TEMPERATURE AFFECTS SELECTIVITY OF CHAOBORUS LARVAE-EATING DAPHNIA

In ponds, a chemical produced by predaceous Chaoborus (Insecta, Dipter a) larvae changes the development of juvenile Daphnia pulex (Crustacea , Branchiopoda) so the juveniles grow spines (neckteeth) on the back o f their head. It is generally assumed that the spined phenotype is (or is an indicator of) a morphological predator defense. The research re ported here tests the hypothesis that the induced neckteeth do in fact increase Daphnia survivorship, over a range of temperatures. Predatio n experiments were conducted over a range of temperatures from 6 to 22 degrees C using fourth instar Chaoborus americanus larvae as the pred ator. The prey were a mixture of spined (induced ''necktooth'' phenoty pe) and unspined (uninduced) juvenile Daphnia pulex. At 6 and 11 degre es C, Chaoborus selected the unspined phenotype over the spined phenot ype, as expected. However, at 22 degrees C, the selectivity was revers ed: significantly fewer on the spined survived compared to the unspine d phenotype. These results suggest that the spined phenotype may eithe r increase or decrease Daphnia pulex survival, depending on temperatur e and clone.