The hydrodynamics of filter feeding in choanoflagellates

Choanoflagellates are filter feeding heterotrophic nanoflagellates. Fluid flow generated by the motion of the flagellum delivers suspended prey particles to the surface of a crown-like filtering apparatus. Hydrodynamic characteristics are consequently of great importance to understanding prey capture in these organisms. This paper reports on the use of video-microscopy to investigate the hydrodynamics of filter feeding in choanoflagellates. The cell and collar morphology, and the flagellar and fluid motion of three species of choanoflagellates, one from each family of the phylum, are described. Inter-species differences in these parameters are compared to one another and to optima theoretically determined by Higdon (1979b). The motion of a flagellum close to an infinite boundary is modelled using a line of stokeslets. This model predicts the occurrence of viscous eddies in the fluid streamlines. Viscous eddies are observed in the far field fluid flow of two species of sessile choanoflagellates. Balancing forces over a control volume and using slender body theory allows estimates of pressure drop across the collar of the three species of choanoflagellate to be made. The magnitude of pressure drop (Deltap) was found to be consistent with previous publications, suggesting that Ap may be a constraining parameter for filter feeders.