Particle size,flow velocity,and suspension-feeding by the erect bryozoansBugula neritina andB. stolonifera

Patterns of feeding by the erect bryozoansBugula neritina andB. stolonifera were studied by assessing ingestion rates of mixtures of polystyrene particles of three sizes present in equal densities at two ambient water-flow velocities. Particle size was found to influence feeding byB. neritina, while feeding byB. stolonifera was influenced by an interaction between particle size and flow velocity. Large and mediumsized particles were ingested at rates disproportionate to their numerical abundance, and small particles were always ingested in low numbers. Disproportionate feeding did not appear to be due to the greater likelihood of directly intercepting or of detecting particles of larger sizes, but may be explained by other size-dependent particle behaviors, active selection or rejection by the bryozoans, and/or the utilization of different feeding techniques. Comparison with a parallel study indicated that patterns of feeding on single-sized suspensions cannot be used to predict patterns of feeding on mixed suspensions. This is one of the few studies to test the combined influence of variation in both suspended particulate matter and in properties of the fluid medium. Such investigations will provide more realistic views of suspensionfeeding performance.