Induced vertical migration in copepods as a defence against invertebrate predation

Abstract

THE adaptive significance of diel vertical migration by planktonic organisms is often explained in terms of daily changes in foraging locations that reduce the risk of depth-stratified predation^1–3. The timing, extent and rate of migration are usually presumed to be more or less invariant (closed⁴) behaviours that have evolved under local predation regimes^5–8. Observational and experimental evidence, however, indicate that migratory patterns in the presence of predators may be altered substantially by short-term reduction in the availability of food^9–13. Behavioural flexibility of food-limited individuals rather than genetic change of populations seems likely¹³. Whether individuals are also sensitive to changes in the risk of predation is unknown, though some field observations are highly suggestive¹⁴. Here I report rapid initiation (<4 h) of vertical migration in previously non-migrating freshwater copepods inside large in situ enclosures on exposure to an invertebrate predator. Responses of copepods to water that had previously held captive predators suggests that a chemical cue is involved. These results show that crustacean zooplankton may be capable of flexible, predator-sensitive foraging and suggest a mechanism for rapid changes in migration patterns.