Behavioural response of Daphnia to olfactory cues from food, competitors and predators

The behavioural response of the freshwater zooplankter Daphnia to chemicals from its food, the green alga Scenedesmus, to algal cells, to the green colour from chlorophyll, to chemicals released from congeners and conspecifics, and to chemicals released from invertebrate (Chaoborus) and vertebrate predators (Leuciscus) was investigated in a Y-tube olfactometer. No preference was observed either for medium that had contained algae, or for medium with algal cells or with the green colour of algae, offered as alternatives to clean medium. In contrast, swimming speed was significantly reduced at high algal concentrations and in the presence of green colour. Moreover, starved animals had lost their rheotaxis. Neither Daphnia magna nor Daphnia pulex had a preference for either clean medium or medium that had contained conspecifics, but D. magna significantly chose clean medium when medium which had been inhabited by D. pulex was the alternative. No avoidance of medium from Chaoborus cultures was found, but D. magna significantly avoided medium that had been inhabited by ides (Leuciscus idus L.). The responses observed could result in aggregation of animals by reduced swimming speed at high algal densities and by avoidance of areas with predators. When these cues have become less important due to food depletion and decreased predation pressure, the interspecific-competitor-related cues might result in desegregation.