In biology, the electric organ is an organ common to all electric fish used for the purposes of creating an electric field. The electric organ is derived from modified nerve or muscle tissue. The electric discharge from this organ is used for navigation, communication, defense and also sometimes for the incapacitation of prey.
In the 1770s the electric organs of the torpedo and electric eel were the subject of Royal Society papers by Hunter, Walsh and Williamson. They appear to have influenced the thinking of Luigi Galvani and Alessandro Volta - the founders of electrophysiology and electrochemistry.
In the 19th century, Charles Darwin discussed the electric organ in his Origin of Species as a likely example of convergent evolution: "But if the electric organs had been inherited from one ancient progenitor thus provided, we might have expected that all electric fishes would have been specially related to each other…I am inclined to believe that in nearly the same way as two men have sometimes independently hit on the very same invention, so natural selection, working for the good of each being and taking advantage of analogous variations, has sometimes modified in very nearly the same manner two parts in two organic beings".
Electrocytes, electroplaques or electroplaxes are cells used by electric eels, rays, and other fish for electrogenesis. They are flat disk-like cells. Electric eels have several thousand of these cells stacked, each producing 0.15 V. The cells function by pumping positive sodium and potassium ions out of the cell via transport proteins powered by adenosine triphosphate (ATP). Postsynaptically, electrocytes work much like muscle cells. They have nicotinic acetylcholine receptors. These cells are used in research because of their resemblance to nerve-muscle junctions.
The stack of electrocytes has long been compared to a voltaic pile, and may even have inspired the invention of the battery, since the analogy was already noted by Alessandro Volta. While the electric organ is structurally similar to a battery, its cycle of operation is more like a Marx generator, in that the individual elements are slowly charged in parallel, then suddenly and nearly simultaneously discharged in series to produce a high voltage pulse.
To discharge the electrocytes at the correct time, the electric eel uses its pacemaker nucleus, a nucleus of pacemaker neurons. When an electric eel spots its prey, the pacemaker neurons fire and acetylcholine is subsequently released from electromotor neurons to the electrocytes, resulting in an electric organ discharge.
The electric organs are usually oriented to fire along the length of the body in most fishes. In stargazers and in rays they are oriented along the dorso-ventral (up-down) axis. In most fishes, the organs lie along the length of the tail within the musculature. Many of them have smaller accessory electric organs in the head. In the electric torpedo ray, the organ is near the pectoral muscles and the gills (see the image). The stargazer's electric organs lie between the mouth and the eye. In the electric catfish, the organs are located just below the skin and encase most of the body like a sheath.
Electric organ discharge
Electric organ discharge (EOD) is the electric field generated by the organs of animals including electric fish. In some cases the electric discharge is strong and is used for protection from predators; in other cases it is weak and it is used for navigation and communication. Communicating through EODs occurs when a fish uses its own electroreceptors to sense the electric signals of a nearby fish. Electric fish navigate by detecting distortions in their electrical field by using their cutaneous electroreceptors 
Further peer-reviewed reading
Review Article on the molecular evolution of the electric organ.
Comprehensive overview of electric organ function and evolution in weakly electric fishes.
Succinct Nature article describing differences in gene expression between sarcomeres and elctrocytes of electric organs.
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