The impact of ultraviolet radiation on the vertical distribution of zooplankton of the genus Daphnia

The vertical migration of zooplankton into lower and darker water strata by day is generally explained by the avoidance of visually orienting predator s, mainly fish(1-4); however, it is unclear why daily zooplankton migration has been maintained in fishless areas(5). In addition to predation, ultrav iolet radiation-a hazardous factor for zooplankton in the surface layers of marine and freshwater environments(6-8) - has been suspected as a possible cause of daytime downward migration(9). Here we test this hypothesis by st udying several Daphnia species, both in a controlled laboratory system and under natural sunlight in an outdoor system. We selected Daphnia species th at differed in their pigmentation as both melanin and carotenoids have been shown to protect Daphnia from ultraviolet light(10,11). All Daphnia specie s escaped into significantly deeper water layers under ultraviolet radiatio n. The extent to which the daphnids responded to this radiation was inverse ly linked to their pigmentation, which reduced ultraviolet transmission. Th ese results suggest that ultraviolet avoidance is an additional factor in e xplaining daytime downward migration. Synergistic benefits might have shape d the evolution of this complex behaviour.