Ultraviolet radiation induced a lower mortality in morphs of A canthodiaptomus denticornis, coloured red by carotenoid pigments, than in translucent morphs. Calculations showed that ultraviolet penetration in a lake of low chlorophyll-a content may cause a hazardous dose in the surface layers. Experiments confirmed this.

We investigated the relative importance of a behavioural defence (refuge use through diel vertical migration) and a life history change (a reduced size at first reproduction) that are used by daphnids to decrease the risk of predation by visually hunting fish. We used an individual based model of a Daphnia population in a stratified lake to quantify the effects of these inducible defences on Daphnia predation‐mortality and the resulting Daphnia population dynamics. Our analysis shows that diel vertical migration (DVM) confers a much stronger protection against fish predation than a reduced size at first reproduction (SFR). DVM allows daphnids to withstand a higher predation pressure in the epilimnion and it decelerates a Daphnia population decline more strongly than a reduced SFR. DVM effectively reduces the (P/B) flow of carbon from daphnids to fish. 
Many theoretical studies have only considered the fitness benefits of DVM above ‘staying up’ in the epilimnion of a lake. Our results suggest that ‘staying down’ in the hypolimnion would confer an even stronger fitness benefit to Daphnia than DVM at times of peak predation risk. Daphnids that remain in the hypolimnion avoid the predation suffered by migrating daphnids around dusk and dawn. Staying down could prevent a Daphnia population decline, while DVM and a reduced SFR can only decelerate the decrease of Daphnia population densities under heavy fish predation. Staying down at high concentrations of fish infochemicals has in fact been observed within a variety of Daphnia clones and species, both in the laboratory and in stratified lakes.

Downward and upward displacement velocity of diel vertical migration in Lake Maarsseveen by the hybrid Daphnia gaZeata x hyalina, was correlated with the relative change in light intensity at dawn and dusk, indicating a phototactic basis for these migrations. The significance of the regression function increased over a period of 4 yr. Changes in environmental factors such as water transparency and food concentration are not responsible.

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