Electric Eels Are Not Eels, And Their Shock Can Stop a Horse.

The name is wrong on two counts. Electrophorus electricus is not an eel. It is a knifefish, more closely related to catfish than to true eels, and it was classified incorrectly when European naturalists first encountered it in South America in the eighteenth century. The name stuck. The animal itself is far stranger than the name suggests.

An adult electric eel can reach 2.5 meters in length and generate electric discharges of up to 860 volts, the highest voltage produced by any living animal. The discharge is powerful enough to stun a horse, and historical accounts from Alexander von Humboldt’s 1800 expedition to Venezuela describe local people driving horses into eel-inhabited waters to exhaust the eels’ charge before fishing them by hand, a technique that killed some of the horses in the process.

The electricity is generated by three specialized electric organs that occupy approximately 80% of the eel’s body length, displacing most of the internal organs to a small section near the head. The electric organs are composed of modified muscle cells called electrocytes, stacked in series like batteries, each generating a small voltage that adds together to produce the total discharge.

Three Organs, Three Functions

Researcher Carlos David de Santana at the Smithsonian Institution has studied electric eel biology extensively and published research in 2019 describing what had been considered a single species as actually three distinct species, each with different maximum voltages and geographic distributions. His work also documented a hunting behavior that had not previously been described in detail.

When an electric eel detects prey hiding in vegetation or leaf litter, it curls its body into a C-shape around the prey, bringing the head and tail close together. This doubles the effective voltage delivered to the prey by creating a more focused electric field. De Santana’s high-speed video documentation of this behavior showed prey fish experiencing involuntary muscle contractions that immobilize them instantly, making capture trivial.

The three electric organs serve different functions. The main organ and Hunter’s organ produce the high-voltage discharges used for hunting and defense. Sachs’ organ produces low-voltage pulses used for electrolocation, sensing the environment by detecting distortions in the electric field the eel generates around itself. This electrolocation system allows the eel to navigate and detect prey in the murky, low-visibility waters of the Amazon and Orinoco basins where it lives.

Breathing Air in an Oxygen-Poor River

The Amazon basin contains vast areas of blackwater rivers and flooded forests where dissolved oxygen levels are extremely low. Electric eels have adapted to this environment by becoming obligate air breathers. They must surface to breathe every 10 minutes or so, obtaining approximately 80% of their oxygen from air rather than water through a highly vascularized mouth lining that functions as a primitive lung.

This air-breathing requirement means electric eels are tied to the water surface in a way that most fish are not, and it shapes their behavior and habitat use. They prefer shallow, slow-moving water where surfacing is easy and where the dense vegetation provides cover and hunting opportunities.

The remaining 20% of their oxygen comes from water passing over the gills, but the gills are reduced compared to fully aquatic fish, reflecting the evolutionary shift toward aerial respiration. An electric eel removed from water will survive considerably longer than most fish, as long as its skin remains moist enough to allow some gas exchange.

The horse-killing voltage, the body that is 80% battery, the air-breathing fish that is not a fish, the animal that doubles its own voltage by folding itself in half — Electrophorus is one of those species where the accurate description sounds like an exaggeration. It isn’t.