Ken Catania, an eccentric professor of biological sciences at Vanderbilt University, likens electric eels with batteries immersed in water. These fascinating long, thin, bony fish of the order Anguilliformes, have intrigued scientists for centuries partly because they seem to so good at keeping secrets. For instance, one of the oddest and longest quests in 18th-century biology was the search for eel testicles. Sigmund Freud himself at one point joined this quest when he received the task as a junior researcher.

Despite their name, electric eels (Electrophorus electricus) are not eels at all. Rather, it’s a kind of knifefish, one of several South American fish, actually, that specializes in producing electric fields. Most, however, produce weak fields which they only use to navigate murky waters or to communicate with peers.

Ken Catania (Steve Green/Vanderbilt)

Ken Catania (Steve Green/Vanderbilt).

The electric eel, the sole species of its genus, does much more than this. With the help of multiple separate organs, the fish can produce much more violent discharges.

Michael Faraday, the 19th-century British physicist who discovered electromagnetic induction, the principle behind the electric transformer and generator, insisted to get an electric eel. He believed the fish would greatly accelerate his study of the role electricity plays in the nervous system. When he finally got ahold of one, the scientist touched the electric eels with his bare hands. “The shock of this animal was very powerful when the hands were placed in a favourable position, i.e. one on the body near the head, and the other near the tail,” he wrote.

More than two centuries later, a contemporary scientist is also experimenting hands-on with the electric potential of these fish.

A shocking touch, enough to put down a horse

Illustration from the book ‘Ichthyology’ by Henry G. Bohn.

Illustration from the book ‘Ichthyology’ by Henry G. Bohn.

Last year, Catania made headlines when he confirmed the plausibility of a 200-year-old myth.

Legend has it, in the year 1800, during the dry season, and German naturalist Alexander von Humboldt was in the Amazon where he was documenting new species, demystifying myths, and all sorts of other scientific endeavors. Among his many interests were eels, owing to their natural electrical generation (remember this was the early 19th century and electricity was still new). He eventually met some Amazonian natives who promised him some live eels he could take back home.

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Humboldt was jubilant and followed the men on horseback to shallow ponds and streams where the natives knew they could find the much sought after eels. The seasoned fishermen then submerged their horses in the shallow stream and the eels immediately attacked the animals’ throats. The barrage of electrical discharges stunned some of the horses causing them to sink in the water where they eventually died but also depleted the eels which function like electric batteries. Then, the natives simply picked the tired eels with their bare hands.

In the years since then, scholars have looked back at this story with skepticism, Catania included. The professor, who previously ‘discovered moles smell in stereo’, gave it a shot. He and colleagues strapped LEDs to a fake crocodile head and a plastic prop arm, which they then submerged in a water basin with a live eel. High-speed cameras captured the shocking interactions such as the moments featured in the video below.

Catania found the eel’s 600-volt pulses could indeed lash onto a horse and immobilize it if needed. This a purely defensive behavior, the biologist says, likely an adaptation that helped the eels cope with predatorial threats during the dry season when the water is shallow, and there’s not much room to escape.

The whole ordeal “was pretty intimidating,” said Catania at the time. “I made a little note to myself to come back and study this.”

Gloves off

He was wearing electrically insulating gloves back then. For his latest work published in Current Biology, the biologist with an eye for unusual animals stuck his bare arm into a tank with a small eel ten times. The point? Catania reckoned this was the only way to accurately measure the circuit created by the animal, arm, and water. The team from Vanderbilt used a device comprised of a copper wire to conduct the electricity from the shock to his arm back to the water, for this purpose.

A series of photos captures Finless, a juvenile electric eel, jumping out of the water to shock Catania’s arm. (Ken Catania/Vanderbilt University)

A series of photos captures Finless, a juvenile electric eel, jumping out of the water to shock Catania’s arm. (Ken Catania/Vanderbilt University)

The trials with the juvenile eel affectionally called Finless proved helpful indeed. The team found that ‘even small eels impart substantial electrical power to their target’, peaking at 40–50 mA, ‘greatly exceeding thresholds for nociceptor activation reported for both humans and horses.’

Extrapolating the results from being shocked by Finless for a big, adult eel, Catania found being attacked by such a fish is equivalent to getting tasered nine times — at the same time. The way the Taser works is pretty simple – it sends strong bursts of electricity to the muscles, overriding the nerves controlling the muscles and making them contract – exactly the way the eel does it.

“Results suggest that the main purpose of the leaping attack is to strongly deter potential eel predators by briefly causing intense pain. Apparently a strong offense is the eel’s best defense,” the authors wrote in their paper.

Previously, in another paper, Catania showed that the eel can totally inactivate its prey in just three milliseconds. The electric eel is very sneaky. The animal, which has very bad eyesight, detects prey by sending a specific electric signal which produces a very rapid contraction in the muscles of its prey. It’s like hunting with a remote control.

If anything, despite 200 years of research, Catania and his colleagues at Vanderbilt show us that there’s still very much to learn about these fascinating creatures. Oh, and about those eel testicles. It seems these slippery fellows don’t acquire genitals of any description until they need them.

“They’re so much more sophisticated than we possibly could have imagined. This animal can generate hundreds of volts, but they’ve also evolved to very efficiently deliver that electricity,” Catania said in a press release. 

[NOW CHECK OUT] The Garden of Eel: the out of this world, mildly disturbing feeding pattern of garden eels