That eels are slippery is no surprise, but that they can literally slip out of a predator's stomach is "truly astonishing," according to researchers, who've recorded footage of the first-of-its-kind behavior, allowing for escape in less than a minute. "It sounds like the plot of a horror movie—a predator swallows its prey only for the creature to burst out of its captor's body," per the Guardian. Yet it's a real survival tactic of Japanese eels (Anguilla japonica), which reside in fresh water and estuaries throughout East Asia. Researchers knew these eels were able to escape certain predators once inside their mouths but had no idea how they managed it until they filmed the behavior with X-ray cameras, Gizmodo reports.
Nagasaki University fisheries researcher Yuha Hasegawa and colleagues were able to peer inside dark sleeper fish in a tank to which Japanese eels were added. A video shows that within seconds of being eaten, an eel—injected with barium sulfate to make it visible to cameras while inside the fish's stomach—moved its tail back up the digestive tract and through the fish's esophagus and gills. By coiling its body, the eel was then able to pull its head through the opening and swim free. This was "the most surprising moment" in the study published Monday in Current Biology, as researchers had expected the eels to "escape directly from the predator's mouth to the gill," says co-author Yuuki Kawabata, also of Nagasaki.
The team recorded fish swallowing 32 different eels. "Some eels, whose entire bodies were completely inside the stomach, exhibited circling behavior along the stomach, seemingly searching for possible escape routes," according to the study. In all, 28 eels tried to escape, 13 managed to get their tail out of the gill, and nine slipped out entirely. The average escape time: 56 seconds. Kawabata notes the Japanese eel is now "the only species of fish confirmed to be able to escape from the digestive tract of the predatory fish after being captured," per the Guardian. The dark sleeper fish weren't harmed. "However, the eels that managed to escape sometimes showed signs of abrasions," Hasegawa says. (More animal behavior stories.)
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