Electric eels don’t seem to be the loners we thought they were.
In a small lake deep in Brazil’s Amazon River basin, scientists recorded for the first time that the fish not only live together, but actively cooperate to forage and take down their prey.
There are even indications that the strategy is working. From Volta’s abundant electric eel (Electrophorus voltai, not real eel but a kind of knife fish) that lived in the lake, many were over four feet long and thriving.
“This is an extraordinary discovery,” said ichthyologist Carlos David de Santana of the Smithsonian Institution’s National Museum of Natural History. “Nothing like this has ever been documented in electric eel.”
Not much is known about Volta’s electric eel. The fish was only recently discovered in a lake along the Iriri River and officially described and recognized as a separate species last year. But it packs a punch, capable of discharging a single shock of more than 860 volts – more powerful than any other electric eel.
De Santana and his team first observed the electric eel hunt in a group in 2012. More than 100 individuals seemed to work together to herd and kill prey so that the entire school could feed. But one observation was not enough to classify hunting as normal behavior.
In 2014, the team returned and found more electric eel from Volta, so they got down to observing and recording the animals. After 72 hours of continuous observation, they watched the electric eels hunt five more times. Not only was this enough to classify the behavior as normal, it also enabled the researchers to observe and record exactly how these “social predation events” occur.
During the day and at night, the electric eels usually rested. At dusk and dawn, dusk, the electric eels moved to hunt. This, the team noted in their paper, is unusual: Typically, Volta’s electric eels are only observed at night and solo.
The difference here is striking. On each occasion, more than 100 individual electric eels gathered and began to swim in circles, basically herding groups of smaller fish, usually characins, into a “ball of prey” that gradually drifted them into shallower water.
Once the prey ball was tightly compressed and nowhere to go, up to 10 of the electric eels moved forward and launched a powerful joint attack, intoxicating the prey – which would jump out of the water before falling back down, meaningless.
“If you think about it, a person of this kind can produce a discharge of up to 860 volts – so in theory if 10 of them could be discharged at the same time, they could produce up to 8,600 volts of electricity,” de Santana said. “That’s about the same voltage it takes to power 100 light bulbs.”
Once the prey was intoxicated, the school could move in and feed at its leisure.
Each hunt, the team found, lasted about an hour and involved five to seven electric attacks.
“Group hunting is quite common in mammals, but it’s actually quite rare in fish,” said de Santana. “Only nine other fish species are known to do this, which makes this finding really special.”
Nevertheless, while the yachts may be normal, the team still believes they can be quite rare. In their interviews with locals, the gathering and hunting behavior of the electric eel was not mentioned. So whether the electric eels gather to hunt or go solo may depend on the right conditions, such as a great abundance of prey and specific locations with a lot of shelter for large numbers of these fish.
While much is still unknown, the team believes the electric eels are likely to return to the lake annually. They have launched a citizen science project called Projeto Poraquê where locals can log sightings; that data can prove invaluable. And the team plans to return to the site in hopes of observing the animals again.
“In addition to trying to find additional populations of eels involved in group foraging, our future field and laboratory studies will investigate social predation in electric eel, focusing on the link between population, social structures, genomics and electrogenesis. they wrote. in their newspaper.
“In summary, this case provides a unique perspective for future studies of the evolutionary interaction between predatory and escape tactics in vertebrates.”
The research is published in Ecology and Evolution.