Abstract
Diel cycles of reproduction and vertical migration were observed in the rotifer Keratella crassa. Egg‐hatching synchrony was pronounced during summer, when 73–93% of eggs hatched during a 6–7‐h period. Reproductive synchrony was evident before the egg development stage and may have arisen during oogenesis in response to diel changes in lake temperature. Vertical migration was generally upward at night and downward during the day. These movements differed in timing, however, for ovigerous and nonovigerous females. Ovigerous animals reached their lowest and highest positions in the water column about 4 and 6 h later than nonovigerous animals. This differential migration produced diel changes in the depth profile of egg ratio. These findings of egg‐hatching synchrony and vertical migration have important implications for studies of zooplankton population dynamics. In the 2 years of this study, sampling at different times of day produced daily variations in egg ratios and calculated instantaneous birth rates that approached an order of magnitude.
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