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A species of Japanese eel has proven to be a master escape artist, capable of wriggling its way through a predator’s digestive tract to freedom.
While it was known previously that Anguila japonica (which, despite its name, can be found off the coasts of Korea, Taiwan, China, Vietnam, and the Philippines, as well as Japan) was capable of escaping from its predators’ insides, it wasn’t clear how they pulled it off.
Now, thanks to some novel camera work, that technique has been documented in a study published today in Current Biology. According to the team, it’s the first time the behavior of prey has been captured while it’s still inside a predator’s digestive system.
“We had no understanding of their escape routes and behavioral patterns during the escape because it occurred inside the predator’s body,” said Yuha Hasegawa, a researcher at Nagasaki University who worked on the study, in a statement.
To figure out exactly what was going on in the fish during the great eel escape, Hasegawa and his colleagues set up a pair of video recording setups near a tub containing a predatory fish (in this case, a type of freshwater fish called a Dark sleeper). One camera captured conventional video, while another used x-rays to see what was happening in the fish’s guts. An A. japonica eel, which had been injected with barium sulfate to make it easier to spot once it was inside the fish, was then introduced into the tub, and was promptly turned into lunch.
Rather than give into fate, the eel got busy. In footage released alongside the study, the fish can be seen lying placidly at the tub’s bottom. Inside it, however, the eel was battling valiantly. One eel could be seen circling through the stomach, as if looking for an exit. It used its tail to probe through the fish’s stomach, eventually sliding it up into the esophagus. Eventually, the tail was used to push through a gill. Coiling the rest of its body inside the fish, the eel was able to eject itself out the gill, living to eel another day.
“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 Hasegawa’s colleague, Yuuki Kawabata. “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.”
The experiment was repeated 32 times, and only four eels didn’t attempt an escape. In total, 13 were able to get their tails out of the gills, and nine fully managed to get free. It was a quick process, too, with the average escaper taking only 56 seconds to liberate themselves.
The Japanese eels aren’t alone in developing a way to survive when all seems lost. A 2006 study reported a parasitic worm called Paragordius tricuspidatus was able to force its way out of a predator’s system when its host organism was eaten. Another paper, from 2020, documented how a species of aquatic beetle turned itself into a living laxative to escape from a frog’s anus after being ingested.
