The circadian rhythm of a behavioral photoresponse in the dinoflagellate Gyrodinium dorsum.

The circadian rhythm of the photoresponse to blue light in the dinoflagellate Gyrodinium dorsum Kofoid was investigated by the use of a closed circuit television system. The initial cessation of movement upon stimulation (stop-response) was used as the index of light reception. Under constant dark conditions cells grown on a 12L:12D regime show an endogenous circadian rhythm in their stop-response with maximum responsiveness occurring approximately one hour before the beginning of the expected light phase. This rhythmic response was only observed if the cells were irradiated with red light (620 nm) prior to stimulation with blue light. After preirradiation both far-red reversibility and the shift in the stop-response action spectrum from 470 nm to 490 nm could also be demonstrated. These findings may be related to the diurnal migration of marine dinoflagellates.