer ampulla canals are able to sample across a broad area and can detect the large electric fields, such as those emitted by the earth. Shorter ampulla canals are much more sensitive and can detect local fields such as those emitted by other fish (Tricas, 2001).
The location of the canals in relation to the electrical source can also help to determine the intensity of the charge. A canal receives the strongest charge when it is oriented parallel to the source of the charge. As canals become less parallel with the source, the strength of the charge also decreases. Thus, it is believed that sharks can sense where the electrical charge is strongest and use this to locate prey (Tricas, 2001). This idea will be discussed in more detail in the following pages.
The structure of the ampullae canals. Drawing by R. Dixon. http://bio.classes.ucsc.edu/bio137/images/amploren.GIF
|STRUCTURE OF THE AMPULLAE|
|THE FUNCTION OF THE GEL IN THE AMPULLAE|
|THE COMPASS SENSE|
|NEURO-ECOLOGY AND PERIPHERAL MORPHOLOGY|
|ELECTRICAL SHARK DETERRENTS|