IMPLICATIONS OF GILL ARCH MOVEMENTS FOR FILTER-FEEDING - AN X-RAY CINEMATOGRAPHICAL STUDY OF FILTER-FEEDING WHITE BREAM (BLICCA-BJOERKNA) AND COMMON BREAM (ABRAMIS-BRAMA)

Previous research shows that the reducible-channel model of filter-fee ding can probably be applied to common bream, but not to white bream. According to this model, zooplankton are retained in the channels betw een the medial gill rakers; the mesh size of the sieve can be reduced by lowering the lateral rakers of the neighbouring gill arch into thes e channels. Gill arch movements may well disturb this mechanism; the d epressed lateral gill rakers will move in and out of the medial channe ls and also shift out of their centre. We have quantified these distur bances by measuring the gill arch movements during fitter-feeding in w hite bream and common bream, using dorsal X-ray films. In both species , the lateral rakers are long enough to bridge the gill slits. It was expected that common bream, which can reduce their channels, would hav e considerably less shift out of the channel centre than white bream, which cannot reduce their channels. However, the predicted shift is 40 -50% of the channel width in white bream and 75% in common bream. A ne w, dynamic retention mechanism is proposed for common bream. According to this hypothesis, once a particle is trapped in a reduced channel, the channel walls release mucus and the particle becomes sticky. Hence , particles need to be retained mechanically only during part of the g ulping cycle. According to the hypothesis, this is achieved by sideway s rotation of the lateral rakers in combination with their tapering sh ape. Retention mechanisms with interdigitating rakers are expected chi efly in facultative filter-feeders, because such mechanisms are easily disturbed by gill arch movements.