Background/Question/Methods Intermediate predators frequently persist with top predators in nature despite consumptive and competitive pressure by top predators.  One mechanism that may promote persistence of top and intermediate predators is the ability of intermediate predators to alter their behavior and/or morphology in a manner that decreases their susceptibility to predation.  The ability to alter traits which reduce susceptibility to predation, however, may be limited in environments where intraspecific competition is important.  Traits reducing predation risk may also reduce competitive ability.  Furthermore, the ability to elicit particular trait changes may vary during ontological development.  We conducted an experiment in which we measured the behavioral and morphological responses of an intermediate predator (larval Ambystoma opacum) to the presence of a nonlethal top predator (larval Anax spp.) at three densities of an intermediate predator during different stages of ontological development.  We hypothesized that the nonlethal presence of Anax would alter the behavior and morphology of larval A. opacum but the effects would be greatest when the density of A. opacum is low.  Furthermore, we expected the greatest effects on behavioral traits during early stages of larval A. opacum development and the greatest changes in morphological traits to be later in larval A. opacum development.        
Results/Conclusions We found that the nonlethal presence of Anax caused A. opacum to develop a shorter tail during intermediate stages of development when A. opacum was present in low abundance but not high abundance and the effects disappeared just prior to metamorphosis.  A. opacum also developed a shorter torso and were less active in the presence of non-lethal Anax regardless of A. opacum abundance during early developmental stages but the effects disappeared just prior to metamorphosis.  Our results indicate that intermediate predators alter their larval phenotypes in response to top predators and future efforts need to evaluate whether these changes reduce intermediate predator susceptibility to top predators.  A high abundance of conspecifics, however, may impose conflicting demands on intermediate predators that prevent the expression of an entire suite of morphological defenses to top predators.