Numerical and experimental study of flow inchoanoflagellates and choanocytes

The present thesis uses mainly three dimensional computational fluid dynamics (CFD) along with experiments to study hydrodynamics of choanoflagellates and choanocytes of leuconoid sponges. Choanoflagellates are unicellular eukaryotes that are ubiquitous in aquatic habitats. They are morphologically similar to choanocytes of sponges and believed to be sister group to animals. Both chaonoflagellates and choanocytes have a single flagellum that creates a flow toward a collar filter composed of filter strands that extend from the cell. Leuconoid sponges are filterfeeders with a complex system of branching inhalant and exhalant canals leading to and from the closepacked choanocyte chambers. Each of these choanocyte chambers hold many choanocytes that act as pumping units delivering the relatively high pressure needed to overcome the system pressure losses in canals and constrictions. In this thesis, we first study hydrodynamics of choanoflagellates, specifically morphological adaptations for their flagellum to create adequate flow through the collar filter. We show that observed feeding flow is inconsistent with hydrodynamics of choanoflagellates based on a ’naked’ flagellum. Instead, addition of a flagellar vane, a winglike structure sporadically observed in some species of chonaoflagellates, to both CFD and theoretical models reasonably accounts for the observed flow. Next, we explore hydrodynamic functionality of the lorica, an elaborate extracellular structure in choanoflagellates the function of which has remained unknown. Our results provide no support for the several previous hypotheses, i.e. an increased flow rate through the collar and slowing down the motion through increasing drag. Instead, we argue that the main function of the lorica is to enhance the capture efficiency, but this happens at the cost of lower encounter rate with motile prey. We subsequently explore hydrodynamics of leucon sponges. We show that simply a collection of choanoflagellates with many flagella cannot account for the relatively high pressure measured and estimated for sponges. Instead, some detailed morphological adaptations and additional design elements, i.e. the minimal gap between flagellar vane and collar, the glycocalyx mesh on the collar, and the secondary reticulum, are crucial to the functionality of the choanocyte pump. Finally, we investigate hydrodynamics of fast swimmer choanoflagellates, a dispersal life form of some species of choanoflagellates, near surfaces and explore the possible hydrodynamic impacts on their trajectory. We find that unless very close to a surface, hydrodynamics does not substantially affect their trajectory, but rather their initial swimming direction (and possible flicks) are the determining factor in directing them toward surfaces. However, we find that hydrodynamics benefit fast swimmers in keeping them close to a surface while they navigate arguably looking for a suitable position to attach themselves to the surface; an attachment that seems purposeful, that is to differentiate into feeding thecate cells, but does not happen in the absence of bacteria.