On ciliary sieving and pumping in bryozoans

Based on video-microscope observations of trajectories of particles in the feeding currents of individual isolated bryozoans Bowerbankia imbricata, Flustrellidra hispida and Electra pilosa the velocity fields above and in the lophophore have been determined. The flow into the lophophore, which is a result of water currents driven out between tentacles by the water pumping lateral cilia, is characterised by nearly parabolic profiles with highest velocity along the centreline of the lophophore. In intact animals, the centreline velocity first increases from its value at the inlet to a maximal value about 20 to 25% down into the lophophore and then decreases to low values as the flow stagnates at the mouth. In a narcotised animal, whose laterofrontal filter was inactive, the centreline velocity was found to decrease monotonously from its inlet value. An approximate expression is derived for the relation between velocity distribution in the lophophore and variation of pumping rate along tentacles. Typical variations are given and compared to those obtained by a more accurate two-dimensional numerical solution. Based on observed velocity distributions in the lophophore, particle tracks and tentacle flicks, a description is given of the feeding mechanisms. Particles entering the central region are brought to the mouth by the high velocity feeding current in the central part of the lophophore. Particles entering further out either escape between tentacles or are stopped by the laterofrontal cilia sieve and, in the distal region of tentacles, are brought back into the central feeding current by flicks of tentacles. The relative velocity between fluid and particle during a flick recovery phase ensures particle release. Particles stopped in the proximal region of the lophophore appear to be transferred to and conveyed by the frontal cilia bands on tentacles. The added load on laterofrontal cilia from viscous drag on a food particle retained by the cilia is found to be of the same order of magnitude or greater than the ‘background load’ of viscous drag from fluid passing the laterofrontal cilia in the absence of a particle. This is hypothesised to stimulate the sensing mechanism triggering observed flicks. The energy cost of pumping is estimated at 1 to 4% of the metabolic power of a ‘standard’ zooid.