THE ACTION SPECTRUM OF <i>DAPHNIA MAGNA</i> (CRUSTACEA) PHOTOTAXIS IN A SIMULATED NATURAL ENVIRONMENT

Abstract

Abstract— The action spectrum of phototaxis in Daphnia magna (Crustacea) was measured in a chamber which simulated a natural angular distribution of underwater light. A 17% step‐down in irradiance was used to stimulate the phototactic response at all wavelengths and irradiances tested. Peaks in the spectral response curves depended on the fluence rate to which the zooplankton were acclimated. The wavelength of maximum response (Z max ) shifted from yellow‐green at the highest acclimation fluence rate (5.1 × 10 −2 Wm −2 ) to blue‐violet at moderate rates. At low acclimation fluence rates, the blue‐violet maximum was retained and another maximum developed in the red. At the lowest fluence rate (1.6 × 10 −5 Wm −2 ), the blue‐violet and red maxima were lost and another maximum developed in the near ultraviolet. The action spectrum indicates the presence of three, and possibly four, photopigments with Z max , at ∼405, 440, 570 and 690nm. The 440 and 690nm maxima may belong to the same photopigment; however, this was not tested. Changes in zooplankton swimming speed, caused either by large changes in irradiance or by mechanical stimuli, were accompanied by changes in the strength of the phototactic response to the −17% stimulus at any irradiance level for white and monochromatic light, and indicated the presence of a mechanism connecting swimming speed and photosensitivity.