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Some electric eels coordinate attacks to eliminate their prey

The electric eel of a Volta – capable of subduing small fish with a shake of 860 volts – is quite scary. Now imagine more than 100 eels circling you, unleashing coordinated electrical attacks.

Such a vision was supposed to be just a nightmare thing, at least for the dams. Researchers have long thought that these eels, a type of knife, are solitary and nocturnal hunters who use their electric sense to find smaller fish while they sleep (SN: 12/4/14). But in a remote region of the Amazon, groups of more than 100 electric eels (Electrophorus voltai) hunt together, gathering thousands of smaller fish to concentrate, shock and devour prey, researchers reported on Jan. 14 in Ecology and Evolution.

“This is enormously unexpected,” says Raimundo Nonato Mendes-Júnior, a biologist at the Chico Mendes Institute for Biodiversity Conservation in Brasilia, Brazil who did not participate in the study. "It's going to show how little we know about how electric eels behave in nature."

Volta's electric eels sometimes hunt in groups (pictured), using their numbers to corral shoals of smaller fish in shallow areas where they can be easily removed.Douglas Bastos

Group hunting is quite rare in fish, says Carlos David de Santana, an evolutionary biologist at the Smithsonian's National Museum of Natural History in Washington, D.C. “I’ve never seen more than 12 electric eels together in the field,” he says. So he was stunned in 2012 when his colleague Douglas Bastos, now a biologist at the National Amazon Research Institute in Manaus, Brazil, reported seeing more than 100 eels congregating and apparently hunting together in a small lake in northern Brazil.

Two years later, de Santana's team returned to the lake to make more detailed observations. The researchers found that eels nearly 2 feet long lay lethargically in deeper waters for much of the day. But at dusk and dawn, these long streaks of black come together, spinning in unison to form a twisted circle of more than 100 forts that brings together thousands of smaller fish in deeper waters.

A new study finds that Volta’s electric eels can come together in groups, working together to cure smaller fish in shallower waters. Then, groups of about 10 eels attack in unison, shaking the fish out of the water and turning into a stupor so they can eat them easily.

After dragging prey, smaller groups of about 10 eels trigger coordinated electrical attacks that can launch fish flights out of the water. Researchers have yet to measure the combined stress of these attacks, but 10 Volta eels shooting together could, in theory, feed something like 100 light bulbs, Santana says. The floating dams of that time helpless make easy choices for the mass of eels. The whole ordeal lasts about two hours.

So far, such aggregations have only been observed in this lake. But de Santana suspects that group hunting may be advantageous in other lakes and rivers with large shoals of small fish. Much of the scope of eels remains unexplored by scientists, so De Santana and colleagues launch a citizen science project with indigenous communities to identify more points where many eels coexist, he explains. "We still don't know much about these organisms."

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