A new explanation of particle capture in suspension‐ feeding bivalve molluscs

Using video endoscopy, we examined water flow through the infrabranchial cavity and observed particle capture by the ctenidia (gills) of several species of suspension-feeding bivalve molluscs. We found that previously published interpretations of the particle capture process did not adequately explain our in vivo observations. Instead, we propose that particle capture in bivalves can be explained in terms of hydrosol filtration theories that have been shown to apply to a wide diversity of aquatic suspension feeders. Particles are captured by direct interception with the ctenidial filament and then transported along the frontal surface of the filament by mucociliary processes. Two primary mechanisms aid in capture. First, the low angle at which particles approach the filaments increases the probability of encounter with frontal cilia. Second, vortical flow patterns set up by the beating of the laterofrontal cilia or cirri reduce or block flow through the interfilamentary spaces and redirect it toward the frontal surface of a filament. This flow pattern further increases encounter efficiency with the frontal cilia and promotes particle retention on the frontal surface. Our studies indicate that the suspension-feeding complex as a whole (incurrent siphon margin, ctenidia, and mantle) functions in a highly integrated manner and is critical for particle capture. This observation calls into question previous explanations developed from examination of surgically altered bivalves.