We have studied stochastic motions of the microorganism Volvox, which
displays phototactic behavior. Volvox colonies in a chamber were stimulated stepwise or
sinusoidally by varying light intensities from both sides of the chamber, and their
responses were systematically measured. We found that the colonies show a linear
response to a relative light intensity difference, expressed as a ratio of the difference
between the light intensities of the two sides to the sum of the intensities. An experimental
formula obtained using our measurements describe the responses under various light
conditions. We also found a linear relationship between fluctuation without the light
stimulus and response to the stimulus; a group of colonies with a large fluctuation in
velocity shows a larger response. This result indicates the existence of a relationship
between fluctuations and responses in the motion of a living organism.
Keywords: Active matter, fluctuation, light stimulus, phototaxis, response, stochastic motion, Volvox.
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