Vertical distribution ofChlamydomonaschanges in response to grazer and predator kairomones

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Individuals in aquatic communities frequently assess their biotic environment through infochemicals. In particular, kairomones are commonly involved in interactions between predator and prey. However, the relationship between individuals and chemicals produced by other organisms that are not direct predators, but may indicate the presence of a predator, is not well characterized. We used experimental microcosms to test whether the unicellular green alga Chlamydomonas reinhardtii alters vertical migration patterns in response to kairomones produced by zooplankton (Daphnia) and planktivores (fish). Our results suggested that phototaxis in C. reinhardtii was strongly affected by the type of kairomone present, the concentration of the kairomone, and the duration of exposure to the kairomone. Kairomones generally increased phototaxis in C. reinhardtii. The adaptive significance of such behavioral changes in natural settings would depend largely on local community composition. The similarity in phototactic responses of C. reinhardtii to Daphnia and fish kairomone suggest that, in at least this species of phytoplankton, the underlying genetic elements responsible for kairomone detection may be responsive to a broad range of chemical stimuli, allowing this species to adjust its phototaxis in response to not only the presence of its grazers, but also to predators of its grazers.

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