Predator-induced alterations in Daphnia morphology

The freshwater cladocerans Daphnia pulex and Daphnia schodleri protect themselves from predation by morphological alterations induced in response to water-soluble chemicals released by their respective predators. Daphnia pulex is induced by larvae of the phantom midge, Chaoborus. Populations of D. pulex which are induced are those most likely to have intense interaction with the predator. This is true both on a broad geographic scale as well as locally. Cephalic expansion in D. schodleri is induced by notonectids, in particular Buenoa sp. This predator prefers larger prey and consequently small instars of D. schodleri show no evidence of induction. Both examples of predator-induced alterations suggest that this type of response is costly to the prey and is manifested only in those individuals and populations most threatened.