Effectiveness of post-capture escape tactics in juvenile Japanese eels against various predatory fishes

Although rare, it is possible for some prey species to actively escape from the digestive tract of predators after being captured. Because these tactics involve escaping after being swallowed by the predator, they are likely constrained by predator-specific factors. However, the limits of the effectiveness and the predator-specific factors that influence its success remain poorly understood. This study examined the effectiveness of a post-capture defensive tactic of juvenile Japanese eels, Anguilla japonica , in which they escape through the predator’s gills by navigating backward from the stomach. We evaluated the escape success of eels through predation experiments using six predator species that differ in habitat, swimming styles, and feeding styles (biting or suction feeding). The results indicated that eels escaped via the gills of all five suction-feeding predators examined (32%, 114/359), including Odontobutidae, Sebastidae, Lateolabracidae, Cyprinidae, and larger conspecific Japanese eels. In contrast, eels never escaped from the grass puffer Takifugu alboplumbeus (Tetraodontidae), the only biting-type predator, as the eels were torn into small pieces and consumed. Our results suggest that post-capture escape in eels is effective against diverse predator species encountered in nature. Meanwhile, predators that bite and kill prey before swallowing, such as tetraodontid fishes, may pose a greater threat to juvenile eels.