Bryozoan filter feeding in laminar wall layers: Flume experiments and computer simulation

Particle paths and velocities have been determined from video recordings ab ove single-line colonies of bryozoans (Celleporella hyalina, Electra pilosa , Alcyonidium hirsutum, Membranipora membranacea, Flustrellidra. hispida) p laced at the bottom of a laminar flow flume in zones of constant velocity g radient (1 to 4 s(-1)). The laminar wall layer simulated viscous sublayers found in the field for smooth surfaces. Incurrents to lines of 3 to 10 zooi ds typically distort paths of particles approaching the colony at heights 1 to 2 mm above the level of lophophore inlets and they capture particles fr om paths 0.7 to 1.2 mm above this level. The experiment was simulated numer ically by computing the full three-dimensional laminar flume flow for the c ase of a line of 10 zooids that were modelled as sink-source pairs. Compute d paths of discrete "fluid particles" show how the fraction of captured par ticles per zooid decreases downstream. Similar results were obtained by com puting the continuous concentration distribution in the flow resulting from specifying uniform upstream concentration and sinks at zooids. Computed pa rticle paths show the cross sectional area of approaching flow cleared of p articles by the 10 zooid line colony to be about 16 times the frontal area of a simulated lophophore. Fluid particles were captured from paths about 1 .3 mm above the sink. At twice the flowrate, the area cleared of particles reduced to about 7 times the frontal area while feeding rate increased by a bout 19%.