In the constant battle between predator and prey, species have evolved various strategies to avoid being eaten. Now, a study published in the journal Current Biology takes predator-prey dynamics to a surprising new level. Researchers from Nagasaki University have captured video evidence of juvenile Japanese eels, known as Anguilla japonica, escaping from the stomachs of their fish predators—an unprecedented glimpse into the lengths prey species go to for survival.
“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,” Yuuki Kawabata, a researcher at Nagasaki University said in a recent statement. “This study is the first to observe the behavioral patterns and escape processes of prey within the digestive tract of predators.”
As Slippery as an Eel
The idea for this study arose from previous research, in which Japanese eels were observed escaping through the gills of predatory fish after capture. Fish gills are respiratory organs that allow fish to extract oxygen from water. Water enters the fish’s mouth and passes over the gill filaments rich in blood vessels. As water flows across the thin membranes of the gill filaments, oxygen is absorbed into the blood while carbon dioxide is removed, enabling the fish to breathe underwater. The gill structure maximizes surface area to ensure efficient gas exchange, but it also gives a literal opening for Japanese eels to escape unharmed.
In a 2021 paper studying this escape tactic, the researchers did not understand how the eels navigated their way out.
“We had no understanding of their escape routes and behavioral patterns during the escape because it occurred inside the predator’s body,” Yuha Hasegawa of Nagasaki University added. The team hoped they could uncover the method by using more advanced technology.
The Art of Escaping
To capture these escape attempts, the team had to inject the eels with a contrast agent, allowing them to visualize the tiny creatures inside the fish predators. After a year of trials, they finally captured conclusive footage of the escape process. Their videos showed that out of 32 eels swallowed by the predatory fish Odontobutis obscura, all but four tried to escape by moving through the digestive tract. Like a scene out of Alien, thirteen got their tails out of the predator’s gills, and nine successfully freed themselves entirely. On average, it took about 56 seconds for the eels to escape.
“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. “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.”
A Survival Tactic Seen Nowhere Else
The research shows further insights into the behavior of prey species after being swallowed, revealing that the eels don’t rely on a single escape route. While many eels approached the gills, others circled within the stomach, seemingly searching for any possible exit. This reveals survival strategies the eels have adapted to survive their many predators.
The researchers believe the X-ray videography methods developed for this study can be used to observe other predator-prey interactions in future research. As for the eels, the team hopes to learn more about the characteristics that enable some individuals to escape while others do not.
As Japanese eels are eaten worldwide, particularly in Asia, their populations feel heavy pressure from overfishing and pollution. In 2014, they were marked as endangered, and since then, scientists have worked hard to understand their reproductive and survival tactics in order to help their populations recover. Now, with this new study, researchers are hopeful to learn more about eel survival to give them a better chance in the future.
Kenna Hughes-Castleberry is the Science Communicator at JILA (a world-leading physics research institute) and a science writer at The Debrief. Follow and connect with her on X or contact her via email at kenna@thedebrief.org