Video: Japanese eels (Anguilla japonica) escapes from dark sleeper fish (Odontobutis obscura) stomach

NEWS - Prey species have survival tactics to avoid being eaten by potential predators, but young Japanese eels (Anguilla japonica) have gone above and beyond in escaping after being swallowed by the dark sleeper fish (Odontobutis obscura).

Researchers from Nagasaki University and the Bunkyo Fisheries Resources Institute in Yokohama used X-ray videography and watched the eels escape backwards, first pushing their tail end through their esophagus and gills before pulling their heads out completely.

“They escape from the predator’s stomach by moving backward through the digestive tract towards the gills after being caught by the predator fish. Previously, we did not understand their escape route and behavioural patterns because it takes place inside the predator’s body,” said Yuuki Kawabata from Nagasaki University.



This study is the first to observe the behavioural patterns and escape process of prey inside the predator’s digestive tract. In previous studies, Kawabata and his team had shown that A. japonica can escape through the predator’s gills after being eaten, but did not know how.

They found a way to see inside O. obscura using X-ray videography equipment. The researchers injected a contrast agent into the eels’ bodies to visualize movement inside their stomachs. It took the team a year to produce convincing video evidence of the escape process involved.

The researchers report that four eels attempted to escape through the digestive tract to the esophagus and gills, 13 eels managed to extend their tails from their gills, and nine managed to escape through their gills. On average, it took about 56 seconds to free themselves from the predator’s gills.



“At the beginning of the experiment, we speculated that the eels would escape directly from the predator’s mouth to the gills. However, watching the eels desperately escape from the predator’s stomach to the gills was truly astonishing to us,” Kawabata said.

Further research found that the eels did not always rely on the same escape route through the gill slits. Some of them also circled around along the stomach, as if looking for a way out. The researchers say the X-ray method could be applied to other predator-prey behavior observations.

Original research

Hasegawa, Yuha et al. (2024). How Japanese eels escape from the stomach of a predatory fish. Current Biology, Volume 34, Issue 17, R812 - R813, DOI:10.1016/j.cub.2024.07.023