Shock discovery – electric eels hunt in packs
A five-day boat ride up the Iriri River, deep in the Amazon rainforest, there is a small brown lake. At the bottom, hidden in the sludgy mud, are more than 100 electric eels, each up to 1.2m long.
These eels do things that eels shouldn’t do: they hunt in a pack.
For most of the day they lie still while, above them, tropical fish swim. But in the jungle twilight, these supposedly solitary animals come together in a swirling churning mass, herding the fish into tighter and tighter balls – until they are ready to zap them with 860 volts of electricity.
The behaviour, recorded for the first time, changes our view of the nature of eels, which are generally viewed as loners. Not only was it a surprise to see electric eels co-ordinating their hunting, it was a surprise to see a fish of any kind doing it. Only nine other species are known to do it.
‘‘This is an extraordinary discovery,’’ David de Santana, from the Smithsonian National Museum of Natural History, said. ‘‘Nothing like this has ever been documented in electric eels. Hunting in groups is pretty common among mammals, but it’s actually quite rare in fishes.’’
Yet, as with wolves or killer whales, he and his colleagues could watch as the eels circled the prey then broke off into smaller groups to go in for the kill.
In a paper in Ecology and Evolution they describe an attack in detail, in which a ‘‘prey ball’’ of tetras, a tropical fish common in aquariums, was repeatedly attacked one evening.
‘‘Prey hit by the electrical discharges were seen jumping out of the water and returning to the water surface stunned and motionless, being quickly swallowed by the eels or, in some cases, other opportunistic predators,’’ they write.
The discharge only lasts two thousandths of a second but is enough to temporarily paralyse muscles.
The researchers observed that the eels seemed to take it in turns, spreading the energy cost of zapping the tetras, and also sharing the spoils. ‘‘The prey ball was attacked each time by different subsets of eels,’’ they write. ‘‘Each event, including the movement from and to the hunting area, took about two hours from start to end, and involved five to seven joint high-voltage predatory attacks.’’
The eels, electrophorus voltai, are themselves a recently discovered species. Dr de Santana first described them in a paper in 2019, when he also revealed that they discharged 210 volts more than any other animal yet seen.
He thinks that their peculiar behaviour is probably extremely unusual because of the very specific set of circumstances in which it takes place.
‘‘This is the only location where this behaviour has been observed, but we think the eels probably show up every year,’’ he said. ‘‘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.’’
The researchers had asked people living locally if they had seen the eels swarming and attacking in this way, but none had. ‘‘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.’’