Particle capture in the crown of the ciliary suspension feeding polychaete Sabella penicillus: videotape recordings and interpretations

An experimental setup was designed for in situ videotape recording of the particle capture process in the crown of the polychaete Sabella penicillus. Intact individuals of S. penicillus (collected in the Gullmornfjord, Sweden in 1992) were exposed to either 6 m Latex spheres or Rhodomonas sp. flagellate cells (6 m). The capture of the added particles was recorded on video. From frameby-frame analyses particle velocities were estimated and the shape of the three-dimensional particle paths was inferred. The mean velocity of particles approaching the crown was estimated to be ca. 1 mm s^-1, increasing to ca. 1.7 mm s^-1 in the interpinnule channel. At the moment of capture the particles were seen to follow a curved, near circular path close to the tips of the latero-frontal cilia. The transport velocities on the frontal side of the pinnules and filaments were estimated to be up to 0.15 and 0.5 mm s^-1, respectively. Counting captured particles relative to particles arriving within the area of the pinnules gave a rough, direct estimate of nearly 100% retention rate when the polychaete was feeding undisturbed. Together with results from clearance measurements in the literature this implies that the worm is able to capture particles down to 3 m entering the interpinnule channel almost 100% effectively. In view of the 80-m wide interpinnule channel and 40-m spacing between the tips of the latero-frontal cilia on both sides of the channel, this result cannot be explained by mechanisms based solely on direct mechanical contact between cilia and particles but must involve fluid mechanical mechanisms. The present work is the experimental basis for ongoing numerical simulations of the particle motion in the interpinnule channel.