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". Since the 20th Century, electric organs have received extensive study, for example Hans Lissmann's pioneering 1951 paper and his review of their function and evolution in 1958. More recently, Torpedo californica electrocytes were used in the first sequencing of the acetylcholine receptor by Noda and colleagues in 1982, while Electrophorus electrocytes served in the first sequencing of the voltage-gated sodium channel by Noda and colleagues in 1984.
Evolution
Electric organs have evolved at least six times in various teleost and elasmobranchfish. Notably, they have convergently evolved in the African Mormyridae and South AmericanGymnotidae groups of electric fish. The two groups are distantly related, as they shared a common ancestor before the supercontinent Gondwana split into the American and African continents, leading to the divergence of the two groups. A whole-genome duplication event in the teleost lineage allowed for the neofunctionalization of the voltage-gated sodium channel gene Scn4aa which produces electric discharges. In all clades except Apteronotus, where the cells are derived from neural tissue, the electrocytes are derived from skeletal muscle.
Electrocytes
Electrocytes, electroplaques or electroplaxes are cells used by electric eels, rays, and other fish for electrogenesis. In some species they are cigar-shaped; in others, 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 potassiumions out of the cell via transport proteins powered by adenosine triphosphate. Postsynaptically, electrocytes work much like muscle cells. They have nicotinic acetylcholine receptors. 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.
Firing
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. The electrocytes fire an action potential using the voltage-gated sodium channels on one or both sides of the electrocyte, depending on the complexity of the electric organ in that species. If the electrocyte has sodium channels on both sides, the depolarization caused by firing action potentials on one side of the electrocyte can cause the sodium channels on the other side of the electrocyte to fire as well.
Location
In most fishes, electric organs are oriented to fire along the length of the body, usually lying along the length of the tail and within the fish's musculature, with smaller accessory electric organs in the head. However, there are some exceptions; in stargazers and in rays the electric organs are oriented along the dorso-ventral axis. In the electric torpedo ray, the organ is near the pectoral muscles and the gills. 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 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 electric organ discharges 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. Electric organ discharges influence mate choice in weakly electric fish, as females have been shown to be attracted to electric discharge characteristics of conspecific males.
