The behavior, used by wolves and orcas to run down fast prey, is rarely seen in fish.
In August 2012, Douglas Bastos, then a graduate student at Brazil’s Instituto Nacional de Pesquisas da Amazônia, was exploring a remote waterway in the Amazon rainforest when he came across a small lake teeming with electric eels.
Electric eels, which despite their name are actually a type of knifefish, were believed to be solitary creatures. And yet before Dr. Bastos’s eyes were more than 100 of them. Then things got even more jolting.
Dr. Bastos watched, astonished, as the writhing mass of eels began corralling groups of tetra fish into tightly packed balls and bombarding them with synchronized electric attacks that sent them flying.
“When I saw the tetras jumping after the attacks, I was in shock,” Dr. Bastos said. “Group hunting is a rare event in freshwater fishes. My first reaction was to run to the boat and get a camera.”
Two years later, Dr. Bastos and researchers from the Smithsonian National Museum of Natural History returned to the area to study this unusual phenomenon. The findings of their study, published Thursday in the journal Ecology and Evolution, overturn the idea that electric eels are exclusively solitary predators and raise new questions about the lives of these little-understood fish.
When the researchers returned to the site, along the banks of Brazil’s Iriri River, they confirmed that the electric eels Dr. Bastos had observed in 2012 were Volta’s electric eels, a recently discovered species that can reach 8 feet in length and are capable of producing 860-volt electric shocks — the strongest electric discharge of any animal…
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