Phototaxis in water fleas (Daphnia magna) is differently influenced by visible and UV light

The effects of vertical illumination with monochromatic lights on phototaxis of Daphnia magna in a test chamber were determined at five levels of equal quantal flux density (between 188 and 6.42 . 10(-5) nEinstein). Visible adaptation light (500 nm) and subsequent spectral test light had the same quantal flux density. The animals reacted to ultraviolet light (260-380 nm) with negative phototaxis. whereas visible light (420-600 nm) caused positive phototaxis. Action spectra were determined, based on the evaluation of different parameters of phototactic behavior. The maximum spectral sensitivity in the ultraviolet was found at 340 nm. The maximum spectral efficiency in the visible varied in dependence on light intensity. Ecological consequences of the results are discussed.