Not-so-simple sieving by ascidians: re-examining particle capture at the mesh and organismal scales

Abstract

The particle capture mechanisms of biological filters determine the particle spectrum that is ingested by filter-feeding animals. Although ascidian feeding has been extensively investigated, the organismal-scale fluid dynamics and mesh-scale particle-filter interactions are not fully characterized. Fluorescein dye visualization of flow through the branchial sac of the ascidian Ciona intestinalis showed organismal-scale flow was laminar and moved both parallel and perpendicular to the mucous mesh. Endoscopic investigations of Herdmania momus revealed the mesh-scale filtration process, including the pre-capture velocities, particle approach angles, and mesh behavior. The mesh speed was variable (range 0–0.4 mm s−1). To determine how particle shape affects hydrosol capture, Styela plicata was fed differently shaped polystyrene particles (ellipsoids and spheres); sampling the inhaled and exhaled water revealed that microellipsoids (0.3 × 0.7 µm) were captured at significantly lower efficiency (32%) than 1 µm microspheres (86%). The capture efficiency of microellipsoids resembled that of microspheres with a diameter similar to the microellipsoids’ minor axis (0.3 µm, 31%) suggesting that the minimum diameter of ellipsoidal particles determines the capture efficiency. Flow near the filter was parallel to the mesh even ~ 10 s of micrometers away, implicating a “crossflow” component to ascidian filtration, where the fluid being filtered is directed along the surface of the filter rather than exclusively perpendicular to it. Collectively, these results suggest that ascidian filtration acts as a hybrid-flow filtration system rather than a classical direct sieve.