Stauroneis sp. from a horizontal view

From a pond near Hohenheim (48°42'32.2"N 9°12'40.3"E) a diatom of the genus Stauroneis was isolated, which was about 180 µm long. It is likely to be Stauroneis phoenicenteron. The picture of its valve can be seen on the left (click to enlarge). The video above left shows the movement in different magnification levels (5x to 40x objective) and time-lapse factors (5x: 450x time-lapse; 10x first scene: 300x time-lapse: 10x second scene: 600x time-lapse; 20x: 300x time-lapse; 40x: 150x time-lapse).

The speed of the diatoms can be very different. In the first video sequence a fast-moving diatom appears at the bottom of the screen.

In a short section of this video you can see diatoms that separate after asexual reproduction (10x objective, 600x time-lapse). Here, the outline is clearly visible in girdle view (girdle is uppermost). Obviously, the diatom in a valve view (a valve is uppermost) cannot lie level on a flat substrate but has only a small contact surface.

Due to the shape of the raphe only slightly curved trajectories usually occur. A detail enlargement from a video showing a typical path can be seen at the top right. At the end of the video a superposition of all frames is shown. Occasionally, stronger curved paths are observed. I suppose adherent EPS lumps are the cause. In such cultures diatoms in girdle position also show an astonishing activity of movement.

Therefore, a calculation of the position of the center of force using path curvature (see contribution to the analysis of trajectories) is not reliable. Also the evaluation of the not very large directional fluctuations around the direction of movement did not provide a clear information for short paths between reversal points. In the case of longer sections between reversal points, the diatom seems to be pushed from a point near the trailing apex. This analysis also corresponds to the visual impression when viewing the fluctuations.

For reliable evaluation, observation from a horizontal perspective in relation to the substrate is appropriate. A typical video is shown on the left side (50x time-lapse). There is a strong similarity to the movement of Craticula cuspidata. In case of longer movements the subsequent apex is closer to the substrate. This position is taken shortly after reversing the direction. It is not very stable as occasional fluctuations can be recognized. Sometimes the diatom moves for a short time with its apical axis parallel to the substrate. When changing direction quickly, no constant inclination of the apical axis to the horizontal is established.

As with Craticula cuspidata, the reversal of direction in the video does not occur as a result of tilting to the other raphe with an opposite direction of its activity. A change in direction is caused by a change in the direction of the transport of the raphe. When in the course of the movement the inclination of the apical axis changes in the way described, the other raphe comes into contact with the substrate. If the direction of transport of the raphes is different, the movement must stop at horizontal position of the diatom. It would then have to reverse itself if the diatom remained in contact with the substrate. A fast jerking back and forth would be the result if the diatom remained approximately in a horizontal position. During the observation period of a few hours there were a few cases where the direction of movement was reversed while the diatom was in an almost horizontal position. It is unclear whether this reversal was caused by the opposite transport direction of the raphes or by a simultaneous change of an identical transport direction. I assume that a reversal of direction as a result of opposite raphe activity rarely or never occurs and that the transport direction of both raphes of a valve is usually the same.

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