Ontogenetic shifts in efficacy of antipredator mechanisms in a top aquatic predator, Anax junius

In fishless ponds, numerous studies have classified large carnivorous invertebrates such as dragonfly nymphs as top predators. In contrast, few studies have looked at the antipredator behavior and associated ecological role of these animals as prey in these systems. We examined the antipredator behavior of a large dragonfly nymph, Anax junius, from a historically fishless pond. These predators displayed a series of distinct aggressive antipredator behaviors when grasped that involved stabbing with lateral and posterior spines, and seizing with labial hooks. Larger (older) nymphs displayed these aggressive behaviors significantly more than smaller (younger) animals in simulated predation trials. During encounters with live larval salamander predators (Ambystoma tigrinum), all large nymphs, but only 12.5% of small nymphs successfully escaped predation attempts by the amphibians through the use of antipredator behavior. Large nymphs were also significantly more active than smaller nymphs in the presence of salamander larvae. Despite often being considered top predators in fishless ponds, our study demonstrates that their true role is more complex, depending on ontogeny and body size, and that the development of effective antipredator behavior is likely necessary for survival in these systems.