Fig. 1

Twenty-four hour rhythmic changes in Daphnia’s environment. Daphnia are exposed to different environmental conditions and stressors as the 24 h day progresses. This variation is a consequence of the daily rising and setting of the sun, the chronobiology of other organisms in the environment, and Daphnia’s pattern of diel vertical migration (DVM) through the water column. The sun brings changes in temperature, ambient light, UV radiation, and increased risk of fish predation (which locate Daphnia visually) in populations found in large bodies of water. Similarly, as Daphnids move down the water column they are exposed to decreasing ambient light of changing wavelengths, and reduced UV radiation, temperature, and oxygen levels. Some parasites of Daphnia live in the sediment at the bottom of water bodies, so risk of being parasitized is increased at times-of-day the Daphnia are lower in the water column (daytime in populations from large water bodies)