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Some water fleas Daphnia change their head morphology to reduce predation risk in response to chemical substances (kairomones) released from larvae of the invertebrate predator Chaoborus (Insecta: Diptera). We tested for evidence of the costs associated with elongation of the head spine in Daphnia galeata and the consequences of these costs on the inducibility of head spine elongation in predictable and unpredictable environments. We exposed D. galeata in outdoor experimental ponds to conditions under which predation pressure by Chaoborus larvae and the concentration of kairomones from this predator were controlled for about 70 days. In the laboratory, we then used Daphnia clones collected from the outdoor ponds to investigate the inducibility of head spine formation in response to Chaoborus kairomones. The inducibility of head spine formation increased in D. galeata from the ponds that had contained both predators and kairomones, whereas in water fleas from the ponds containing only kairomones the plasticity (inducibility) of head spine formation decreased compared with that in the control ponds. These results suggest that the production of a defensive head spine, its phenotypic plasticity, or both entail some costs. Contrary to our predictions, exposure to Chaoborus kairomones in the laboratory resulted in head lengths that were not significantly different among any of the clones from the three outdoor treatments. We found no evidence for costs associated with head spine elongation in terms of fecundity, time to maturity, or intrinsic rate of natural population increase. Average within-clone partial correlations calculated for head length and intrinsic rate of natural population increase, corrected for body length, were not significantly negative, indicating no cost of defence. This was probably because food conditions in the laboratory were so good that the costs of defence could not be detected. Furthermore, community-level changes, such as changes in food conditions caused by manipulation of the predation regime in the outdoor ponds, might have worked as selection pressures and confounded the results.
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