Fluctuations and Responses in Stochastic Motions of Volvox Colonies

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.