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In vertebrates, electric organs evolved independently from skeletal muscle at least six times – in South American Gymnotiformes (weak and strong EO), African Mormyridae (weak EO), the catfish Malapterurus (strong EO), stargazers (strong EO), rays (strong EO) and skates (weak EO). The Gymnotiformes originated ~120-100 MYA in Western Gondwana, and radiated rapidly into the five living families including the ancestors of the modern Electrophorus,Sternopygus and Eigenmannia. The differing anatomy and gene expression profiles show that electrocytes of each lineage evolved in different ways since their divergence from a common electrogenic ancestor.
Electric fishes contain electric organs made up of electrocytes, a novel organ and cell type not found in any other animal. These cells respond to acetylcholine secretion from adjacent neurons with the initial activation of nicotinic acetylcholine receptors followed by sodium channels and depolarizations that ultimately result in the release of electrical power directed into the water outside of the animal. Many hundreds of species of weak voltage electric fishes use the emitted electricity for navigation and communication in dark waters, and in addition, in case of a handful of fish species with strong voltage discharges (e.g., Electrophorus electricus, the strong voltage electric eel) for the killing of prey or predators. No other organism is capable of generating and using such large amounts of electric power.
Based on their embryological origin and physiological character, electrocytes are thought to have evolved from muscle cells. Evolutionary origin of electric organs perplexed Darwin, while developing his theory of natural selection. He wrote in Origin of Species - “The electric organs of fishes offer another case of special difficulty; it is impossible to conceive by what steps these wondrous organs have been produced” (2). Nearly 100 years later, Lissmann recorded weak electric signals from the tail of an African Mormyroidea, Gymnarchus niloticus, and noted it as a solution to Darwin's dilemma. All gymnotiform and mormyrid species generate weak electric organ discharges (EOD’s) that never exceed more than a few hundred millivolts. The electric eel also produces a strong EOD (up to 600 V), which is used to stun prey items and deter potential predators. These discharges are produced from the Main electric organ and the anterior two-thirds of the more ventrally positioned Hunter's electric organ. The strength of these discharges is correlated with body length, increasing by about 100 V for each 30 cm of total length. The trains of strong discharges are under voluntary control and are used in aggressive and predatory behaviors. The posterior one-third of the Hunter's Organ, together with the Organ of Sachs lying dorsal and posterior to the other two electric organs, generate a continuous, variable, low frequency (1.5 Hz) low amplitude (1-10 V) electric discharge. The continuous train of weak electric discharges in E. electricus is composed of head positive monophasic pulse that generates the electric field used in navigation and sexual communication