Meeting Abstract
Mobulid fishes (manta and devil rays) are massive ram filter feeders that prey primarily on zooplankton that are routinely smaller than the apertures of their porous gill rakers. These gill rakers, supported by 5 chevron shaped arches inside the oropharyngeal cavity, consist of rows of lobes that are arranged at an acute angle to the central raphe that connects them. Pores, fissures between successive lobes, are the site of filtration. Prior studies with scaled up biomimetic gill raker models documented vortices at the filter pore that occlude the pore and may centrifugally transport particles smaller than the pore, back into the free stream that progresses posteriorly toward the esophagus. Furthermore, adjusting the lobes’ leading edge shape, angle with respect to flow and surface microstructure have been shown to alter the vortex diameter. We tested actual sized models by embedding them inside a mock oropharyngeal cavity and exposing them to particle-laden fluid flow and adjusting the fluid velocity, lobe angle of attack and surface microstructure. The cavity had a simulated gill slit (ventral opening to the model) and an esophagus (opening downstream to the model) where particles were collected using a fine mesh. We used a homogenous mix of spherical artemia eggs (200-500 μm) that are heavier than water and smaller than the model pore size (1.7 mm), as particles. The simplest model (smooth) retained 4 times more particles at the esophagus while resisting half the flow compared to when the model was removed from the gill slit. These results suggest that these mobulid gill rakers use vortical filtration, a distinct mode of particle filtration among previously described systems, to concentrate particles smaller than the pore size inside the oropharyngeal cavity and to prevent the filters’ clogging.
