In August 2012, Douglas Bastos, then a PhD student at the Brazilian Instituto Nacional de Pesquisas da Amazônia, was exploring a remote waterway in the Amazon rainforest when he came across a small lake full of electric eels.
Electric eels, which despite their name are actually a kind of knife fish, were considered loners. And yet before Dr. Bastos’ eyes more than 100 of them. Then it got even choppier.
Dr. Bastos watched in amazement as the writhing mass of eels began to group groups of tetra fish into tightly packed balls and bombard them with synchronized electrical attacks that made them fly.
“When I saw the Tetras jumping after the attacks, I was shocked,” said Dr. Bastos. “Group hunting is a rare occurrence among freshwater fish. 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 investigate this unusual phenomenon. The results of their study, published Thursday in the journal Ecology and Evolution, overturn the idea that electric eels are solely solitary animals and raise new questions about the lives of these poorly understood fish.
When the researchers returned on the banks of Brazil’s Iriri River, they confirmed that the electric eels that Dr. Bastos observed in 2012 that it was Volta’s electric eel, a recently discovered species that can reach 8 feet in length and produce 860-volt electric shocks – the strongest electrical discharge of any animal.
For the past 250 years, scientists believed that all electric eels were from the same species. In 2019, research by C. David de Santana, a Smithsonian researcher, showed that there were at least three species, the largest and most electrified, the Volta electric eel.
According to Dr. de Santana, a co-author of the new study, had never documented such behavior in electric eels. “It was pretty unexpected,” he said.
Usually, electric eels hunt alone, sneaking up on sleeping fish and shocking them into submission. But hunting in groups can allow predators to hunt down prey that would otherwise be too fast, such as the tiny tetras. Although many mammals, including wolves and orcas, are known to hunt in groups, the strategy of fish is rarely used. It is known that only nine species of fish, including the golden saddle goat fish, hunt this way.
Dr. Bastos and Dr. de Santana analyzed over 70 hours of recordings of Volta’s electric eels conducting highly coordinated group hunts. At dusk, the slimy, serpentine creatures gathered in the shallow water and began to swim together in great circles. After balling thousands of tiny fish into dense balls, the eels split up into cooperative hunting groups of two to ten members.
These parties would then surround the terrified Tetra’s schools and launch joint electrical attacks that would cause the Tetra to jump out of the water. When the electrocuted fish splashed down, the eel quickly devoured them.
Although the researchers were unable to measure the voltage of the coordinated electrical attacks, they estimate that 10 voltas of electrical eels working together could produce an electrical current strong enough to power 100 lightbulbs.
The researchers suspect that these electric eels orchestrate their attacks by communicating via low-voltage electrical discharges.
While it is unclear whether other species of electric eel hunt in groups, experts say it isn’t unlikely. “It is possible for all electric eel species to hunt cooperatively,” said Kory Evans, a fish ecologist at Rice University.
Dr. de Santana and his colleagues plan to return to the Iriri River to collect tissue samples from the electric eels and tag them with radio tags so that they can determine if the family relationship is a factor in the fish working together, as it does with others The case of packs is hunters. He also has plans to collect some of the eels from the wild so that he and his staff can learn more about how these animals communicate.
“There is so much to learn,” he said.