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When an Eel Takes a Bite, Then an Octopus Might Lose an Arm but Take an Eye

The conger eel was the favorite, weighing at least three times more than its eight-armed opponent. But by the time the video footage begins, the underdog octopus had already asserted its toughness, blocking off the eel’s eyes and stuffing arms into its mouth and out the gill hole.

“I thought that with such difference in size it would be hard for the octopus to avoid death,” said Jorge Hernández-Urcera, a marine ecologist at the Institute of Marine Research of the Spanish National Research Council.

The common octopus not only defended itself, but also seemed to come out on top. The divers who made the video — not scientists — broke up the brawl, and the two animals survived, the octopus tearing off in a cloud of ink.

“It was very impressive to see,” said Dr. Hernández-Urcera, who collects amateur diving videos and analyzes them for previously undescribed behavior. He believed this video, recorded in 2008 off the coast of Galicia in northwestern Spain, showed “the intelligence of octopus and the big repertoire of defensive behavior.” But it was only a single video, not enough to suggest that this eight-armed technique was a regular form of octopus martial arts.

More recently, Dr. Hernández-Urcera acquired additional video footage. Enough, he believes, to show that octopuses will choke, blind and sacrifice limbs in an effort to defend themselves from much bigger eel foes. He published his research in March in the journal Ecology and Evolution.

In this 2022 video, taken off the coast of the northern Spanish region of Asturias, another common octopus uses the same tactics seen in the first video to defend against a conger eel attack. In response, the eel rapidly spins to get loose. At this point the octopus escapes, but it manages to pop out one of the eel’s eyeballs with the power of a sucker.

In each video, the common octopus may sacrifice arms, much as lizards drop their tails to distract predators, Dr. Hernández-Urcera said. In the first video, the octopus loses three arms while the one in the second video loses two — but they can fully regrow limbs in about 45 days, some lab tests show.

The octopus doesn’t always win. In a third video, taken in 2023 near Galicia again, the conger eel grabbed the cephalopod from the head, then spins the octopus around in a death hold, slamming it onto rocks. The octopus appears stunned, if not dead, and then the eel swims off with its prey.

Piero Amodio, a biologist and comparative psychologist with the Anton Dohrn Zoological Station in Naples in Italy, thinks that when plugging the eel’s gills, the octopus’s arms may operate instinctually — literally with a mind of their own because of the many neurons in their appendages.

“My hunch is that the arms can do that in a non-completely conscious way,” Dr. Amodio said.

He said he had seen fights between octopuses where one will fill the gill hole of the other with an arm, asphyxiating its foe.

Peter Tse, a professor of cognitive neuroscience who works with octopuses at Dartmouth College, calls it a “remarkable adaptation” and points to a scene in the documentary “My Octopus Teacher,” where an octopus resists an attack from a shark by climbing onto the back of its head.

“If it’s not an instinct but something that they think of individually, it’s even more remarkable,” Dr. Tse said, “because it takes quite a bit of insight to understand that the only way to escape this mouth is to get behind it.”

Dr. Hernández-Urcera isn’t sure whether the fighting techniques in his videos are instinctual or learned behavior. But that’s partly because conger eels typically hunt at night, whereas these videos were taken in the day. He believes these encounters may happen frequently.

“We think from the beginning of their lives,” he said, “octopus have to face these kinds of attacks from conger eels.”

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