How Japanese eels escape from the stomach of a predatory fish

Predation shapes diversity in the defensive tactics of prey. One specialized defensive tactic is to escape the digestive system of the predator after capture^{1,2,3,4,5,6,7,8}. While most of these defensive tactics involve passive ejection alive from predators' mouths and vents^1,2,3,4,5, active escape from the digestive tracts of predators has recently been observed in certain invertebrate species^6,7 and fish⁸. However, no study has yet uncovered the behavioral patterns and escape routes of the prey within a predator's digestive tract. Here, we report the sequential escape processes of the Japanese eel Anguilla japonica from capture to escape via the gills of predatory fish Odontobutis obscura using an X-ray video system. All captured eels had at least one portion of their bodies swallowed into the stomach of the predator. Surprisingly, after being swallowed, most individuals attempted to escape by going back up the digestive tract towards the esophagus and gill, and some of them succeeded in escaping via the predator's gill. Some eels, whose entire bodies were completely inside the stomach, exhibited circling behavior along the stomach, seemingly searching for possible escape routes. An electro-anesthetization experiment revealed that eels utilize various escape routes through gill clefts, rather than just one.