The movements of the latero-frontal cilia and the mechanism of particle retention in the mussel

Abstract

1.1. A method is described whereby the movements of the cilia on the gills of intact, transparent young mussels can be studied.2.2. A full description of the movements of the latero-frontal cilia is given. These cilia are straightened during their effective beat and curved during the recovery-beat (fig. 4). The effective beat is started from a position roughly parallel to the gill surface and is usually ended when an arc of about 90° (sometimes less) has been described. During the transition from the recovery- to the effective beat the movement of the cilia is considerably slower than in the outer beat phases. Exceptionally, another type of movement, described by Gray (1982) as flexural movement (fig. 7), is performed. It is more frequently observed in excised gill pieces than in intact animals.3.3. The beat of the latero-frontal cilia is co-ordinated in such a way that the movements of adjoining cilia differ about half a phase (fig. 8), so that metachronic waves as such are not discernable. The wavelike movement which is observed is ascribed to an optical artefact.4.4. The beat-frequency of the latero-frontal cilia varies between 4 and 9 beats per second and usually is 7 to 8 beats per second. There are small local and temporal variations in the beat-frequency of these cilia; the major variations seem to occur more or less parallel with those of the lateral cilia (fig. 10). The frequency of the latero-frontal ciliary beat is affected by the resistance caused by the water current.5.5. The gill filter may be compared to a lime-twig mechanism as described by Tammes and Dral (1955); food particles stick to the latero-frontal cilia.6.6. Water may be pumped through without apparent retention of food particles. In these non-filtering animals the latero-frontal cilia are no longer sticky. The beat may remain co-ordinated or not; it may also stop. In the latter case various positions of the resting cilia may be observed.7.7. Normally the tips of the latero-frontal cilia of neighbouring filaments approximately touch oneanother when they are in the position from where the effective beat is started. As a result the latero-frontal cilia bridge the ostia between the filaments completely.8.8. It follows from the arrangement, movements and beat-frequency of the latero-frontal cilia that at low pumping rates all particles of 3 μ and over can be retained when water passes the gills. At high pumping rates all particles larger than 6 μ are retained. The “critical” pumping rate is defined as the lowest velocity allowing the passage of particles of 6 μ. This rate is compared with the actual rates measured for mussels of various sizes (fig. 13).9.9. The mussel can lower the efficiency of the gill-filter by shifting the range of beat of the latero-frontal cilia towards the frontal side (fig. 5), so that these cilia no longer bridge the ostia, and suspended particles have a chance to pass the free space thus formed.