Electric eel

The electric eel, Electrophorus electricus, is a species of fish. It is capable of generating powerful electric shocks, which it uses for both hunting and self-defense. It is an apex predator in its South American range. Despite its name it is not an eel at all but rather a knifefish.

Anatomy
A typical electric eel has an elongated square body, a flattened head, and an overall dark grayish green color shifting to yellowish on the bottom. They have almost no scales. The mouth is square, placed right at the end of the snout. The anal fin continues down the length of the body to the tip of their tail. It can grow up to 2.5 m (about 8.2 feet) in length and 20 kg (about 44 pounds) in weight, making them the largest Gymnotiform. 1 m specimens are more common.

They have a vascularized respiratory organ in their oral cavity. These fish are obligate air-breathers; rising to the surface every 10 minutes or so, the animal will gulp air before returning to the bottom. Nearly 80% of the oxygen used by the fish is taken in this way.. Despite its name, the electric eel is not related to eels but is more closely related to catfish.

Scientists have been able to determine through experimental information that E. electricus has a well developed sense of hearing. They have a Weberian apparatus that connects the ear to the swim bladder which greatly enhances their hearing capability.

Physiology
The electric eel has three abdominal pairs of organs that produce electricity. They are the main organ, the hunter's organ, and the sachs’ organ. These organs take up 4/5 of its body. Only the front 1/5 contains the vital organs.|1 These organs are made of electrocytes lined up in series. The electrocytes are lined up so the current flows through them and produces an electrical charge. When the eel locates its prey, the brain sends a signal through the nervous system to the electric cells. This opens the ion channel, allowing positively-charged sodium to flow through, reversing the charges momentarily. By causing a sudden difference in voltage, it generates a current. The electric eel generates its characteristic electrical pulse in a manner similar to a battery, in which stacked plates produce an electrical charge. 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). The organs give the electric eel the ability to generate two types of electric organ discharges (EODs), low voltage and high voltage. The shock could be harmful for an adult human.

The Sachs organ is associated with electrolocation. Inside the organ are many muscle-like cells, called electrocytes. Each cell can only produce 0.15V, though working together the organ transmits a signal of about 10V in amplitude at around 25 Hz. These signals are what is thought to be used for communication as well as orientation; useful not only for finding prey, but also in finding and choosing a mate.

High-voltage EODs are emitted by the main organ and the Hunter's organ that can be emitted at rates of several hundred Hz. These high voltage EODs may reach up to 650 volts. The electric eel is unique among the gymnotiforms in having large electric organs capable of producing lethal discharges that allows them to stun prey. There are reports of animals producing larger voltages, but the typical output is sufficient to stun or deter virtually any other animal. Juveniles produce smaller voltages (about 100 volts). Electric eels 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, it is capable of producing these intermittent electrical shocks over a period of at least an hour without signs of tiring. The species is of some interest to researchers, who make use of its acetylcholinesterase and ATP.

The electric eel also possesses high-frequency sensitive tuberous receptors patchily distributed over the body that seem useful for hunting other Gymnotiformes.

Habitat
They tend to live on muddy bottoms in calm water. They are also found in swamps, coastal plains, and creeks. It can be found in South American rivers and in the ocean.

Feeding ecology
Electric eels feed on invertebrates, while adult eels feed on fish and small mammals. First-born hatchlings will even prey on other eggs and embryos from later batches..

Taxonomic history
The species is so unusual that it has been reclassified several times. Originally it was given its own family Electrophoridae, and then placed in a genus of Gymnotidae alongside Gymnotus.