Electrophorus electricus is the best-known species of electric eel. It is a South American electric fish. Until the discovery of two additional species in 2019, the genus was classified as the monotypic, with this species the only one in the genus. Despite the name, it is not an eel, but rather a knifefish. It is considered as a freshwater teleost which contains an electrogenic tissue that produces electric discharges.
Taxonomic history
The species 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, David de Santana et al. suggested the division of the genus into three species based on DNA divergence, ecology and habitat, anatomy and physiology, and electrical ability: E. electricus, E. voltai sp. nov., and E. varii sp. nov. The study found E. electricus to be the sister species to E. voltai, with both species diverging during the Pliocene.
Anatomy
E. electricus has an elongated, cylindrical body, typically growing to about in length, and in weight, making them the largest of the Gymnotiformes. Their coloration is dark gray-brown on the back and yellow or orange on the belly. Mature females have a darker 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, E. electricus 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.
Physiology
E. electricus has three pairs of abdominal organs that produce electricity: the main organ, Hunter's organ, and Sachs' organ. These organs occupy a large part 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. The three electrical organs are developed from muscle and exhibit several biochemical properties and morphological features of the muscle sarcolemma; they are found symmetrically along both sides of the eel.
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.
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, at which point the electric eel applies a suction-feeding bite.
Sachs' organ is associated with electrolocation. Inside the organ are many muscle-like cells, called electrocytes. Each cell produces 0.15 V, the cells being stacked in series to enable the organ to generate nearly 10 V at around 25 Hz in frequency. These signals are emitted by the main organ; Hunter's organ can emit signals at rates of several hundred hertz.