THE SPECTRAL SENSITIVITY OF EYE-MOVEMENTS IN RESPONSE TO LIGHT-FLASHES IN DAPHNIA-MAGNA

The crustacean D. magna responds to a flash of light with a ventral rotation of its compound eye: this behavior is termed eye flick. The spectral sensitivity for the threshold of eye flick in response to light flashes has 3 different spatial characteristics: full-field, extending 180.degree. from dorsal to ventral in the animal''s field of view; dorsal, 30.degree. wide and located in the dorsal region of the visual field and ventral, same as dorsal but located ventrally. All 3 stimuli extended 30.degree. to the right and to the left of the animal''s midplane. 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 -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 maximum 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. Animals which had their nauplius eyes removed by means of laser microsurgery had the same special sensitivity to full-field illumination as normal animals. The results are discussed in terms of the current knowledge of the spectral classes of photoreceptors found in the Daphnia compound eye.