|Electric eel at the New England Aquarium, United States|
T. N. Gill, 1864
The electric eel (Electrophorus electricus, other species proposed) is a South American electric fish. Until 2019, it was classified as the only species in its genus. Despite the name, it is not an eel, but rather a knifefish.
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 them the largest species of the Gymnotiformes. Their coloration is dark gray-brown on the back and yellow or orange on the belly. Mature females have a darker color on the abdomen.
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 maintains the fish's buoyancy.
E. electricus has a vascularized respiratory system with gas exchange occurring through epithelial tissue in its buccal cavity. As obligate air-breathers, electric eels must rise to the surface every ten minutes or so to inhale before returning to the bottom. Nearly eighty percent of the oxygen used by the fish is obtained in this way.
Despite its name, the electric eel is not closely related to the true eels (Anguilliformes) but is a member of the neotropical knifefish order (Gymnotiformes), which is more closely related to the catfish.
The electric eel has three pairs of abdominal 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 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 finds 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 electric potential difference. Electric eels are also capable of controlling their prey's nervous systems with their electrical abilities; by controlling their victim's nervous system and muscles via electrical pulses, they can keep prey from escaping or force it to move so they can locate its position.
In the electric eel, some 5,000 to 6,000 stacked electroplaques can make a shock up to 860 volts and up to 1 ampere of current. This level of current is reportedly enough to produce a brief and painful numbing shock likened to a stun gun discharge, which due to the voltage can be felt for some distance from the fish; this is a common risk for aquarium caretakers and biologists attempting to handle or examine electric eels.
Electric eels use electricity in multiple ways. Low voltages are used to sense the surrounding environment. High voltages are used to detect prey and, separately, stun them. Pairs of high voltage pulses separated by 2 milliseconds are used to detect and locate prey by causing them to twitch involuntarily; the electric eel senses this movement. A string of high voltage pulses at up to 400 per second are then used to attack and stun or paralyze the target, at which point the electric eel applies a suction-feeding bite.
The Sach's organ is associated with electrolocation. Inside the organ are many muscle-like cells, called electrocytes. Each cell can only produce 0.15 V, though the organ can transmit a signal of nearly 10 V overall in amplitude at around 25 Hz in frequency. These signals are emitted by the main organ; the Hunter's organ can emit signals at rates of several hundred hertz.
The electric eel is unique among the Gymnotiformes in having large electric organs that can produce potentially lethal discharges that allow them to stun prey. Larger voltages have been reported, but the typical output is sufficient to stun or deter virtually any animal. Juveniles produce smaller voltages (about 100 V). They can vary the intensity of the electric discharge, using lower discharges for hunting and higher intensities for stunning prey or defending themselves. They can also concentrate the discharge by curling up and making contact at two points along its body. When agitated, they can produce these intermittent electric shocks over at least an hour without tiring.
The electric eel also possesses high frequency–sensitive tuberous receptors, which are distributed in patches over its body. This feature is apparently useful for hunting other Gymnotiformes.
Electric eels have been used as a model in the study of bioelectrogenesis. The species is of some interest to researchers, who make use of its acetylcholinesterase and adenosine triphosphate.
Michael Faraday extensively tested the electrical properties of an electric eel, imported from Suriname. For a span of four months, Faraday carefully and humanely measured the electrical impulses produced by the animal by pressing shaped copper paddles and saddles against the specimen. Through this method, Faraday determined and quantified the direction and magnitude of electric current, and proved the animal's impulses were in fact electrical by observing sparks and deflections on a galvanometer.
Researchers at Yale University and the National Institute of Standards and Technology argue artificial cells could be built that not only replicate the electrical behavior of electric eel cells, but also improve on them. Artificial versions of the eel's electricity-generating cells could be developed as a power source for medical implants and other microscopic devices.
Ecology and life history
Electric eels inhabit fresh waters of the Amazon and Orinoco River basins in South America, in floodplains, swamps, creeks, small rivers, and coastal plains. They often live on muddy bottoms in calm or stagnant waters.
Electric eels feed on invertebrates, although adult eels may also consume fish and small mammals, such as rats. First-born hatchlings eat other eggs and embryos from later clutches. The juveniles eat invertebrates, such as shrimp and crabs.
The electric eel is known for its unusual breeding behavior. In the dry season, a male eel makes a nest from his saliva into which the female lays her eggs. As many as 3,000 young hatch from the eggs in one nest. Males grow to be larger than females by about 35 cm (14 in).
In zoos and private collections
These fish have always been sought after by some animal collectors, but catching them is difficult, because the only reasonable option is to make the eels tired by continually discharging their electricity. The fish's electric organs eventually become completely discharged, allowing the collector to wade into the water in comparative safety.
Keeping electric eels in captivity is difficult and mostly limited to zoos and aquaria, although a few hobbyists have kept them as pets.
The Tennessee Aquarium in the United States is home to an electric eel. Named Miguel Wattson, the eel's exhibit is wired to a small computer that sends out a prewritten tweet when it emits electricity at a high enough threshold.
The species is so unusual that it has been reclassified several times. When originally described by Carl Linnaeus in 1766, he used the name Gymnotus electricus, placing it in the same genus as Gymnotus carapo (banded knifefish) which he had described several years earlier. It was only about a century later, in 1864, that the electric eel was moved to its own genus Electrophorus by Theodore Gill.
In September 2019, C. David de Santana et al. published work strongly suggesting division of Electrophorus electricus into three species based on DNA divergence, ecology and habitat, anatomy and physiology, and electric ability. The proposed three species are E. electricus, E. voltai sp. nov., and E. varii sp. nov.
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