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.
“We thought the eel escaped from the mouth of the predator, but in the first footage we recorded the eel escaped from the stomach of the predator,” Dr. Hasegawa said. “It moved back up the digestive tract towards the gill of the fish.”
Of the 32 eels the researchers recorded being swallowed whole, all but four tried to escape by wriggling their way, tail first, out of the digestive tract, up the esophagus and out the gills.
And of those 28 who tried, only nine succeeded.
Time played a large role in the success of these escapes. The young eels could spend roughly three minutes inside the fish before dying, the researcher found. Those that escaped successfully spent just under a minute liberating themselves.
According to the researchers, these eels are built for such escapes. “Burrowing and swimming backward are both natural behaviors,” Dr. Kawabata said. Not only are eels naturally slimy, but their slender, muscular bodies are designed to fit into tight crevices in a river bed. If any species could pull off a breakout of this caliber, the researchers argue, it’s these eels.
“It’s an impressive feat,” says Kory Evans, a fish biologist at Rice University in Houston who wasn’t involved in the study.
He noted that the fish’s gills have bony arches and other obstacles.
“Oftentimes, fish have teeth on their gill surfaces, so slipping past that can be quite a challenge,” Dr. Evans said, adding that the study offered “another reminder that eels are really cool.”
The escape strategy used by these young eels is fairly uncommon in the world of wildlife. Only a handful of other species are known to be capable of such feats, and they use very different methods. For example, when a bombardier beetle finds itself in the stomach of a predatory toad, it will eject a noxious fluid from the tip of its abdomen that forces the amphibian to throw up.
The findings of the eel study have raised many questions about the ability of eels and other slender-bodied fishes to snatch victory from the hungry jaws of defeat. The researchers are now investigating whether young Japanese eels use this strategy when swallowed by other predatory species. They are also interested in finding out whether this behavior has evolved in other species.
It may take years to get the answers they seek. But if their research has taught these scientists anything, it’s to never give up.
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