This thesis explores the filter-feeding mechanisms of the American paddlefish, Polyodon spathula. The data indicate that paddlefish use a filtration method termed cross-step filtration, which utilizes vortices formed behind the branchial arches by the incoming water flow. Videos of these vortices were obtained and quantified for the first time inside the oral cavities of preserved paddlefish. Since the paddlefish can move the branchial arches within the oral cavity, the effects of changing branchial arch angle on vortex characteristics were studied in 3D models. The vortices in three models with different arch angles were quantified. The data demonstrated that a change in arch angle affects vortex characteristics in the 3D models. Therefore, paddlefish could be changing the angles of their branchial arches to influence the flow within their oral cavities
