It has long been known that the waterflea Daphnia hyalina exhibits diel vertical migration in the water column, but the chemical that triggers this behaviour has not been identified. We find that trimethylamine (TMA), which is a major component of the odour produced by decaying fish, induces Daphnia to migrate to greater depths during the day, presumably to avoid predation by fish1,2. We observed a gradual increase in average depth of Daphnia with increasing TMA concentration. Changes in light intensity are known to trigger migration3, and chemicals produced by their predators must also be present4. Because migration has demographic and physiological costs, this chemical cue ensures that zooplankton migration occurs only when fish are present.
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