In a 1st, Japanese eels seen escaping predator’s stomach after getting swallowed

Oceans are places where silent battle rages. Predators hunt prey, and prey develop ingenious tactics to survive. But one species has taken survival to a whole new level: the Japanese eel.

Researchers at Nagasaki University have captured the first video proof of the Japanese eel’s unique survival tactic. Using an X-ray videography device, they witnessed an extraordinary escape.

Surprisingly, juvenile Japanese eels were seen escaping from the stomachs of their predatory fish. The eels escaped by inserting their tail ends into the esophagus and gills of the predator and then pulling their heads out.

“We have discovered a unique defensive tactic of juvenile Japanese eels using an X-ray video system: they escape from the predator’s stomach by moving back up the digestive tract towards the gills after being captured by the predatory fish,” said Yuuki Kawabata of Nagasaki University in Japan.

“This study is the first to observe the behavioral patterns and escape processes of prey within the digestive tract of predators,” Kawabata added.

The escape route

Researchers’ previous study revealed that Japanese eels were capable of escaping from their predator’s gills. However, the mechanism behind their escape remained unknown.

“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, one of the researchers.

To understand the eels’ (Anguilla japonica) escape mechanisms, researchers introduced them to predatory fish (Odontobutis obscura) . For the visualization, the team injected the eels with a contrast agent to make them visible inside the predator’s stomach.

Using X-ray technology, they watched as the eels navigated the predator’s digestive tract. It took the researchers almost the whole of a year to get clear video footage of the escape process.

Takes only 56 seconds to escape

As per the press release, the researchers observed 32 captured eels , all of which were partially swallowed by their predatory fish.

Interestingly, most of the eels attempted to escape by reversing their direction and traveling back up the digestive tract towards the gills. Only 9 of the 13 eels were able to totally escape via the gills.

The eels typically took about “56 seconds” to free themselves from the predator’s gills.

“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,” Kawabata said.

The author added: “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 videos showcased that the eels didn’t always use the same route through the gill cleft. Some of them were seen circling the stomach, possibly seeking a different way out.

Some of them explored other options, such as circling the stomach.

This is the first time such a remarkable escape behavior has been documented.

“It’s also the first time any study has captured the behaviors of any prey inside the digestive tract of its predator, according to the researchers,” the press release noted.

Also, the researchers propose that this X-ray technique could be utilized to examine other predator-prey relationships.

The findings were published in the journal Current Biology.