The common name, electric eel, is a misnomer. Despite its snake-like appearance and its name, the creature is not a true eel at all. It is a highly specialized fish that evolved its elongated form and electric capabilities independently of the marine eels known to most people.
The True Classification of the Electric Eel
The electric eel belongs to the order Gymnotiformes, a group of teleost bony fishes commonly known as the South American knifefish. This places the electric eel, now classified under the genus Electrophorus, in a close evolutionary relationship with catfish and carp, and far removed from the true eels of the order Anguilliformes. Following a 2019 taxonomic revision, the genus is recognized to contain three distinct species, including Electrophorus electricus, E. varii, and E. voltai.
The serpentine body shape of the electric eel is an example of convergent evolution, where two unrelated species evolve similar physical traits due to comparable environmental pressures. Like all knifefish, the electric eel lacks a dorsal fin, pelvic fins, and a caudal fin, instead relying on an exceptionally long anal fin that runs nearly the entire length of its body.
This fin is undulated in a wave-like motion, allowing the fish to maneuver forward and backward with great precision in the murky, slow-moving waters of the Amazon and Orinoco basins. All its vital organs are concentrated within the first fifth of its body, near the head, which allows the massive electrical organs to dominate the rest of its trunk.
Key Differences Defining True Eels
True eels belong to the taxonomic order Anguilliformes, and their biology fundamentally differs from that of the electric knifefish. A defining feature of their life cycle is the leptocephalus stage, a larval form that is transparent, ribbon-like, and highly compressed. These larvae drift in open ocean currents, such as the Gulf Stream, for months or even over a year, before undergoing a metamorphosis into a juvenile “glass eel”.
Anatomically, true eels are characterized by their single, continuous fin, formed by the fusion of the dorsal, caudal, and anal fins, which runs along the back, around the tail, and underneath the body. While true eels also lack pelvic fins, their migration patterns are primarily catadromous, meaning they spawn in the ocean and their offspring must return to freshwater or coastal habitats to mature. The electric eel, by contrast, is a purely freshwater fish that spends its entire life in South American rivers and swamps.
How Electric Eels Produce Their Electrical Charge
The electric eel’s ability to generate powerful electrical discharges is rooted in specialized cells called electrocytes, which are modified muscle cells. These flattened, disc-shaped cells are stacked in columns, much like the individual cells in a battery, within three pairs of electric organs: the Main organ, Hunter’s organ, and Sachs’ organ. The arrangement is important, as stacking the electrocytes in series longitudinally multiplies the voltage, while arranging the stacks in parallel boosts the current.
When the eel decides to discharge, its brain sends a synchronized signal that causes neurotransmitters to be released onto one side of the electrocyte. This chemical signal triggers the opening of ion channels, allowing a rapid influx of positively charged sodium ions into the cell. This sudden, temporary reversal of electrical polarity across the cell membrane generates a small voltage gradient of about 150 millivolts per cell.
The cumulative, simultaneous firing of thousands of these electrocytes produces the massive electrical output, which can reach up to 860 volts in the most powerful species. The eel uses low-voltage pulses, typically around 10 volts from the Sachs’ organ, for electrolocation and communication in its dark, turbid environment. The high-voltage discharge, generated by the Main and Hunter’s organs, is used to stun prey or deter predators by remotely controlling their nervous and muscular systems.