Japanese Eel Escapes Dark Sleeper Fish Through Its Gills in X-Ray Video

The astonishing, Houdini-like antics of a slippery eel have at last been exposed - and captured on camera for everyone to witness.

Using X-ray footage, scientists have shown that juvenile Japanese eels (Anguilla japonica) can wriggle backwards from the stomachs of predatory dark sleeper fish (Odontobutis obscura) after being eaten, before escaping through the gill openings.

Japanese eel escapes revealed by X-ray video

"This study is the first to observe the behavioral patterns and escape processes of prey within the digestive tract of predators," marine biologists Yuha Hasegawa and Yuuki Kawabata of Nagasaki University told ScienceAlert.

"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."

Several years earlier, Hasegawa, Kawabata and ecologist Kazuki Yokouchi of the Japan Fisheries Research and Education Agency had first identified the eel's remarkable escape tactic. They saw juvenile eels, called elvers, emerging from dark sleepers' gills after the fish had captured them.

Active escapes from a predator's digestive system have been documented only a handful of times, and mostly among invertebrates. Examples include a beetle that exits a frog's rear end uninjured after being swallowed, and a parasitic worm that flees both its insect host and the predator that consumed that host through virtually any available opening.

Testing how elvers avoid digestion

The team sought to understand more about how Japanese eels evade being digested, as it was not known whether their escape method was deliberate. They therefore designed an experiment using X-rays to observe events inside a dark sleeper once an eel had vanished into its mouth.

Lab-reared elvers were injected with barium sulphate, a contrast substance that made them readily visible on X-ray images. The researchers then introduced the elvers, individually, into a tank containing a dark sleeper and recorded each predator-prey encounter with X-ray video.

Of 32 eels captured by the fish, 13 reached the gills and began an attempt to get out. Nine successfully finished the escape and squirmed free.

A tail-first route through the dark sleeper

What especially astonished the scientists was the means of escape visible in the X-ray recordings. Before escaping, the fish had swallowed the prey completely; the eels then wriggled and twisted backwards, tail first, up through the fish's oesophagus.

"At the beginning of the experiment, we speculated that eels would escape directly from the predator's mouth to the gill," the researchers said. "However, contrary to our expectations, witnessing the eels' desperate escape from the predator's stomach to the gills was truly astonishing for us."

The behaviour also appears to be highly active. Eels that failed to escape were nevertheless seen moving around the fish's stomach, seemingly searching for an exit as rapidly as possible. On average, each eel remained active inside the fish for 211 seconds before succumbing to the hostile digestive conditions.

"This discovery has provided us with new insights: muscle strength and tolerance to highly acidic and anaerobic environments, as well as their elongated and slippery morphology, are necessary for eels to quickly escape from the digestive tract before being digested," Hasegawa and Kawabata said.

Japanese eels do not succeed 100 percent of the time, but attempting to wriggle out of danger is clearly far preferable to not trying.

Although a bold escape from the literal jaws - and stomach - of death appears to be an uncommon survival tactic in animals, other creatures may possess the same ability.

The researchers say investigating the small number of known examples could reveal which traits make such escapes possible.

"The X-ray method utilized in this study is applicable for observing the post-capture active escape of other prey inside a predator and identifying the specific organs that pose challenges for their passage," they write in their paper.

"Such knowledge will provide valuable insights into the kinematic, physiological, and behavioral traits that are crucial for successful escapes, thereby offering new insights into the evolution of post-capture anti-predator tactics in prey."

The research was published in Current Biology.