Volta’s electric eel can come together in groups and work together to correlate smaller fish in shallower waters, a new study finds. Then smaller groups of about 10 eels attack along with high-voltage discharges.
It has long been believed that electric eels were predators only, preferring to hunt and kill their prey alone by sneaking up on unsuspecting sleeping fish at night and subduing them. But according to a recent article published in the journal Ecology and Evolution, there are rare circumstances in which eels rather use a social hunting strategy. Specifically, researchers observed more than 100 electric eels in a small lake in the Brazilian Amazon basin that forms cooperative hunting parties to catch small fish, tetras.
“This is an extraordinary discovery,” said co-author C. David de Santana of the Smithsonian National Museum of Natural History. “Something like this has never been documented in electric eels. Hunting in groups is pretty common among mammals, but it’s actually very rare in fish. There are only nine other fish species that do, which makes this finding really special.”
Electric eels are technically knife fish. The eel produces its characteristic electrical discharges – both low and high voltage, depending on the purpose of the discharge – via three pairs of abdominal organs consisting of electrocytes, symmetrically along both sides of the eel. The brain sends a signal to the electrocytes, opens ion channels and briefly reverses the polarity. The difference in electric potential then generates a current, just like a battery with stacked plates.
Douglas Bastos
This is not the first time that researchers have made surprising discoveries about electric eel. The 19th-century physicist Michael Faraday, for example, conducted several experiments in 1838 with electric eels. He noted that he experienced only mild shocks because the water dispersed the effluent so quickly. Kenneth Catania, biologist and neuroscientist at Vanderbilt University, is nowadays one of the most famous scientists studying electric eel. He found that the creatures can vary the degree of voltage in their electrical discharges by using lower voltage for hunting purposes, and higher voltage to stun and kill prey. That higher voltage is also useful for detecting potential prey, similar to how bats use echolocation.
And in 2016, Catania reported evidence in support of Alexander von Humboldt’s 1800 version of how at that time Venezuelan natives used wild horses to lure and trap electric eels (“horse fishing”). The bumping and snoring of horses in the shallow waters favored by electric eels caused the latter to jump up and stun the horses with a series of high voltage electric discharges as a defense mechanism. Once the eel was exhausted, the natives could easily catch it by using small hairpins on ropes.
Physicists have rejected Humboldt’s report for centuries because no one has noticed such behavior since – until Catania spotted eels in his laboratory, as did Humboldt’s description on the net used to transfer the eels from their cages to the room that Catania used for experiments. He did experiments with LEDs mounted on a fake crocodile head (equipped with conductive tape to visualize the discharges). The eel aggressively attacked the fake crocodile, just as described by Humboldt. Catania believes the reaction begins under certain conditions, such as when eels are trapped in small bodies of water with the sudden arrival of the dry season.
Until 2019, scientists thought the electric eel was the only species of its kind. This was the year in which de Santana published an article, effectively tripling the number of known types of electric eels. Under the topic was the topic of the current article: Volta’s electric eel (Electrophorus voltai).
“A person of this species can produce a discharge of up to 860 volts, so in theory, if ten of them are discharged simultaneously, they can produce up to 8,600 volts of electricity,” de Santana said. “It’s about the same voltage needed to power 100 light bulbs.” He has been shocked in the field several times and notes that, although it only takes a fraction of a second, a shock can still cause painful muscle spasms.
DA Bastos et al., 2021
De Santana and his co-authors noticed the unusual group hunting behavior on a field expedition of 2012 to explore the diversity of fish in the Iriri River, when team member (and co-author) Douglas Bastos found a small lake with more than 100 electric lake eels. A second expedition in 2014 found a similar group at the same location, and the team would eventually record 72 hours of continuous observation and record the behavior of the eel.
The eel mostly hung in the deeper side of the lake and occasionally showed up to breathe. But the eel became active at dusk and dawn. De Santana’s team noticed how the eels would work together to herd tetra schools in densely packed areas in the shallow waters, by swimming in a large circle to create the equivalent of a bead. Then the eel spits in smaller hunting parties of about 10 eels, surrounds the ball of tetras and demonizes the small fish with synchronized high-voltage discharge. This made it very easy to snap up the anesthetized tetras.
“This is the only place where this behavior is observed, but at the moment we think the eels probably occur every year,” de Santana said. “Our initial hypothesis is that this is a relatively rare event that only occurs in places with a lot of prey and enough shelter for large numbers of adult eels.” If the behavior was commonplace, he reasoned, it would have come to the fore in their interviews with the locals.
De Santana and his team will continue their investigation into this unusual behavior; they hope to make direct measurements of the synchronized discharges during their next expedition. And they launched a civil science program called Project Poraque to track down additional packets of electric eels in the region. The team will also collect eight to ten adult eels and bring them to a laboratory in Germany, the better to study them among other controlled institutions.
DOI: Ecology and Evolution, 2021. 10.1002 / ece3.7121 (on DOIs).
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