Numerous prey animals have developed strategies to avoid becoming meals for their predators. However, a groundbreaking study published in the Cell Press journal Current Biology on September 9, 2024 has taken this concept further by providing the first video footage of juvenile Japanese eels successfully escaping after being consumed by their fish predators. Utilizing X-ray videography, researchers discovered that the eels maneuver their way out by first pushing their tails through the esophagus and gills before freeing their heads.
“We have identified a remarkable defensive strategy employed by juvenile Japanese eels with the help of an X-ray video system: they escape from the stomach of predators by moving back up the digestive system toward the gills after being eaten by predatory fish,” stated Yuuki Kawabata from Nagasaki University in Japan. “This is the first study to document the behavior and escape mechanisms of prey within the digestive systems of predators.”
In a previous investigation, researchers including Kawabata and Yuha Hasegawa demonstrated that Japanese eels could escape from the gills of their captors after being caught. However, the exact method was previously unknown.
“We did not comprehend the routes or behaviors of their escape because it took place within the body of the predator,” said Hasegawa.
In this latest research, they utilized an X-ray videography tool to look inside the predatory fish (Odontobutis obscura). To visualize the eel after ingestion, they first needed to inject it with a contrast agent. It took the team a year to gather convincing video evidence of the escape process.
The footage revealed that all 32 eels captured had at least a part of their bodies swallowed into the stomachs of their fish predators. After being ingested, all but four attempted to escape by retracing their way through the digestive tract to the esophagus and gills. Among these, 13 eels managed to extract their tails from the predator’s gills, and nine successfully escaped through them. On average, the eels took about 56 seconds to release themselves from the gills of their captors.
“The most surprising moment in this study occurred when we captured the first footage of eels escaping by moving back up the digestive tract toward the predator’s gills,” Kawabata remarked. “Initially, we had assumed that eels would directly flee from the predator’s mouth to the gills. However, we were astounded to witness the eels’ desperate attempt to escape from the stomach to the gills.”
Additional observations indicated that, despite some commonalities, the eels did not always take the same escape route via the gill cleft. Some eels also circled within the stomach, seemingly searching for an exit. The findings are the first to illustrate that the eel Anguilla japonica can employ a specific behavior to escape from its predator’s stomach and gills after being ingested. This marks the first time any research has documented behaviors of prey within the digestive tracts of their predators, according to the investigators.
The researchers believe that the X-ray techniques used in this study can be applied to examine other predator-prey interactions. In future research, they aim to uncover more about the traits that contribute to the eels’ successful escapes.
