Cilia make direct use of the differences in drag between cylinders moving normal compared to tangential to the fluid in generating motion. The movement of the cilia consists of a power stroke in which the cilia forms a cylinder with a motion normal to the fluid, and a recovery stroke where most of the cilia moves tangentially with respect to the fluid (Figure 13.4). Whereas flagellates typically have only one or few flagella, ciliates must have thousands of cilia to produce motion. They are typically arranged in rows in which the cilia beat in metachronical waves, i.e., waves formed by a slight phase lag between adjacent cilia. These waves seem to be fluid dynamical in origin because they can be explained largely through the viscous coupling of adjacent cilia [45,46].

Another important aspect of the functioning of the cilia is the proximity of the cell wall. During the power stroke, when the cilium is extended away from the cell wall, it is capable of carrying along with it a large envelope of fluid. During the recovery stroke, the cilium moves close to the surface of the cell, and because of the viscous interactions with the wall, the cilium cannot carry as much fluid with it. Hence, there is a net movement of fluid down the surface of the cell, which contributes to the motion of the organism [47]. The fact that the ciliates move by moving fluid over the cell surface results in a much steeper velocity gradient over the surface than that found in inert bodies being pulled through the fluid by an external force, such as sedimenting organisms. Bodies pulled in this way carry more fluid along with them, and so they disturb the fluid much more than swimming cells [47,48]. Because predators may perceive prey through fluid dynamic signals such as shear [49,50], this reduces the visibility of the ciliates.

FIGURE 13.4 Movement of a cilium. The movement from position 1 to 3 constitutes the power stroke and from 3 over to 4 and 5 and back to 1 constitutes the recovery stroke. Movement purely related to the recovery stroke is drawn with dashed lines.

Flagellates, which pull an inert cell body through the fluid with a flagellum, do not have this advantage, but they are also typically smaller and slower. Hence, it is not clear under which conditions flagellates are hydrodynamically more visible than ciliates. Ciliates that have the cell body only partially, as compared to fully, covered by cilia could also generate a larger scale flow field around the cell body. Thus, it seems that the exact mechanism by which protozoa generate motion influences their relative visibility toward different types of predators and so influences their relative predation rates. Though this has important ecological implications, there have to my knowledge not been any thorough investigations of this phenomenon on protozoa. Only work on how foraging behaviors influence hydrodynamic visibility in copepods [51] and on how size and velocity of an assumed nonciliated particle affects its visibility [49] have been carried out so far.

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