Scientists were surprised to find that electric eels sometimes hunt in groups

A new study has found that electric volta eels can congregate in groups and work together to trap small fish in shallow water. Then, smaller groups of about 10 snakes attack in unison with a high voltage discharge.

Electric snakes have long been believed to be solitary predators, preferring to hunt and kill their prey on their own by sneaking up on comfortably sleeping fish at night and ramming them to force them into submission. But according A recent paper Published in Ecology and Evolution, there are rare circumstances where eels use a social fishing strategy instead. Specifically, researchers observed more than 100 electric eels in a small lake in the Brazilian Amazon River Basin forming cooperative fishing groups to capture small fish called tetras.

“This is an extraordinary discovery.” Co-author C. David de Santana, From the Smithsonian National Museum of Natural History. “Nothing like this has been documented in electric eels. Hunting in groups is very common among mammals, but it is actually very rare in fish. There are only nine other species of fish known to do so, which makes this discovery really special.”

Electric eels are technically knife thickness. The eel produces its characteristic electrical discharge – both low and high voltages, depending on the purpose of the discharge – via three pairs of abdominal organs made up of electrophysiological cells located symmetrically on either side of the eel. The brain sends a signal to the electrical cells, opening ion channels and reversing polarity for a while. The difference in voltage generates current much like a battery with stacked panels.

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Zoom in / Multiple Volta electric snakes attack as a group, slamming the fish out of the water and entering into a stupor until they can be eaten easily.

Douglas Bastos

This isn’t the first time researchers have made surprising discoveries about electric eels. For example, the nineteenth century physicist Michael Faraday He performed several experiments With electric eels in 1838. He indicated that he felt only mild shocks because the water dissipated the discharges so quickly. Biologist and neuroscientist at Vanderbilt University Kenneth Catania It is one of the most prominent scientists who study electric snakes these days. He found that creatures can The degree of voltage varies In its electrostatic discharge, using low voltages for hunting purposes, high voltages to stun and kill prey. These higher voltages are also useful for tracking potential prey, similar to how bats are used for echolocation.

And in 2016Catania I mentioned the evidence Support for Alexander von HumboldtAn 1800 report of how Venezuelans used wild horses at the time to attract and confine electric eels (“horse hunting”). Horses’ stamping and snoring in the shallow waters preferred by the electric eel caused the horses to jump and stun the horses with a series of high-voltage electrical discharges as a defense mechanism. Once exhausted, the snakes can be easily captured by the natives with small spears on the ropes.

For centuries, naturalists rejected the Humboldt description, because no one had observed such behavior since – until Catania spotted snakes in his laboratory reacting a lot to Humboldt’s description upon seeing the net used to transport snakes from their cages into the room that Catania used to conduct experiments. Experiments with LEDs mounted on a fake alligator head (supplied with conductive tape to photograph the vacuum). The snakes attacked the fake crocodile’s head with force, as Humboldt described it. Catania believes the response begins under certain conditions, such as when snakes are stranded in small bodies of water with the sudden arrival of a dry season.

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Until 2019, scientists believed that the electric eel was the only species in its own genus. That was the year of de Santana Published a paper Triple the number of known species of electric snakes. Among these topics was the topic of the present paper: the electric eel in Volta (Electrophorus voltai).

“An individual of this type can produce up to 860 volts of discharge, so theoretically, if 10 of them are discharged at the same time, they can produce up to 8,600 volts of electricity,” Di Santana said. “This is the same voltage required to power 100 light bulbs.” He’s been shocked multiple times in the field and noted that while it only lasts for a fraction of a second, the shock still causes painful muscle contractions.

Schematic illustration of the stages of social predation observed in electric eels.
Zoom in / Schematic illustration of the stages of social predation observed in electric eels.

DA Bastos et al., 2021

De Santana and colleagues first noticed the unusual mass fishing behavior on a field expedition in 2012 to explore the diversity of fish in the River Erie, when team member (and co-author) Douglas Bastos found a small lake filled with more than 100 electric fish and eels. A second expedition in 2014 found a similar-sized group at the same site, and the team will eventually record about 72 hours of continuous monitoring, recording eels’ behavior.

Most of the time, snakes were hanging out at the deep end of the lake, sometimes floating around to breathe. But snakes become active at dusk and dawn. De Santana’s team observed how eels would work together to raise tetras in densely populated areas in shallow water, by swimming in a large circle to create the equivalent of a barn. Then the eels spit out in small fishing groups of about 10 snakes, surround the tetra reel and stun the young fish with a synchronous high voltage discharge. This made it very easy to catch the amazing tetras.

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“This is the only place this behavior has been observed, but at the moment we think eels may appear every year.” Di Santana said. “Our initial hypothesis is that this is a relatively rare event that only occurs in places where there is a lot of prey and adequate shelter for large numbers of adult snakes.” If the behavior had been commonplace, he said, it would have appeared in their interviews with locals.

De Santana and his team will continue their investigations into this unusual behavior. They hope to make direct measurements of the simultaneous discharges on their next expedition. And they launched a Citizen Scientist program called Project Poraque to track down additional packs of electric eels in the area. The team will also collect eight to 10 adult snakes and bring them to a laboratory in Germany, best studied under more controlled conditions.

DOI: Ecology and Evolution, 2021. 10.1002 / ece3.7121 (About DOIs).

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