The Terrifying Way That Eels Escape a Hungry Fish’s Stomach

For most animals, ending up in a predator’s stomach means all is lost. But not for Japanese eels.

In a study published on Monday in the journal Current Biology, scientists filmed juvenile Japanese eels staging Houdiniesque feats of escape from inside a predatory fish. After being swallowed and deposited into the fish’s stomach, the young eels swam up the hunter’s esophagus and escaped through an opening in its gills, much to the fish’s displeasure.

This is the first time scientists have been able to see exactly how a prey species escaped a predator’s stomach. Researchers now wonder if other slender-bodied species use the same lifesaving technique.

In 2021, Yuha Hasegawa, then a graduate student at Nagasaki University, released a young Japanese eel into a tank with a dark sleeper, a predatory river-dwelling fish. He watched as the small eel was swallowed whole.

Less than a minute later, the eel suddenly reappeared in the tank.

He assumed that the eel must have escaped while still in the fish’s mouth, but he had no idea how. Now an assistant professor, Dr. Hasegawa and his colleague Yuuki Kawabata, an associate professor at Nagasaki University, spent the next three years trying to solve this mystery.

It took a device that recorded X-ray movies to reveal the eel’s escape technique. The researchers injected dozens of young eels with a substance that made them stand out under X-rays and put them into a tank with a dark sleeper. As soon as the fish ate the eels, the researchers would lightly anesthetize the fish and put it under their recording device. What they saw shocked them.