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.
|Place of Publication||Kgs. Lyngby|
|Publisher||Technical University of Denmark|
|Number of pages||111|
|Publication status||Published - 2019|
|Series||DCAMM Special Report|