Watch an eel climb up its predator's digestive tract and wriggle to freedom through its gills

Japanese eels have found an ingenious way to escape a fish's stomach after being swallowed — backing up the digestive tract then squeezing themselves out of the predator's gills.

In a scientific first, researchers in Japan have used X-ray videography to capture the behavior of live prey inside its predator's digestive tract.

The dark sleeper fish (Odontobutis obscura) swallows its prey whole and alive. The team discovered that juvenile Japanese eels (Anguilla japonica) wolfed down by the fish can insert the tip of their tails through the fish's esophagus and gill, then wriggle it out of the gill, pulling themselves backward until they're entirely free. The researchers published their findings Monday (Sept. 9) in the journal Current Biology.

In previous work, the authors had noticed that juvenile eels can escape through their predator's gills — but they didn't know exactly how. "Before the X-ray video experiment, we hypothesized that eels might escape directly from the predator's mouth through the gills," co-author Yuuki Kawabata, an associate professor in the Graduate School of Fisheries and Environmental Sciences at Nagasaki University, told Live Science in an email.

When the scientists saw the tiny eels wriggle toward the gills from the predator's stomach — rather than escaping through its mouth — they were "absolutely astonished," Kawabata said. "We were anticipating more straightforward escape routes, but their ability to navigate back up the digestive tract was truly surprising," he said.

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It took the team a year to film compelling evidence showing how the eels were pulling off their great escape.