The vertical migration of zooplankton into lower and darker water strata by day is generally explained by the avoidance of visually orienting predators, mainly fish; however, it is unclear why daily zooplankton migration has been maintained in fishless areas. In addition to predation, ultraviolet radiation-a hazardous factor for zooplankton in the surface layers of marine and freshwater environments-has been suspected as a possible cause of daytime downward migration. Here we test this hypothesis by studying several Daphnia species, both in a controlled laboratory system and under natural sunlight in an outdoor system. We selected Daphnia species that differed in their pigmentation as both melanin and carotenoids have been shown to protect Daphnia from ultraviolet light. All Daphnia species escaped into significantly deeper water layers under ultraviolet radiation. The extent to which the daphnids responded to this radiation was inversely linked to their pigmentation, which reduced ultraviolet transmission. These results suggest that ultraviolet avoidance is an additional factor in explaining daytime downward migration. Synergistic benefits might have shaped the evolution of this complex behaviour.

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