The electric eel has an elongated, cylindrical body, typically growing to about 2 m (6 ft 7 in) in length, and 20 kg (44 lb) in weight, making it the largest species of the Gymnotiformes. The coloration is dark gray-brown on the back and yellow or orange on the belly. Mature males have a darker color on the belly. They have no scales. The mouth is square, and positioned at the end of the snout. The anal fin extends the length of the body to the tip of the tail. As in other ostariophysan fishes, the swim bladder has two chambers. The anterior chamber is connected to the inner ear by a series of small bones derived from neck vertebrae called the Weberian apparatus, which greatly enhances its hearing capability. The posterior chamber extends along the whole length of the body and is used in buoyancy. Electrophorus has a well-developed sense of hearing. This fish has a vascularized respiratory organ in its oral cavity.[citation needed] As obligate air-breathers, it rises to the surface every 10 minutes or so, and will gulp air before returning to the bottom. Nearly 80% of the oxygen used by the fish is taken in this way.
Physiology
The electric eel has three abdominal pairs of organs that produce electricity: the main organ, the Hunter's organ, and the Sach's organ. These organs make up four-fifths of its body, and are what give the electric eel the ability to generate two types of electric organ discharges: low voltage and high voltage. These organs are made of electrocytes, lined up so a current of ions can flow through them and stacked so each one adds to a potential difference. When the eel locates its prey, the brain sends a signal through the nervous system to the electrocytes. This opens the ion channels, allowing sodium to flow through, reversing the polarity momentarily. By causing a sudden difference in electric potential, it generates an electric current in a manner similar to a battery, in which stacked plates each produce an electrical potential difference. In the electric eel, some 5,000 to 6,000 stacked electroplaques are capable of producing a shock at up to 500 volts and 1 ampere of current (500 watts). It would be extremely unlikely for such a shock to be deadly for an adult human, due to the very short duration of an eel's discharge (<2 ms). Electrocution death is due to current flow; the level of current would be fatal in humans depending on the path the current takes through the human body, and the duration of current flow. Heart fibrillation (which is reversible via a heart defibrillator) can take place from currents ranging from 70 to 700 mA and higher, provided that the current flows for more than approximately 30 ms.
The electric eel is unique among the Gymnotiformes in having large electric organs capable of producing lethal discharges that allows them to stun prey. Larger voltages have been reported, but the typical output is sufficient to stun or deter virtually any other animal. Juveniles produce smaller voltages (about 100 V). They are capable of varying the intensity of the electrical discharge, using lower discharges for "hunting" and higher intensities for stunning prey, or defending themselves. When agitated, they are capable of producing these intermittent electrical shocks over a period of at least an hour without signs of tiring.
