Scientists in Japan have discovered that Japanese eels can escape from the digestive tract of predatory dark sleeper fish after being swallowed. The study, led by Yuha Hasegawa from Nagasaki University, was published in the journal Current Biology.
The researchers found that these eels can move backwards up the predator’s digestive tract and push their tails through the gills to free their heads.

Previous discovery

In an earlier study, the researchers had found that some eels escaped through a predator’s gills after capture but did not understand the exact mechanism behind this behaviour.
“However, contrary to our expectations, witnessing the eels’ desperate escape from the predator’s stomach to the gills was truly astonishing for us,” Hasegawa said, as quoted by The Guardian.

X-ray videography used

To investigate this phenomenon, the team used an X-ray videography device to observe dark sleeper fish, Odontobutis obscura, after they swallowed juvenile Anguilla japonica eels. The eels were injected with a contrast agent to enhance their visibility. It took the researchers a year to gather enough clear footage of the escape process. They observed the eels backing up, inserting their tail tips into the predator’s esophagus, and eventually pulling their heads free.

“The most surprising moment was seeing the first footage of eels escaping by moving back up the digestive tract toward the predator’s gills,” said researcher Kawabata.

104 Japanese eels involved

The study involved 104 Japanese eels placed in a tank with 11 dark sleeper fish. Using an X-ray video system, researchers tracked the eels injected with barium sulfate. Out of 32 eels captured by the sleeper fish, nine managed to escape using the tail-first approach. Hasegawa noted that the predatory fish were not harmed, although some eels showed signs of abrasions.

The researchers also observed that some eels failed to escape fully, with four getting their tails out without completing the exit and two moving in the wrong direction. Some eels swam in circles inside the predator’s stomach, seemingly searching for an exit.

‘Only species confirmed to escape’

The findings are unprecedented, showing that Japanese eels can escape alive from a predator’s stomach, unlike previous studies where other eel species died during escape attempts.
“At this point, the Japanese eel is the only species of fish confirmed to be able to escape from the digestive tract of the predatory fish after being captured,” co-author Yuuki Kawabata mentioned.

Further experiments planned

The researchers suggest that the eels’ elongated shape might facilitate their escape by increasing the likelihood that their tail remains in the predator’s esophagus when swallowed headfirst. Further experiments are planned to explore factors aiding these escapes.