Exceptional images: Japanese eels manage to escape from the stomach of their predator

As we know, many animal species have developed sometimes very elaborate techniques to escape their predators. Here is once again the proof, with this new study, published in the journal Cell Press Current Biology. Scientists have thus succeeded in filming juvenile Japanese eels for the first time (Anguilla japonica) escaping from the stomach of the fish which had swallowed them.

An incredible survival instinct

In a previous study, Japanese researchers showed that Japanese eels can escape through the gills of their predator after capture. What they didn't know was how. “We had no understanding of their escape routes and their behaviors during flight, that this was happening inside the predator's body.”says Yuuki Kawabata, of Nagasaki University in Japan, who worked on the earlier study as well as the new one.

It was through the use of X-ray videography that scientists finally discovered the technique of these eels. To successfully visualize them inside the predator's body, the researchers injected them with a contrast product each time.

“We discovered a unique defensive tactic in juvenile Japanese eels using X-ray video: they escape from the predator's stomach up the digestive tract towards the gills after being captured by the predatory fish, stated Yuuki Kawabata. This study is the first to observe patterns of prey behavior and escape processes in the digestive tract of predators.”

A device that could be extended to other species

It took the authors of the study a year to obtain a sufficiently good quality video proving their theory. Their videos show that the 32 eels captured had at least part of their bodies swallowed into the predator's stomach. After being swallowed, all but four attempted to escape by going up through the digestive tract toward the esophagus and gills, they report.

Of these, 13 managed to pull their tails out of the fish's gills, and nine managed to escape this way. It took on average approximately 56 seconds for the “eels on the run” to free themselves from the predator through the gills.

“The most surprising moment of this study was when we observed the first images of eels escaping up the digestive tract towards the gills of predatory fish, reveals Yuuki Kawabata. At the beginning of the experiment, we thought that eels entering the predator's mouth would escape directly through the gills, without going into the stomach and digestive tract. However, contrary to our expectations, we witnessed the desperate escape of eels from the predator's stomach to the gills, and this was truly a stunning discovery for us.”

These results are the first to show that eel Anguilla japonica may use a specific behavior to escape from its predator's stomach and gills after being eaten. It is also the first time that a study has shown the behaviors of a prey inside the digestive tract of its predator, according to the researchers.

They say that this new method of observation using X-rays having proven itself, it can now be used to study other predator-prey behaviors.

This article was published on September 9.