Electric eels don’t seem to be the lonely ones we thought they were.
In a small deep lake in the Amazon River Basin in Brazil, scientists have for the first time recorded fish not only living together, but also actively working together to feed and demolish their prey.
There is even evidence that the strategy works. Of the abundant electric eels of Volta (Electrophorus voltai, not a real eel, but a type of knife fish) found living in the lake, many were over 1.2 meters long and thrived.
“This is an extraordinary discovery,” said ichthyologist Carlos David de Santana of the Smithsonian Institution’s National Museum of Natural History. “Nothing like electric eels has ever been documented.”
Not much is known about the Volta electric eel. The fish was only recently discovered in a lake along the Iriri River and last year was officially described and recognized as a different species. But it has a punch capable of discharging a single discharge greater than 860 volts, more powerful than any other registered electric eel.
De Santana and his team first observed electric eel hunting in a group in 2012. More than 100 individuals appeared to be working together to row and kill prey so that the entire bank would feed. But one observation was not enough to classify hunting as normal behavior.
In 2014, the team returned and found even more Volta electric eels, so they set to work observing and recording the animals. For more than 72 hours of continuous observation, they watched as the eels engaged in five more hunts. Not only was it enough to classify behavior as normal, but it allowed researchers to observe and record exactly how these “social predation events” occur.
During the day and night, the electric eels rested mostly. At dusk and dawn, at dusk, the electric eels shuffled to hunt. This, the team noted in their work, is unusual: Normally, Volta’s electric eels are only observed looking for food at night and alone.
The difference here is staggering. On each occasion, more than 100 individual electric eels grouped together and began swimming in circles, effectively grouping smaller groups of fish, mostly prey, into a “catch ball” that gradually whistled into shallow water. .
Then, once the ball of the prey was stretched hard to where to go, up to ten of the electric eels advanced and launched a powerful joint attack, impressing the prey, which would jump out of the water before falling again, meaningless.
“If you think about it, an individual of this species can produce a discharge of up to 860 volts, so in theory, if ten were discharged at once, they could produce up to 8,600 volts of electricity,” he said. dir Santana. . “It’s about the same voltage needed to power 100 light bulbs.”
Once the dam was caught, the bank was able to enter and feed freely.
The team found that each hunt lasted about an hour and involved five to seven electric shocks.
“Group hunting is quite common among mammals, but it’s actually quite rare in fish,” Santana said. “There are only nine other known fish species that do, which makes this finding really special.”
Although, while hunting may be normal, the team still believes they could be quite rare. In their interviews with locals, the congregation of electric eels and hunting behavior were not mentioned. Therefore, whether electric eels gather to hunt or go solo may depend on appropriate conditions, such as the high abundance of prey, and specific places with plenty of shelter for a large number of these fish.
Although many are still unknown, the team believes electric eels will likely return to the lake each year. They have launched a citizen science project called Projeto Poraquê, where locals can record observations; these data could prove invaluable. And the team plans to return to the site in hopes of observing the animals again.
“In addition to attempting to locate additional populations of eels involved in group feeding, our future field- and laboratory-based studies will investigate social predation in electric eels by focusing on the link between population, social structures , genomics and electrogenesis, “they wrote. to their role.
“In summary, this case offers a unique perspective for future studies on the evolutionary interaction between predatory and escape tactics between vertebrates.”
The research has been published in Ecology and evolution.