Changes in post- and pre-capture escape ability over development in juvenile Japanese eels

Predation is one of the major forces driving the evolution of defensive tactics in prey. Recently, post-capture active escape behaviors of different prey animals from the predator’s digestive tract have been reported. However, no studies have yet examined when these escape behaviors are developed through the ontogeny of the prey. This study examined the ontogenetic change in a unique defensive tactic of juvenile Japanese eels Anguilla japonica in which they escape via the predator’s gills after being captured. We used A. japonica ranging in stage from pre-settlement glass eels to post-settlement yellow eels (based on their habitat change from tidal estuaries to freshwater rivers). The results indicated that individuals in the 2 earliest stages of glass eels (stages VI A0 and VI A1 ) never escaped after being captured, while 28.1% of individuals after the middle glass eel stages (VI A2 and later stages) escaped via the predator’s gill. The ontogenetic timing of the development of escape ability coincides with when eels settle into benthic riverine and estuarine habitats as suggested by previous studies. Additionally, the pre-capture attack avoidance ability of Japanese eels improved rapidly with growth in the subsequent elver and yellow eel stages. These results suggest that the unique post-capture defensive tactic of eels may be particularly important during the vulnerable period around the settlement phase when they are less capable of avoiding predator attacks. Our study offers valuable insights into the behavioral ecology and conservation of anguillid eels, which have faced considerable population declines.