Electric eels usually hunt alone at night.
CNN  — 

Electric eels have been found to hunt in packs in the first documented case of its kind, which researchers have called an “extraordinary discovery.”

Scientists working in the Brazilian Amazon River basin were surprised to find a small lake containing more than 100 adult electric eels, which are usually found to be solitary animals, and stunned to see them cooperating to hunt fish, according to a press release from the Smithsonian Natural History Museum on Thursday.

“This is an extraordinary discovery,” said the museum’s fish research associate David de Santana, who led the research team, in the press release. “Nothing like this has ever been documented in electric eels.”

The electric eels – a type of knifefish – were living in a lake on the Iriri river in Pará state, northern Brazil.

The electric eels observed at the lake can produce 860-volt shocks.

They would work together to push small tetra fish into tight shoals, before hunting parties of up to 10 eels would split from the main group and attack the fish, stunning them.

This behavior is reminiscent of packs of wolves and pods of killer whales, according to the press release, and is novel in electric eels.

In this situation, with so many prey fish around, “hunting alone would be less efficient,” de Santana told CNN. “By acting in groups the eels have more chance of success.”

While hunting in groups is common in mammals, it’s quite rare in fish. De Santana said in the press release there were only nine other species of fish known to hunt in such a way “which makes this finding really special.”

Volta’s electric eels (Electrophorus voltai) can grow up to 8 feet long and produce 860-volt shocks – the strongest of any animal on Earth and enough to power 10 light bulbs.

These shocks last around two-thousandths of a second and can cause painful muscle spasms strong enough to knock a human off their feet, according to the press release.

Hunting in packs is quite rare among fish.

Normally, electric eels use these shocks when they hunt resting fish at night, stunning them so they are easy to eat, but hunting tetras in packs makes sense in some environments.

“Our initial hypothesis is that this is a relatively rare event that occurs only in places with lots of prey and enough shelter for large numbers of adult eels,” said de Santana.

However, there are likely many places in the Amazon Basin with these characteristics, de Santana told CNN, adding that he saw a small congregation of electric eels during a trip to Suriname in 2018.

Now we are having much more access to those remote areas in the Amazon because of deforestation,” he said. “So maybe it will be more often that we are going to find this kind of behavior or that we’re going to hear local people talk about that.”

The findings were published in the journal Ecology and Evolution on Thursday.