Swimming dynamics and surface behavior of a uniflagellate puller swimmer

Trypanosomatids are unicellular, uniflagellate parasites that can infect humans and livestock, resulting in significant health and economic problems. In this project, we investigate the swimming behavior of the insect parasite Crithidia fasciculata, a model organism featuring an ellipsoidal body with a front-mounted flagellum. C. fasciculata propels itself by propagating tip-to-base planar bending along its flagellum. Experiments show that the “polarization plane” of this wave can change instantaneously. Using the method of regularized Stokeslets, we simulate their swimming motion and obtain translation velocity and flow fields that closely resemble experimental results. The far-field flow matches that produced by a negative force dipole, in agreement with the theory of puller-type motion. Distinctive near-field flow features appear in the coronal and sagittal planes, which we attribute to the planar nature of the flagellum beating and the reactive body rocking. For a given body shape, we find that swimming efficiency is maximized when the flagellar wavelength matches its contour length, again in good agreement with experimental data across several trypanosomatid species, suggesting an evolutionary optimization of their motility. The near-wall swimming dynamics are strongly affected by the detailed beating pattern and body geometry, in agreement with prior experimental and simulation studies on bacteria and sperm cells.