Ampullae of Lorenzini
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The ampullae of Lorenzini are special sensing organs, forming a network of jelly-filled canals found on elasmobranchs (sharks and rays) and Chimaera. Each ampulla consists of a jelly-filled canal opening to the surface by a pore in the skin and ending blindly in a cluster of small pockets full of electroreceptor cells. The ampullae are mostly clustered into groups inside the body, each cluster having ampullae connecting with different parts of the skin, but preserving a left-right symmetry. The canal lengths vary from animal to animal, but the electroreceptor pores' distribution is approximately species-specific. The ampullae pores are plainly visible as dark spots in the skin. They provide sharks and rays with a sixth sense capable of detecting electro-magnetic fields as well as temperature gradients.
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[edit] Electro-magnetic field sensing ability
The ampullae detects electric fields in the water, or more precisely the difference between the voltage at the skin pore and the voltage at the base of the electroreceptor cells. A positive pore stimulus would decrease the rate of nerve activity coming from the electroreceptor cells and a negative pore stimulus would increase the rate of nerve activity coming from the electroreceptor cells.
Sharks may be more sensitive to electric fields than any other animal, with a threshold of sensitivity as low as 5 nV/cm. That is 5/1,000,000,000 of a volt measured in a centimeter-long ampulla. Since all living creatures produce an electrical field in muscle contractions, it is easy to imagine the shark may pick up weak electrical stimuli from the muscle contractions of animals, particularly prey, on the other hand, the electrochemical fields generated by paralyzed prey were sufficient to elicit a feeding attack from sharks and rays in experimental tanks, therefore muscle contractions are not necessary to attract the animals. Shark and rays can locate prey buried in the sand, or DC electric dipoles simulating the main feature of the electric field of a prey buried in the sand.
The electric fields produced by oceanic currents moving in the magnetic field of the earth are of the same order of magnitude as the electric fields that sharks and rays are capable to sense. Therefore, sharks and rays may orient to the electric fields of oceanic currents, and use other sources of electric fields in the ocean for local orientation. Additionally, the electric field they induce in their bodies when swimming in the magnetic field of the earth may give them electric clues about their magnetic heading.
[edit] Temperature sensing ability
Early in the 20th century the purpose of the ampullae was not clearly understood and electrophysiological experiments suggest a sensibility to temperature, mechanical pressure and maybe salinity. It was not until 1960 that the ampullae was clearly identified as a receptor organ specialized in sensing electric fields. The ampullae may also allow the shark to detect changes in water temperature. Each ampulla is a bundle of sensory cells containing multiple nerve fibres. These fibres are enclosed in a gel-filled tubule which has a direct opening to the surface through a pore. The gel is a glyco-protein based substance with the same resistivity of seawater, and it has electrical properties similar to a semiconductor, allowing it to essentially transduce temperature changes into an electrical signal that the shark may use to detect temperature gradients.
[edit] Electronic shark repellent
Dr. Graeme Charter and Norman Starkey developed the “POD” (or Protective Oceanic Device), which is the first successful electronic shark repellent for scuba divers. By producing an electromagnetic field, the POD irritates the Ampullae of Lorenzini of a great white shark. Shark nets, which traditionally protected divers, can harm or kill the shark, but these more primitive deterrents may soon be out of date.
