| Craticula cuspidata observed from a perpendicular direction of view with respect to the substrate (5x time-lapse) |
Craticula cuspidata observed from a horizontal direction of view with respect to the substrate (8x time-lapse) |
Craticula cuspidata from a horizontal view
The video above left shows the view of a culture of Craticula cuspidata (found in Ebnisee 48°55'25.5"N 9°36'32.8"E, Length approx. 120 µm), as you are used to from an inverse microscope. The viewing direction is perpendicular to the substrate. A diatom of the species Craticula cuspidata can be seen to the right of it in a horizontal view with respect to the substrate. The diatom changes the direction of the movement several times. With longer movement in one direction, the diatom is elevated against the horizontal, with the trailing apex closer to the substrate. Therefore, the diatom is pushed. The point P determined by means of the described method from the trajectory is in accordance with this direct observation.
 |
More important is the behavior of the diatom at the points of motion reversal. As Craticula cuspidata has two raphes on each valve (see picture on the left; click to enlarge), one might assume that the driving raphe loses contact with the substrate and the other raphe comes into contact with the substrate by tilting the diatom. In case of the opposite direction of transport of mucilage, the direction of movement would be reversed.
This recording and several other videos show that the sequence is different. In the first step, the driving raphe changes its direction of transport so that the diatom moves in the opposite direction. Thereafter, the inclination angle of the diatom to the substrate gradually changes until finally the trailing apex is close to the substrate. Three points seem to be essential:
- The diatom is not generally pushed, but is pulled in a short time frame after the movement reversal. A statistical evaluation of the trajectory as described earlier can hardly reveal this because it requires a sufficiently long distance and directional fluctuations in the vicinity of the reversal.
- The diatom is always tilted to the other raphe after the change of direction, without a second reversal of the direction being observed. Obviously the two raphes operate synchronously in the same direction. Strictly speaking, this observation can make a statement about the synchronicity only for the time frame shortly after the change of direction.
- The diatom systematically changes its angle of inclination after the change of direction. It is not recognizable which mechanism causes this effect. Possibly the water flow (small Reynolds numbers) could play a role. It would be interesting to know whether there is a correlation between the velocity of the diatom and the angle of inclination. I consider it more likely that the drive mechanism in the local environment of the contact results in a displacement of the contact point.
In some diatoms spp. one observes that often after long unidirectional trajectories, a rapid back and forth jerking occurs until a longer movement section follows. A possible explanation is that in these cases the raphes work against each other. If the activity of the other raphe was opposed after a change of direction, a new change of direction would occur as soon as the diatom reached the horizontal position. Then the diatom would jerk back and forth until a uniform direction of the activity of the raphe takes place. So far I have not been able to observe this situation in horizontal view.
Note on requirements for a theory of the locomotion
A complete theory of the movement must not only be able to explain how a change of direction takes place at a molecular level. It also has to explain why some diatoms are pulled and others are pushed, how the synchronization between the raphe systems is accomplished and how the shift of the contact point occurs when the direction of movement changes.
It would be important to see whether the sequence for Navicula is analogous to that of Craticula cuspidata. Here, however, it would always have to be changed into a pulling position after reversal of the direction in which the leading apex is closer to the substrate.
Video recordings of a species of the genus Rhopalodia show a tilting of the diatoms on the opposite apex without a direction change. Craticula cuspidata does not show such a tipping.