Ciliary Structures and Particle-Capture Mechanisms in Marine Filter-Feeding Bivalves

The minimum size of particles being efficiently captured in the gills of filter-feeding bivalves differs between mussels with well-developed laterofrontal cirri (lfc) and scallops having only simple pro-laterofrontal cilia (pro-lfc). The presence of branching compound lfc increases the particle retention efficiency below the lower limit of about 4 µm for 100% retention, whereas the simple pro-lfc cilia in scallops are less efficient with decreasing retention efficiency for particles smaller than about 7 µm. To understand the particle capture mechanisms in bivalves, attention must be paid to the ciliary structures and water flow in flat gills (mussels) versus plicate gills (scallops, oysters). Here, we briefly review the literature on particle capture mechanisms in filter-feeding marine bivalves with large lfc (mussels, clams), short lfc (oysters), and with only pro-lfc (scallops), and then we describe our present understanding of these processes. This is carried out along with comments on a long-lasting and current controversy on particle-capture mechanisms in filter-feeding bivalves. We rebut the hypothesis of “hydrosol filtering” proposed by Ward et al. (1998), where the approach angle of a particle towards the gill is 30° and the particle is captured by direct interception with a gill filament, whereas lfc generate “zones of blocked through-flow”. No further test of the hydrosol hypothesis has so far been made, but nevertheless, it has been cited in many publications over the last 25 years.