Some electric eels coordinate their attacks to zap prey

One Volta’s electric eel is scary enough. It can subdue small fish with an 860-volt jolt. Now imagine more than 100 of them swirling about, unleashing coordinated electric attacks.

Such a sight was assumed to be only the stuff of nightmares.

These critters aren’t actually eels. A type of knifefish, they belong to the species Electrophorus voltai. Researchers had long thought they were solitary, nocturnal hunters. The fish tend to use their electric sense to find smaller fish as they sleep. But scientists didn’t have the whole story. In a remote part of the Amazon, groups of more than 100 electric eels hunt together. They corral thousands of prey. Then they shock and devour the now-concentrated fish.

Researchers described the hunting tactic January 14 in Ecology and Evolution.

“This is hugely unexpected,” says Raimundo Nonato Mendes-Júnior. He’s a biologist in Brasilia, Brazil. He works at the Chico Mendes Institute for Biodiversity Conservation and was not involved in the study. “It goes to show how very, very little we know about how electric eels behave in the wild,” he says.

Group hunting by fishes is quite rare, notes Carlos David de Santana. He is an evolutionary biologist in Washington, D.C. There, he works at the Smithsonian’s National Museum of Natural History. “I’d never even seen more than 12 electric eels together in the field,” he says. So he was stunned in 2012 by a report from his colleague Douglas Bastos. Bastos is now a biologist at the National Institute of Amazonian Research in Manaus, Brazil. Bastos described more than 100 eels congregating in a small lake. They seemed to be hunting as a group.

Two years later, de Santana’s team returned to this lake in northern Brazil. They wanted to make more detailed observations. By day, the nearly 2-meter- (2-yard-) long eels tended to lay in deeper waters, the team found. But at dusk and dawn, these long streaks of black came together. Swirling in unison, they’d form a writhing circle more than 100 strong. Such a group could herd thousands of smaller fish into shallower waters. 

After corralling the prey, groups of about 10 eels unleashed coordinated electric attacks. Those attacks sometimes sent shocked fish flying from the water. The researchers haven’t yet measured the combined voltage of such attacks. However, 10 Volta’s eels firing together could, in theory, power something like 100 light bulbs, de Santana says. Shocked and helpless, floating prey made easy pickings for the mass of eels. The whole hunt lasted about two hours.

So far, such groups have been observed in only this one lake. But de Santana suspects that group hunting may offer benefits in other lakes and rivers with large shoals of small fish. Scientists have not yet explored much of the eels’ range. So de Santana and his colleagues are launching a citizen-science project. They will work with Indigenous communities to identify more spots where many eels live together, he explains. “There is still so much we don’t know about these organisms.”