Phototaxis, gravitaxis and vertical migrations in the marine dinoflagellateProrocentrum micans

The phototactic orientation of the marine dinoflagellate Prorocentrum micans was studied at three different ages and at several light intensities. High irradiances caused the cells to show negative phototaxis and low irradiances caused positive phototaxis. The precision of negative phototaxis reached a maximum in the early afternoon, while the precision of positive phototaxis was found to peak in the morning and at night. The cells also showed a pronounced negative gravitactic orientation, which had a maximum in precision in the early afternoon. The degree of gravitaxis was found to be constant over time when the cells were confined to a closed cuvette for up to 9 h. As a consequence of the orientation strategies, populations of Prorocentrum micans showed daily vertical migrations in a 3-m Plexiglas column. They accumulated in the top layers in the afternoon and were almost randomly distributed during the rest of the day.