"Unique tactics"

Eels escape the stomach of a predatory fish

The Japanese eel is not an easy prey.(Bild: opencage)

Researchers have made a spectacular observation: Recordings document for the first time how an animal manages to escape from the digestive tract of a predator. Young Japanese eels can snake back into the gullet with their thin tail tip in front and from there continue backwards through the gills to freedom. 

Researchers at Japan's Nagasaki University were able to document this behavior for the first time using video X-ray images. This defense tactic is unique, said research team member Yuuki Kawabata.

First documented escape from a predator's stomach
"This study is the first to observe the behavioral patterns and escape processes of prey in the digestive tract of predators," it said. In a previous study, the team had already shown that juvenile Japanese eels (Anguilla japonica) escape from the gills of the freshwater predator Odontobutis obscura. But the exact escape route was unclear because the researchers were unable to look inside the fish.

Nine out of 32 eels eaten managed to escape
For the new study, which was published in the journal "Current Biology", the researchers injected the young eels with a contrast agent to make them visible on the images. In total, they observed how 32 young eels were at least partially swallowed. 28 of them tried to escape via the esophagus. Nine managed to escape.

An adult Japanese eel(Bild: Wikipedia/Ffish.asia (CC BY 4.0))

"At the beginning of the experiment, we assumed that the eels would escape directly from the predator's mouth into the gills," said Kawabata. "However, contrary to our expectations, we were amazed when we observed the desperate escape of the eels from the predator's stomach into the gills."

Eels only have a few minutes to escape
The escape can also succeed because the predatory fish quickly swallow their prey whole together with water. If the young eels remain in the digestive tract, they are killed by the very acidic and anaerobic environment. After three to four minutes, the eels in the stomach normally stop moving - unless they find the exit before then.

Eels are highly endangered worldwide
Japanese eels are widespread in Japan and China. However, their populations are declining and they are listed as critically endangered on the Red List.

Like European eels, young Japanese eels swim into rivers from the sea and ascend to the upper sections of rivers and mountain lakes. After several years in freshwater, they migrate downstream and reach the sea, where they spawn.