Downstream collecting in ciliary suspension feeders: the catch-up principle

Based on observations of feeding structures and currents in the polychaete Spirorbis tridentatus, the entoproct Loxosoma pectinaricola and the cycliophore Symbion pandora, which all possess compound cilia, it is hypothesized that their capture mechanism is based on the catch-up principle. According to this principle, the compound cilia constitute the pump which generates a flow with suspended particles that enters the ciliary region. In this region the same cilia, during their power stroke, catch up with suspended particles and transfer the particles to a food groove, or a mouth cavity. In the particle-size retention spectrum, the lower limit depends on spacing between cilia in phase, while the upper end depends on cilia length which may or may not allow particles to enter the ciliary region. On the basis of fluid mechanical considerations and literature descriptions of structure and function of the ciliary bands of some rotifers and of the various types of trochophora larvae of annelids, molluscs and entoprocts, it is hypothesized that the feeding mechanisms of these organisms are based on the catch-up principle.