Light responses of Daphnia pulex Stephen C. Steams’ Laboratory of Limnology, University of Wisconsin, Madison 53706 Abstract Individual Daphnia l?tlZex were tested for activity responses in different wavelengths of light at the same intensity and at different times of day in white light at fixed intensity. The distance a claphnid moved in 5 min varied invcrscly with the wavelength of incident light. Daphnids tcndecl to move vertically between 480 nm (blue) and 735 nm (red) and hori- zontally at 440 nm (violet) and in white light; this observation corroborates Smith and Baylor’s description of color dances. Daphnia pulex has a circadian activity rhythm. Total activity was low at night, increased to a peak in the morning, then declined through the afternoon. The color of ambient light is probably not a reliable indicator of the presence of phytoplankton in the water. Many workers have studied the bchav- ioral responses of Daphnia to the color and intensity of incident light. Although there is now little doubt that a change in the in- tensity of light triggers circadian vertical migrations (Harris and Wolfe 1955; Mc- Naught and IIasler 1964) and that changes in photoperiod trigger diapause (Stross 1966a,b; Stross and IIill 1965)) the adaptive significance of daphnid color responses re- mains poorly understood. Ttaving documented color dances in the Cladocera, Smith and Baylor ( 1953) inter- preted them as a means of locating food. They observed that at wavelengths of inci- dent light below 500 nm Daphnia magna tended to roam horizontally, while at wave- lengths above 600 nm they tended to keep station with a vertical, hop-and-sink mo- tion. The total amount of time an individ- ual daphnid spent doing each dance was quite sensitive to the proportion of long to short wavelengths in the incident light. Smith and Baylor proposed that such bc- havior would tend to concentrate daphnids under algae, where the light was thought to be reddish, while in clear water, where the light is dominated by blue-green wave- lengths, daphnids would roam horizontally until they encountered food. My study had three purposes: to check the Smith and Baylor observations by de- 1 Present address: Institute of Animal Resource Ecology, University of British Columbia, Van- coIIvcr VGT lW5. termining responses of individual daphnids to different wavelengths at a constant in- tensity, to see if daphnids have a circadian response to light intensity that could bias laboratory results taken at different times of the day, and to assess critically the re- liability of the color of ambient light as a sign pointing the way to food for plank- tonic grazers. I thank S. Dodson, J. Magnuson, J. Ncese, T. Northcote, W. Neil], J. Ringelberg, and S. Katona for their constructive criticism of this work. I was supported by an FWQA training grant administered by A. D. IIas- ler. J. IIailman kindly lent me a set of fil- ters. Materials and methods I obtained Daphnia pulex from a culture, probably a chc, that had been maintained for over 2 years by G. Kass. Cultures were maintained in two 5-liter aquaria in a light box with a 14L:lOD photoperiod provided by a 40 W, 110 V GE bulb. The light inten- sity at the water surface of the rearing aquaria varied between 2.0 and 5.6 X 10” ergs cm-2 s-l. Temperature varied between 23.0 and 26.0°C, and pI1 between 7.5 and 7.9. Every morning I fed the daphnids al- gae (a mixture of Selenastrum and Ankis- trodesmus spp.) grown in Hoagland’s solu- tion and diluted 15 : 1 with tapwater aged at lcast 2 days. In the photographic darkroom where the experiments were done, the tcmpcrature LIMNOLOGY AND OCEANOGRAPIIY 564 JULY 1975, V. 20(4)