- Previous studies suggested electric eels were solitary creatures
- But the fish have been spotted hunting in a group in the Amazon River basin
- The behaviour raises questions about how electric eels communicate With the ability to deliver deadly zaps of up to 600 volts, electric eels are without a doubt some of the most fearsome creatures on the planet.
Now, the terrifying fish have been spotted hunting in groups for the first time, delivering a deadly combined zap of up to 8,600 volts to unsuspecting prey.
The 'extraordinary' behaviour was spotted in the Brazilian Amazon River basin, and raises new questions about how electric eels communicate.
Researchers say that if 10 of the eels discharged at the same time, they could be producing up to 8,600 volts of electricity – around the same voltage needed to power 100 light bulbs.
Now the scientists who made the 'extraordinary' discovery deep in the Brazilian Amazon River basin say the never previously seen behaviour raises new questions about how electrical eels communicate.
The team discovered a small, river-fed lake filled with more than 100 adult electric eels, many of whom were upwards of four feet long.
In the lake along the banks of the Iriri River, the researchers saw the eels working together to herd small fish called tetras into tightly packed balls.
Then groups of up to 10 eels periodically split off to form cooperative 'hunting parties' - not unlike packs of wolves or pods of killer whales.
The smaller groups then surrounded the prey ball and launched simultaneous electric attacks, stunning the tetras into submission.
Study senior author Dr David de Santana, fish research associate at the Smithsonian National Museum of Natural History in the US said: 'This is an extraordinary discovery.
'Nothing like this has ever been documented in electric eels.'
The team's findings, published in the journal Ecology and Evolution, overturn the notion that the serpentine fish are exclusively solitary predators.
Dr de Santana said: 'Hunting in groups is pretty common among mammals, but it's actually quite rare in fishes.
'There are only nine other species of fishes known to do this, which makes this finding really special.'
The discovery is the latest in a string of revelations driven by Dr de Santana's investigations of the mysterious lives of South America's electric fish.
His pioneering expeditions into the remote waters of the Amazon and its many tributaries have brought to light 85 new species of electric fishes.
Just last year, he tripled the number of known species of electric eels, which had stood at one for around 250 years.
One of the new species, Volta's electric eel (Electrophorus voltai), is capable of producing 860-volt electric shocks - the strongest electric discharge of any animal on Earth and 210 volts higher than the previous record.
Volta's electric eel, which can reach lengths of up to eight feet, is also the species behind the social hunting strategy at the centre of Dr de Santana's new research.
He said: 'If you think about it, an individual of this species can produce a discharge of up to 860 volts - so in theory if 10 of them discharged at the same time, they could be producing up to 8,600 volts of electricity. That's around the same voltage needed to power 100 light bulbs.'
Direct measurements of the simultaneous shocks are one of the things Dr de Santana and his colleagues hope to collect on their next expedition.
Fortunately for Dr de Santana, who has been shocked more than once by individual eels in the field, the shock only lasts about two-thousandths of a second, but it is enough to cause a painful muscle spasm that might knock a person off their feet.
His team first witnessed Volta's electric eel hunting in groups during a field expedition in August of 2012.
A subsequent expedition in October 2014 found a similar collection of Volta's electric eels in the same locality, which allowed the team to confirm that it was not just a one-off event.
For the majority of the day and the night, the eels lay almost motionless in the deeper end of the lake, only occasionally heading to the surface to breathe. But at dusk and dawn the swarm began to stir.
In the twilight hours, the eels started interacting with each other and then began swimming in a big circle.
The churning circle of electric eels corralled thousands of tetras into tighter and tighter shoals.
The research team watched the group herding the concentrated tetras from the deeper end of the lake – around 12 feet deep – to shallow, three-foot deep waters.
Groups of two up to 10 eels would separate, move in closer and then launch joint electric attacks on the prey ball.
The electric shocks sent the tetras flying out of the water, but when they splashed down the small fish were stunned and motionless.
The attacking eels and their compatriots then easily picked off the defenseless prey.
Dr de Santana says each dawn or dusk hunting ritual took around an hour and contained between five to seven high-voltage attacks.
He said: 'This is the only location where this behaviour has been observed, but right now we think the eels probably show up every year.
'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.
'These animals can be eight feet long and produce 860-volt electric shocks; if 100 of them being in one place was a common occurrence, I think we would have heard about it before now.'
But when the conditions are right, Dr de Santana explained that the hunting technique allows the eels to subdue huge quantities of prey that are normally too evasive to capture.
Electric eels customarily feed alone at night by sneaking up on sleeping fish and jolting them into an easy-to-eat torpor.
Dr de Santana and his team hope that a newly launched citizen scientist programme called Projeto Poraquê may help locate more of the special group of eels.
The project, named after an Indigenous Brazilian word for electric eel, will allow users to report sightings and log observations.
With the Amazon under threat from deforestation, fire and climate change, Dr de Santana says there is a profound sense of urgency to accelerate biodiversity assessment in the region.
He added: 'Electric eels aren't in immediate danger, but their habitats and ecosystems are under immense pressure.
'This paper is an example of how much we still don't know, how many organisms whose life histories we don't yet understand.'