Predator induction of crests in morphs of the Daphnia carinata King complex

Temperature, turbulence, oxygen, and notonectid predators ( Anisops calcaratus ) were studied experimentally as possible factors influencing crest growth in six different forms of the Daphnia carinata complex. Predators were the most potent influence, inducing crest development in four of the morphs. Temperature had only a secondary effect which may have been indirect. When the notonectids were partitioned off from the Daphnia in the same container, crest development still resulted, suggesting that the stimulus was a water‐soluble secretion from the predator. If the notonectids were starved this secretion was either not produced or was ineffective. Experiments showed that predators could induce crest development in adult daphnids that were initially uncrested and that a sustained stimulus from the predator was necessary if crested juvenile Daphnia were to produce crested adults. Under experimental conditions, daphnids with a crest were less susceptible to notonectid predation (mainly because of superior evasion powers) than were those without one. Results of a regression analysis of field data were broadly consistent with experimental results, especially in indicating that predation was a potent factor influencing crest development, and the distribution and abundance of morphs.