The electric eel, a creature of the Amazon and Orinoco river basins in South America, possesses the power to generate substantial electrical discharges. Despite its common name, the electric eel (Electrophorus species) is not a true eel, but rather a type of knifefish, making it more closely related to catfish and carp. This fish, which can grow up to 8 feet in length, uses its electrical capability for navigation, hunting, and defense within the murky, slow-moving freshwater where it resides. Its ability to produce both high and low-voltage shocks has long fascinated scientists.
The Electric Organ System
The biological apparatus responsible for this electric power is a sophisticated system of three paired organs, which occupy up to 80% of the eel’s long, cylindrical body. These organs are the Main Organ, the Hunter’s Organ, and the Sach’s Organ, and they are composed of specialized cells called electrocytes. Electrocytes are modified muscle cells that have lost the ability to contract but are instead dedicated to generating an electrical potential. The Main and Hunter’s organs are the high-voltage producers, designed for delivering powerful shocks to stun prey or deter predators. The Sach’s Organ, located in the tail section, is the low-voltage generator, used to emit weak electrical pulses for sensing the environment.
Generating the Charge
The mechanism for creating a large electrical discharge is based on the synchronized movement of ions across the electrocyte membranes. Each electrocyte acts like a tiny biological battery, maintaining a resting potential across its membrane by actively pumping sodium and potassium ions. This process creates a significant concentration gradient, which is the stored energy source for the shock. The individual voltage created by a single electrocyte is small, only about 0.15 volts.
To achieve a powerful shock, the eel’s brain sends a neural signal that simultaneously reaches thousands of electrocytes. This signal causes specific ion channels on one side of each cell to open rapidly. Positively charged sodium ions then flood into the cell, causing a massive, synchronized depolarization that reverses the electrical polarity of the membrane.
The electrocytes are arranged in long columns, stacked in series, much like the cells inside a flashlight battery. Because the cells are connected in series, the small voltage of each cell adds up, generating a powerful net voltage. A large electric eel may have 5,000 to 6,000 electrocytes stacked in this manner, allowing it to produce hundreds of volts. Other stacks of electrocytes are arranged in parallel to increase the current output.
Uses of the Electric Field
The low-voltage pulses, generated by the Sach’s organ, are used for electrolocation, acting as a form of active sonar. By emitting these weak electrical discharges, the eel can sense distortions in the resulting field, allowing it to navigate murky water and identify objects or prey in its environment.
The high-voltage discharge, produced by the Main and Hunter’s organs, serves as a powerful weapon for hunting and defense. When the eel detects prey, it releases a volley of high-frequency, high-voltage pulses to quickly stun or kill the target. This high-voltage shock can also be used as a tracking system, as the electrical pulses stimulate the prey’s motor neurons, causing involuntary muscle contractions.
This involuntary twitching of the prey reveals its location to the eel’s highly sensitive electroreceptors, even if the prey is hidden from sight. The eel can further amplify the effect by coiling its body, which concentrates the electric field between its head and tail, delivering a more intense shock.
Impact on Prey and Humans
The electric eel is the most powerful of all electric fish, capable of generating discharges that can exceed 600 volts. For small fish and other aquatic prey, the high-voltage discharge is an instant incapacitating strike, often leading to rapid death. The shock is delivered in very short bursts, lasting only a few milliseconds, but the intensity is sufficient to override the prey’s nervous system.
For humans, a shock from a large electric eel is rarely fatal, primarily because the short duration of the pulse and the resistance of the surrounding water limit the current that passes through the body. However, the shock is intensely painful and can cause severe muscle spasms and temporary loss of muscle control. The greatest danger to a person is the risk of secondary drowning or injury if the shock causes them to lose consciousness or control while submerged.