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Electroception is the biological ability either to create or to detect electric charges. It is found most frequently in ocean animals because of the superior ability of water to conduct electricity. Examples of animals with electroception include sharks, rays, eels, and weakly-electric fish. Monotremes, including echidnas and platypi, are the only mammals that have the ability.
Electroception is used to increase the animal's awareness of its surrounding environment and sometimes to detect prey. In electric eels, it is even used as a high-voltage weapon. Lightning bugs, despite their name, do not possess electroception.
The animal that uses electricity most intimately is probably the electric eel (Electrophorus electricus). This Amazonian freshwater predator is not actually an eel, but a knifefish, or gymnotiform. Gymnotiformes are a lineage of fish that have evolved to exploit bioelectricity and electroception. Besides the electric eel, they include the black ghost (Apteronotus albifrons), the glass knifefish (Eigenmannia virescens), and the tiger knifefish (Gymnotus carapo).
Gymnotiformes generate an electric field of about 1 volt on a 24 hour basis. Electric eels, the most powerful gymnotiformes, have electroception fields of about 10 volts. The effect continues strong throughout sleep. For peak hunting or self-defense bursts, the voltage of the field can reach as high as 600 V. This easily kills small fish in the surrounding area.
Electric eels are quite large, growing as long as 2.5 m (8 ft) and weighing 25 kg (55 lbs). They use their Sachs organ, a stack of electroplaques, to generate a charge. The electroplaques are disc-like cells stacked on top of each other to produce an additive charge.
ATP-powered transport proteins pump positive sodium and potassium ions out of the cell, creating a negative charge, which corresponds to electrification. A nucleus of neurons in the fish's brain called the pacemaker nucleus fires when prey or a threat is sighted, releasing acetylcholine which stimulates the cells to "fire". The basic principle of activation is the same as that of muscle cells.
Any fish or shark that lives in muddle water has something to gain from an extra sense. As a result, many fish are weakly electroceptive. It has been shown that sharks can be stimulated to attack by mere manipulation of the electric field of the water, and the presence of visible prey or blood is not required.