LACK OF ENDOGENOUS RHYTHMICITY IN DAPHNIA DIEL VERTICAL MIGRATION

A population of Daphnia galeata x hyalina was induced to perform diel vertical migration (DVM) by the addition of fish-kairomones to a large plankton tower. Vertical profiles of zooplankton abundance were measured every 12 h. When the tower was covered to shield the water column from the light, DVM ceased and vertical distributions were not statistically different from the night profiles that had been observed before the tank was covered. This result suggests than endogenous rhythms do not trigger DVM, but that a change in light intensity must occur.