FLUID MOTION AND PARTICLE RETENTION IN THE GILL OF MYTILUS-EDULIS - VIDEO RECORDINGS AND NUMERICAL MODELING

Particle trajectories of 6.4 mum Latex spheres were recorded by video, both near an isolated blue mussel, Mytilus edulis, gill filament and, in place of an intact interfilamentary canal, in a model canal of wid th 200, 100 or 70 mum, formed by a transparent plate positioned next t o a gill filament. Each arrangement was placed in a 2 x 10 x 10 cm tes t vessel filled with seawater. Serotonin (nerve-transmitter) stimulati on was used to activate lateral cilia and to either lock latero-fronta l cirri at the end of an active stroke (10(-5) M), or to activate them (10(-6) M), yielding lateral cilia beat frequencies of 19 and 16 Hz, respectively. With latero-frontal cirri locked, image analysis of part icle tracks gave maximum velocities of ca. 2.9 +/- 0.2 mm s-1 close to the tips of lateral cilia, for both isolated filament and model canal cases. Experimental velocity profiles along the 200-mum wide model ca nal were recorded and used as good approximations to the fluid velocit y because of the low Reynolds number. A two-dimensional steady model w as proposed for the gill pump, assumed to only comprise lateral cilia. This model was solved numerically for the experimental model canal in the vessel and the results showed satisfactory agreement with experim ental velocity profiles from particle tracks. The numerical approach w as also applied to a model of a single interfilamentary canal in the v essel. The resulting mean velocity in the canal was 1.70 mm s-1, but t he resistance to flow in the model was less than that in an intact mus sel gill. Video graphs of particle tracks indicated that active latero -frontal cirri play a role in the transfer of particles from through c urrent to frontal current, probably by means of a strong interaction t hrough the motion of intervening fluid rather than through a direct ph ysical contact. M. edulis specimens used in the present study were col lected in 1990 at Helsingor and in 1991 at Kerteminde, Denmark.