Frequency distribution of residence time. The motions were recorded near the upper surface (a, b, and c), in the middle (d, e, and f), and near the base (g, h, and i). YM4 (a, d, and g), YM42 (b, e, and h), and NMB102 (c, f, and i) were used in this ex-periment. The bars refer to the measured data. The number of data points is 200 for each case. Data are not shown in the two left-most columns of each graph, since tra-jectories that had 20 frames (2/3 s) of residence time were not analyzed. The line represents an exponential function used to fit the measured data . From Magariyama et al. (2005) with permission.

 10 In summary, backward swimming cells tend to stay close to a surface longer than forward swimming cells, and this asymmetry between the forward and backward motions appears only when the cells swim under the influence of a surface