Algal food identity affects morphological anti‐predatory defense in <scp> <i>Daphnia pulex</i> </scp>

Abstract Microcrustaceans of the genus Daphnia have evolved various inducible anti‐predator defenses; however, it is largely underexplored how the bottom‐up factor food quality may affect the extent of these defenses. A well‐studied example of an inducible defense is the deployment of neckteeth in Daphnia pulex in response to infochemicals from predatory larvae of Chaoborus , which prey on juvenile instars of Daphnia . In general, vulnerability depends on the body size in the respective juvenile Daphnia instars in relation to the mouth gape diameter of Chaoborus larvae. We hypothesized that algae that prolong the time in which juvenile animals remain in the vulnerable size also increase the predation risk and therefore increase the extent of induced neckteeth. To test this, we supplied three D. pulex genotypes with three food algae and quantified somatic growth rates, time in vulnerable instars, and neckteeth in response to Chaoborus infochemicals. Food algal identity affected the extent of induced neckteeth by up to 66%. However, this effect was not positively related to the time spent in vulnerable instars, indicating that the time that juveniles remain in a vulnerable size is not used to assess predation risk. We demonstrate that food identity may significantly affect the extent of induced neckteeth in D. pulex in a genotype‐dependent way. This strongly suggests that in nature, changing phytoplankton composition may affect the degree of morphological defense and thus constitutes a bottom‐up control of chemical predator–prey communication. This is another example of how environmental factors modulate the temporal dynamics of phenotypic plasticity.