Meeting Abstract
Mobulids, mantas and devil rays, use filter feeding to capture zooplankton prey that can be smaller than the pore size of their gill rakers. Filtering structure inside their dorso-ventrally compressed oral cavity consists of five branchial arches with each epi- and ceratobranchial surface containing two mirrored arrays of gill rakers. Crossflow filtration is the primary mechanism of particle entrapment, funneling particles toward the esophagus by way of tangential shearing between the flattened filter lobes and the parallel flow stream. Prior experiments with four fold enlarged 3D models of mobulid filter lobes have shown that a secondary mechanism, cyclonic filtration, acts to resuspend small particles that enter filter pore back into the flow stream. We studied the flow dynamics of mobulid gill raker models at actual size and in four fold enlarged models while varying flow velocity, gill raker orientation, angle of attack and secondary structures commonly found on filter lobes. We induced cyclonic filtration in actual sized models that mimicked the filter morphology of mobulid filter feeders by suspending models in a flow chamber with a die stream aimed at the filter lobes. Vorticity in the filter pore did not vary with scale, however the number of vortices increased with the flow velocity and decreased with increasing angle of attack. At smaller angles of attack the vortices stayed continuous within the pores, but extended in the lateral direction at steeper angles. This complex behavior of the vortices further strengthens the assertion that vortex filtration must play a significant role in the filter feeding mechanics of Mobulids.
