Flexibility foils filter function: structural limitations on suspension feeding.

Suspension feeders rely on filter structures of a variety of forms to capture food particles. Much effort has been devoted to examining the operation of such filters, but mechanistic evaluations have generally represented filter elements with artificially stiff cylinders. We extended this previous work to investigate how bending affects the function of flexible cylindrical filter elements. Scaled models of filters were constructed from materials with elastic moduli comparable to material stiffnesses of invertebrate appendages (1-177 GPa). These models were mounted on a sled to mimic the protrusion of filters away from an animal's body or from the substratum, and were towed through a vat of syrup to generate relative fluid motion at low Reynolds numbers (Re <10(-3), based on cylinder diameter and tow speed). Flow between filter elements was quantified at multiple positions along their lengths, and a hydrodynamic index of filter performance ('leakiness') was calculated. Leakiness generally increased with cylinder Re and distance from the filter base. At higher flexibilities, however, streamwise bending and lateral narrowing of the filter reduced projected area and slowed flow between elements. This effect decreased leakiness and reversed the otherwise monotonic trend for increased leakiness at higher cylinder Re. Additional experiments showed that filters composed of stouter elements were less susceptible to bending but experienced lower leakiness because of their reduced ability to transcend boundary layers formed over surfaces to which they attached. These findings indicate that filter bending can strongly alter the performance of particle capture apparatus in suspension feeders.