POPULATION VARIATION IN THE ONTOGENY OF PREDATOR-INDUCED VERTICAL MIGRATION OF COPEPODS

DIEL vertical migrations (DVM) of zooplankton are nearly ubiquitous and vary widely in timing and magnitude 1-5. Predation can be both an evolutionary and proximal ('inducing') agent in promoting this variation 6-14. Because aquatic predators are strongly size-selective 15, vulnerability to predation changes during an individual's ontogeny. Accordingly, natural selection on phototaxis, timing and magnitude of migration should also vary during ontogeny. Furthermore, biogeographic variation in the composition, density and behaviour of predators would probably select for different ontogenies of DVM. Whether predator-induced or developmentally fixed (canalized) migrations would be favoured probably depends on the variability of predatory risk 16-19. Here I compare experimentally DVMs of calanoid copepods from populations with contrasting histories of vertebrate and invertebrate predation. Results show predator- and population-specific migrations that differed with life history stage/size. Both developmentally fixed and predator-induced migrations were found at various life-stages in the same group of individuals. Ontogenetic patterns, however, differed among populations adapted to different suites of predators. Genes coding for differences in phenotypic expression of migratory behaviour seem to have been selected by local predation at each stage/size.