Is an Electric Eel Really an Eel? Knifefish Explained

What is an electric eel, then?

The electric eel belongs to the South American knifefishes, a group of more than 200 species living in the rivers and swamps of Central and South America. Knifefishes have long, flattened or rounded bodies and swim by rippling a long fin along the underside. Every one of them can make an electric field, though most produce only weak signals for sensing and communication. The electric eel is the giant of the group and the only one that delivers powerful shocks.

Knifefishes are part of a huge group of mostly freshwater fishes, with more than 10,000 species, that also includes catfishes, carps, minnows, tetras and piranhas. Members of this group share an unusual feature: the Weberian apparatus, a chain of tiny bones that links the swim bladder to the inner ear and gives them sharp hearing. The electric eel has one. True eels do not. Among these relatives, catfishes are thought to be the knifefishes’ closest kin.

What are true eels?

True eels form the order Anguilliformes, with about 1,000 species. They include the freshwater eels, such as the European eel, as well as conger eels, moray eels, snake eels and garden eels. Most live in the sea.

True eels start life as leptocephali, flat, transparent, leaf-shaped larvae that drift in the ocean, sometimes for months. They share this larval stage with their closest relatives, the tarpons and bonefishes. European eels, for example, hatch in the Sargasso Sea and drift for many months toward Europe before growing up in rivers and returning to the sea to spawn once and die, a journey explained in where do eels come from?

The line leading to true eels split from the line leading to knifefishes and catfishes well over 100 million years ago, near the base of the family tree of bony fishes.

How to tell an electric eel from a true eel

The fins are the quickest clue. An electric eel has no dorsal fin at all and no tail fin of its own. A single anal fin, with more than 400 fin rays, runs along most of the belly to the tip of the tail. The eel holds its body almost rigid and sends waves along this fin, which lets it move forward or backward with ease. This rigid posture also keeps its electric organs lined up while it senses its surroundings.

Most true eels have a long dorsal fin that runs down the back and merges with the tail and anal fins into one continuous fin. They swim by bending their whole body in waves, a style that biologists call anguilliform swimming, after the eels themselves.

Breathing is another big difference. The electric eel gets up to about 80 percent of its oxygen by gulping air at the surface, and it will drown if it cannot reach the air. True eels breathe mainly with gills, although freshwater eels can absorb some oxygen through their skin when they wriggle across wet ground.

Why do they look so alike?

The similar shape is an example of convergent evolution, in which unrelated animals evolve similar bodies because they face similar challenges. A long, flexible body without large fins is a good design for squeezing into burrows, crevices, tangled roots and soft mud, which is exactly where both groups often live.

The same body plan has evolved many times in fish, as well as in snakes, legless lizards and caecilians. That is why so many unrelated animals end up being called eels, and why even eels and snakes can be confused, as explained in eel vs snake.

Not one electric eel but three

For more than two centuries, scientists recognized a single electric eel, described by Carl Linnaeus in 1766. In 2019 a study of DNA, skull shape, habitat and voltage split it into three species: Electrophorus electricus in the Guiana Shield of northern South America, Volta’s electric eel (E. voltai) in the Brazilian Shield, and Vari’s electric eel (E. varii) in the lowland Amazon basin.

From the outside, the three look almost the same. They differ in the shape of the skull and head and in the strength of their shocks. Volta’s electric eel produced 860 volts, the highest voltage recorded from any animal, as described in how do electric eels make electricity?

Other “eels” that are not eels

The electric eel is not the only misnamed fish. Swamp eels, found in warm fresh waters of Asia, Africa and the Americas, and the spiny eels of Asia and Africa belong to a separate order of fish. The wolf eel of the North Pacific is a wolffish with a long body. Lampreys and hagfish, sometimes called lamprey eels or slime eels, are jawless fish that are only very distantly related to any of these.

Even early scientists noticed the problem. When the surgeon John Hunter dissected an electric eel for the Royal Society in 1775, he remarked that it looked very much like an eel but had none of the features that define one. The name stuck anyway.