Variation in the neonate size of Daphnia pulex: the effects of predator exposure and clonal origin

Abstract

Daphnia pulex clones originating from twelve small pond habitats were exposed to chemical cues from a size-selective predator, larvae of the phantom midge Chaoborus. Exposure delayed the onset of reproduction and increased the size at first reproduction. On the other hand, the neonates produced by these larger mothers were slightly smaller than the neonates produced by the smaller control mothers. In cladocerans, neonate size is usually positively correlated to the size of the mother. Thus exposure to Chaoborus kairomone apparently had direct effects on neonate size counterbalancing the maternal effects. Daphnia clones collected from Chaoborus-free and Chaoborus-rich environments exhibited different responses. In first adult instar, the clones from Chaoborus inhabited environments increased their offspring size under Chaoborus exposure whereas clones from Chaoborus-free environments did not. This may reflect clonal adaptation to the predation prehistory of their original habitat since larger neonates more quickly reach a size protected from the predator.