The spectral sensitivity of eye movements in response to light flashes inDaphnia magna

Summary The crustaceanDaphnia magna responds to a flash of light with a ventral rotation of its compound eye; this behavior is termed eye flick. We determined the spectral sensitivity for the threshold of eye flick in response to light flashes having three different spatial characteristics: (1) full-field, extending 180° from dorsal to ventral in the animal's field of view; (2) dorsal, 30° wide and located in the dorsal region of the visual field; (3) ventral, same as dorsal but located ventrally. All three stimuli extended 30° to the right and to the left of the animal's midplane. We found that spectral sensitivity varies with the spatial characteristics of the stimulus. For full-field illumination, the relative sensitivity was maximal at 527 nm and between 365 nm and 400 nm, with a significant local minimum at 420 nm. For the dorsal stimulus, the relative sensitivity was greatest at 400 nm, but also showed local maxima at 440 nm and 517 nm. For the ventral stimulus, the relative sensitivity maxima occurred at the same wavelengths as those for the full-field stimulus. At wavelengths of 570 nm and longer, the responses to both dorsal and ventral stimuli showed lower relative sensitivity than the full-field stimulus. No circadian or other periodic changes in threshold spectral sensitivity were observed under our experimental conditions. Animals which had their nauplius eyes removed by means of laser microsurgery had the same spectral sensitivity to full-field illumination as normal animals. Our results are discussed in terms of our current knowledge of the spectral classes of photoreceptors found in theDaphnia compound eye.