Imagine you're a Japanese eel, swimming around just minding your own business when—bam! A predatory fish swallows you whole and you only have a few minutes to make your escape before certain death. What's an eel to do? According to a new paper published in the journal Current Biology, Japanese eels opt to back their way out of the digestive tract, tail first, through the esophagus, emerging from the predatory fish's gills.

Per the authors, this is the first such study to observe the behavioral patterns and escape processes of prey within the digestive tract of predators. “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 Yuha Hasegawa at Nagasaki University in Japan told New Scientist.

There are various strategies in nature for escaping predators after being swallowed. For instance, a parasitic worm called Paragordius tricuspidatus can force its way out of a predator’s system when its host organism is eaten. There was also a fascinating study in 2020 by Japanese scientists on the unusual survival strategy of the aquatic beetle Regimbartia attenuata. They fed a bunch of the beetles to a pond frog (Pelophylax nigromaculatus) under laboratory conditions, expecting the frog to spit the beetle out. That's what happened with prior experiments on bombardier beetles (Pheropsophus jessoensis), which spray toxic chemicals (described as an audible "chemical explosion") when they find themselves inside a toad's gut, inducing the toad to invert its own stomach and vomit them back out.

But R. attenuata basically walks through the digestive tract and escapes out of the frog's anus after being swallowed alive. It proved to be a successful escape route. In the case of the bombardier beetles, between 35 and 57 percent of the toads threw up within 50 minutes on average, ensuring the survival of the regurgitated beetles. R. attenuata's survival rate was a whopping 93 percent. In fact, 19 out of 20 walked out of the frog, unharmed, within an hour, although one industrious beetle bolted out in just five minutes. Granted, the beetles often emerged covered in fecal pellets, which can't have been pleasant. But that didn't stop them from resuming their little beetle lives; all survived at least two weeks after being swallowed.

Hasegawa co-authored an earlier study in which they observed Japanese eels emerging from a predator's gills after being swallowed, so they knew this unique strategy was possible. They just didn't know the details of what was going on inside the digestive tract that enabled the eels to pull off this feat. So the team decided to use X-ray videography to peer inside predatory fish (Odontobutis obscura) after eels had been eaten. They injected barium sulfate into the abdominal cavity and tail of the Japanese eels as a contrast agent, then introduced each eel to a tank containing one O. obscura. The X-ray video system captured the interactions after an eel had been swallowed.

Out through the gills

O. obscura swallow their prey whole along with surrounding water, and a swallowed eel quickly ends up in the digestive tract, a highly acidic and oxygen-deprived environment that kills the eels within 211.9 seconds (a little over three minutes). Thirty-two of the eels were eaten, and of those, 13 (or 40.6 percent) managed to poke at least their tails through the gills of their predator. Of those 13, nine (69.2 percent) escaped completely within 56 seconds on average, suggesting "that the period until the tails emerge from the predator's gill is particularly crucial for successful escape," the authors wrote. The final push for freedom involved coiling their bodies to extract their head from the gill.

It helps to be swallowed head-first. The researchers discovered that most captured eels tried to escape by swimming back up the digestive tract toward the esophagus and gills, tail-first in the cases where escape was successful. However, eleven eels ended up completely inside the stomach and resorted to swimming around in circles—most likely looking for a possible escape route. Five of those managed to insert their tails correctly toward the esophagus, while two perished because they oriented their tails in the wrong direction.

“The most surprising moment in this study was when we observed the first footage of eels escaping by going back up the digestive tract toward the gill of the predatory fish,” said co-author Yuuki Kawabata, also of Nagasaki University. “At the beginning of the experiment, we speculated that eels would escape directly from the predator’s mouth to the gill. However, contrary to our expectations, witnessing the eels’ desperate escape from the predator’s stomach to the gills was truly astonishing for us.”

Current Biology, 2024. DOI: 10.1016/j.cub.2024.07.023  (About DOIs).