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When darkness comes, electric eels emerge from South American river bottoms to attack their prey with up to 860 volts of electricity—enough to kill a person. Now, scientists have revealed the snakelike fish don’t always go it alone: They hunt in packs, similar to wolves, orcas, and some species of tuna. The finding, a first among electric fishes, may open the way for new studies to investigate when social predation evolved among fishes.

“I was shocked,” says Douglas Bastos, a biologist at the National Institute of Amazonian Research who first saw a group attack in 2012. Usually the eels, which can grow as long as a broomstick and weigh up to 20 kilograms, prey alone at night, targeting single resting fishes, he notes. “This behavior is unprecedented for electrical eels and also rare among freshwater fishes.”

Bastos witnessed the attack on an expedition looking for endangered fish in northern Brazil. Two years later, he went back for a closer look. Camping alone in the middle of the Amazon—a 5-day journey by boat from the nearest city of Altamira—he filmed hundreds of electric eels in groups of 10 repeatedly attacking small tetra fishes on the Iriri River, a tributary of the Xingu River (which is a tributary of the Amazon River) in northern Brazil. The eels were Electrophorus voltai, which deliver the strongest shock ever measured in a living animal.

Most of the time, the eels rested at the bottom of the river. But at dawn and dusk, they moved to a shallow pool at the riverbank of the Iriri where they herded shoals of fish and launched joint shock attacks, immobilizing and devouring their prey (see video, above). Each hunt usually took 2 hours and involved up to seven attacks the researchers report today in Ecology and Evolution.

So far, the group attacks have only been recorded in the low-water season, which runs from June to November. During those months, which see a significant reduction in rainfall, fishes in the region’s rivers are squeezed into far less water than normal, making them easy pickings for the eel packs. The eels simply have to find a pool full of fish and ambush them. In the high-water season, prey would likely have a much better chance of keeping a safe distance—and getting away, says co-author Carlos David de Santana, a co-author and biologist at the Smithsonian Institution’s National Museum of Natural History.

The researchers don’t know how this behavior evolved, how widespread it is, and whether it happens only during the low-water season. But Santana has some thoughts on how the eels might pick out their groups. “This social predation could have emerged based on a simple individual-level rule: Keep foraging with the same individuals when successful, and then a stable group forms.”

The study is “a fascinating example of careful natural history detective work,” says William Crampton, a biologist at the University of Central Florida who was not part of the work. He has been studying electric fishes for 25 years and says the findings could spur further research into how the eels use electric discharges to navigate and communicate.

Bastos and his colleagues next plan to sequence the genomes of the eels, in hopes of identifying genes linked to social predation shared by other animals that display this behavior, including mammals like whales. This could give them clues to better understand the history of this trait’s evolution among a wide variety of animals.

Crampton suspects other species of electric eels may utilize the same hunting strategy. On a recent trip to Guyana, he says he heard reports from Indigenous peoples about shoals of fish jumping up out of the water near large groups of electric eels of another species (E. electricus). “I’ve never witnessed this behavior myself, but if these stories are true, it suggests that the behavior might not be restricted to E. voltai.”

In any event, the researchers will have to act fast, because human destruction of the Amazon is wiping out these habitats, Santana says. “It is possible that in 5 or 10 years, these locations will not be preserved anymore.”