What do you call a group of electric eels? It sounds like the start of a joke, but it’s a legitimate question on the lips of researchers who made the world-first discovery that electric eels will hunt for prey in groups. Based on findings from a small lake deep in the Amazon basin, the incredibly rare observation was published in the journal Ecology and Evolution.
The study, led by the Smithsonian's National Museum of Natural History fish research associate C. David de Santana, details the discovery of a small lake containing over 100 electric eels. You might think that the eels were making up in number for what they lacked in size, but many of them were more than 1.2 meters (4 feet) long. Electric eels, who aren’t true eels but actually a type of knifefish (#EelOrNoEel) weren’t thought to aggregate in this way. Usually solitary creatures, the discovery of so many electric eels was unusual in itself, but the sighting only got spicier from there.
The eels in this river-fed lake, along the banks of the Iriri River in Brazil's state of Pará, were working together to herd small fish called tetras into tightly packed balls. Much like a pack of wolves, the eels pursued tetras in groups of 10, and would sometimes break up into smaller swarms to surround and simultaneously zap their prey. The behavior has never before been documented in electric eels, previously thought to be strictly solitary hunters, though the researchers don’t yet know how widespread the behavior is.
"Hunting in groups is pretty common among mammals, but it's actually quite rare in fishes," de Santana said in a statement. "There are only nine other species of fishes known to do this, which makes this finding really special… If 100 of [the eels] being in one place was a common occurrence, I think we would have heard about it before now."
De Santana is something of a legend among the electric fish community, having discovered 85 new species of electric fishes in South America and tripling the number of known electric eel species in just last year (electric eels wait for no pandemic). One of de Santana’s discoveries, described in a 2019 paper, is the same species to have been found hunting in groups, a disconcerting thought in the context that these eels were found to have the strongest electric discharge of any animal on Earth. The Volta’s electric eel, Electrophorus voltai, can produce a shock of 860 volts.
"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," de Santana said. "That's around the same voltage needed to power 100 light bulbs."
The next step for de Santana and his team is to take direct measurements of the shocks delivered by simultaneous attacks to better understand the full force of the zap that’s used to stun tetras. While their voltages are high, the shock lasts for around three milliseconds according to de Santana, who in an email to IFLScience described getting zapped by an eel, “I usually feel numbness in the arm that touched the electric eel. It is a strong but very short discharge.”
So, just one question remains...