For the first time, electric eels were seen as a group that hunted prey

Eels don't seem to be the loners we think they are.

In a small lake deep in the Brazilian Amazon, scientists have documented for the first time that these fish not only live together, but also actively forage together and lay down their prey.

There is even evidence that the strategy is working. Discover a large number of eels (Electropus voltai, not real eels, but a kind of knife fish) living in the lake, many of which are more than 1.2 meters (4 feet) long and long.

Carlos David de Santana, a ichthyologist at the Smithsonian Institution's National Museum of Natural History, said: "This is an extraordinary discovery." "Nothing like this is recorded in an electric eel."

Little is known about the eels of The Volta. The fish was only recently discovered in a lake along the Eriri River and was officially described last year as a recognized unique species. But it comes with a punch capable of unleashing a single electric shock of more than 860 volts – more powerful than any other eel on record.

De Santana and his team first observed a herd of eels hunting in 2012. About 100 people work together to graze and kill prey so that the entire shoal can forage. However, one observation alone is not enough to classify hunting as normal behavior.

In 2014, the team returned and found more volta's electric eels, so they began work on observing and documenting the animals. After 72 hours of continuous observation, they saw the eels hunted five more times. Not only is this enough to classify behavior as normal, but it also allows researchers to observe and accurately document how these "social predatory events" occur.

During the day and night, the eels mostly rest. At dusk and dawn, twilight, electric eels stirred up to hunt. The team noted in the paper that this is unusual: usually, Volta's eels are only observed foraging at night and alone.

The difference here is striking. In each case, more than 100 individual eels gather and begin to swim, effectively gathering flocks of small fish (mainly characins) into a "ball of prey", which then gradually turns into shallow water.

Then, once the prey ball is tightly bound and has nowhere to go, up to 10 electric eels move forward and launch a powerful joint strike, shocking the prey – the prey will jump out of the water and then fall down, meaningless.

"If you think about it, an individual of this species can produce up to 860 volts of discharge — so, theoretically, if 10 of them are discharged at the same time, they can produce up to 8,600 volts of electricity," Santana said. Approximately the same voltage required to power 100 bulbs. ”

Once the prey is stunned, the shoals can enter and forage leisurely.

The team found that each hunt took about an hour and involved five to seven electric shocks.

Santana said: "Hunting in groups is very common in mammals, but rare in fish." "Only nine other species of fish can do that, which makes this discovery really special."

Still, while hunting may be normal, convoys still think they may be rare. In interviews with locals, there was no mention of the gathering and hunting of eels. Therefore, whether eels gather to hunt or forage alone may depend on the right conditions, such as the abundance of high prey and specific locations where there are plenty of shelters for such fish.

Although there are still many unknowns, the team believes that the eels may return to the lake every year. They launched a citizen science project called Projeto Polaraquê, where locals can record observations. These data may prove invaluable. The team is planning to return to the site, hoping to observe the animals again.

"In addition to trying to find more collective foraging eel populations, our future field- and laboratory-based research will investigate the social predation of electric eels, focusing on the link between populations, social structures, genomics, and electrogenesis," they wrote.

In short, this situation provides a unique perspective for future studies of the evolutionary interaction between predatory and evasive strategies between vertebrates. ”