X-ray video captures eels escaping from predator’s stomachs

For prey, entering the stomach of a predator is usually the end of the road. But, in a world first, a research team from Japan has used X-ray videography to show how Japanese eels escape after being swallowed.

Unlike land-based predators that bite or tear at prey, many predatory fish feed by sucking smaller prey into their mouths and swallowing them whole. This includes the dark sleeper (Odontobutis obscura), a freshwater fish native to East Asia that eats juvenile Japanese eels (Anguilla japonica).

For prey like Japanese eels, being swallowed whole allows evolution a brief window to select for specialised tactics like active escape from the digestive system.

In a previous study, the researchers showed that juvenile Japanese eels could escape from dark sleepers by exiting through the gills of their would-be predator. But the researchers didn’t know how the eels accomplished this feat or how deep they went into the digestive system.

“We had no understanding of their escape routes and behavioural patterns during the escape because it occurred inside the predator’s body,” says first author Yuha Hasegawa of Nagasaki University.

In the current study, published in Current Biology, Hasegawa and colleagues used X-ray videography to observe and document juvenile Japanese eels inside dark sleepers.

First, they reared juvenile eels under laboratory conditions. Next, they injected them with barium sulfate – a common contrast agent that would allow the eels to be visible to X-rays within the dark sleeper. Then, they introduced one eel and one dark sleeper into a tank and allowed nature to take its course under a specialised X-ray video set up.

It took the researchers a year to solve all the logistical challenges involved in documenting prey inside of a predator.

“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,” says lead author Yuuki Kawabata of Nagasaki University.

An X-ray image and schematic showing an eel exiting tail-first from the predator’s gill. Credit: Current Biology, Hasegawa et al.

“At the beginning of the experiment, we speculated that eels would escape directly from the predator’s mouth to the gill,” adds Kawabata.

Instead, the researchers observed that all 32 captured eels had at least one portion of their body reach the dark sleeper’s stomach. Incredibly, the typical escape process involved eels navigating tail-first up through the oesophagus and then out through the gills. Last, it pulls its head out of the predator and swims away.

The authors hypothesize that the tail-first escape behaviour likely evolved because predatory fish swallow their prey head-first. This means that the tail is more likely to remain in the oesophagus, especially for an elongated eel.

For the unfortunate eels whose entire bodies were engulfed by the predator’s stomach, the authors observed circling behaviour, as if the eels were searching for the oesophagus.

Of the 32 captured eels, nine escaped – a 28% survival rate. But the chances of survival increased to 69% once an eel successfully navigated their tail out of the predator’s gill.

“Witnessing the eels’ desperate escape from the predator’s stomach to the gills was truly astonishing for us,” adds Kawabata.

Results from the research confirm that the eels were racing against the clock. Once swallowed whole, the unsuccessful eels gradually stopped moving after two and a half minutes on average. These eels succumbed to the stomach’s environment, which is both highly acidic and low in oxygen.

This research is the first to document the escape behaviour that allows a prey species to navigate out of a predator’s digestive tract. The authors assert that their experimental set up could be used to understand other examples of prey escaping their predator’s digestive systems.