The ciliate Stentor coeruleus exhibits photodispersal, that is, these cells swim away from light sources and collect in dimly lighted areas. We imaged and reconstructed the tracks of 48 Stentor to determine which swimming behaviors produced their photodispersal. We observed that their photodispersal is not due to a change in their swimming speed but rather to a change in the frequency with which they reorient their swimming direction. Therefore, their photodispersal must be due to either (1) a gradual reorientation of the organism's swimming direction determined by the direction of the light beam (phototaxis) or (2) multiple randomly directed reorientations in swimming direction that occur less frequently when the cell is swimming away from the light source (biased random walk). Sixteen (19%) of the 83 observed forward swimming tracks lasting three or more seconds exhibited a gradual bending away from the light source consistent with a phototaxis. However, most tracks were interrupted repeatedly by abrupt reorientations resulting from ciliary reversals and “smooth turns” that caused cells to reorient through 5.4 times as many degrees as were needed to direct them away from the light source. When cells were swimming away from the light source, their probability of reorienting was reduced and photodispersal resulted.
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Vol. 80 • No. 3