Effect of developmental time on Chaoborus-induced phenotypic plasticity

Due to Daphnia's cosmopolitan distribution and the co-occurrence with various predators, it has developed highly diverse antipredator defenses. In response to chemical cues of Chaoborus larvae, a major predator, neckteeth are induced in vulnerable juvenile instars of Daphnia pulex. As only early juvenile instars of D. pulex are vulnerable to predation by Chaoborus sp., increased developmental time extends the time span that D. pulex is in the vulnerable size, and thus increases the risk of being preyed upon. Here, we hypothesize that increased time spent in vulnerable instars leads to a higher degree of neckteeth formation in vulnerable D. pulex instars. To test this, we created a gradient of growth conditions for Daphnia that would cause an increase in developmental time by means of decreasing the temperature or increasing the proportion of dietary cyanobacteria in separate experiments. We determined the body size during the juvenile instars and calculated the time spent in vulnerable instars. Correlations of neckteeth induction to times spent in vulnerable instars were significant for the data set of the temperature experiment and the combined data set, but not for the data set of the cyanobacteria experiment. However, we cannot exclude that an increased bacterial degradation of the kairomone at elevated temperatures has contributed to this relationship, and dose–response curves revealed that neither the sensitivity to the kairomone nor maximum neckteeth induction in D. pulex was reduced at the elevated temperature. Our results suggest that neckteeth induction is affected by the time spent in vulnerable instars, based on the factors temperature and dietary toxic cyanobacteria, but its universal validity needs to be tested further by including other factors.