Predator-prey relationships mediated by chemicals (kairomones) in lake zooplankton communities are reviewed and discussed. Many zooplankton species change their morphology (anti-predator devices) in response to kairomones released from potential predators. Development of protruding structures (neckteeth, high helmets, long tail-spines) by the cladoceran Daphnia is induced by kairomones during the embryonic or juvenile stage. These structures are lost when the kairomones disappear. Rotifers are, on the other hand, affected by kairomones indirectly through the mother's oogenesis. The effect of kairomones on morphology is influenced by temperature, food conditions for the prey, and by the nutritional state and density of the predators. Chemicals other than predator kairomones, such as chemicals released from non-predatory zooplankton and pesticides, can also induce morphological changes. The predator kairomones affect the prey's behavior as well as morphology. Some zooplankton species migrate vertically to avoid predators in response to the kairomones. Cladoceran species show deterioration of life history parameters and a reduced population growth rate in the presence of kairomones, probably because they use energy for the morphological and behavioral responses. Kairomones reduce the tolerance of cladocerans to environmental stresses such as high temperature, food shortage, pesticide contamination and oxygen deficiency. As a result, kairomones may alter the population dynamics of cladocearns, and in turn, may affect the whole aquatic ecosystem. Investigation of chemical communication between organisms is an important topic of research to help clarify the dynamics of lake ecosystems.
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