By KOICHI AMANO/ Staff Writer
September 12, 2024 at 17:13 JST
NAGASAKI—A team of researchers here has captured astonishing X-ray footage of young eels making a dramatic, Houdini-like escape from the stomach of a large predatory fish.
After being swallowed whole, these resilient eels actively resist digestion and slither their way back up the predator’s gullet to freedom.
The study, published in the Sept. 9 issue of the scientific journal Current Biology, was led by assistant professor Yuha Hasegawa and associate professor Yuki Kawabata, who both specialize in ecology at Nagasaki University’s graduate school.
Intrigued by eels’ ability to wriggle through narrow spaces, the researchers hypothesized that they might exhibit unique escape behaviors when preyed upon.
To test this theory, in 2021, they placed young Japanese eels in a tank with dark sleepers, which are larger, carnivorous freshwater fish.
They observed that more than half the eels, after being eaten by the dark sleepers, would miraculously “reappear”—by escaping through the predator’s gills.
To further examine the eels’ behavior within the predator’s body, the researchers used X-ray imaging in their latest study.
Although eels have very thin bones, which initially made it difficult for the X-ray to pick up their movements, the team overcame this challenge by injecting the eels with barium sulfate, a contrast agent commonly used in medical imaging. This allowed the team to clearly see the eels’ actions inside the predator.
The X-ray footage revealed a fascinating escape strategy.
The eels would initially swim in circles within the predator’s stomach before probing into its gullet with their tails. The intrepid escape artists would then use their tails to navigate backward through the fish’s digestive tract toward its mouth and eventually escape tail first through the predator’s gills.
This behavior is similar to how eels typically enter narrow spaces in their natural habitat.
The researchers originally thought that the eels had been escaping through the predator’s gills while still in its mouth. However, seeing the eels break free even after being swallowed whole into the fish’s deadly digestive tract was even more remarkable and unexpected.
Kawabata noted that while many organisms, such as pufferfish, rely on toxins to deter predators, the eel escape behavior is a rare strategy.
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A young eel escapes from the stomach of a predator. (Provided by assistant professor Yuha Hasegawa and associate professor Yuki Kawabata from Nagasaki University)
