SHAPE OF ZOOPLANKTON AND RETENTION IN FILTER-FEEDING - A QUANTITATIVECOMPARISON BETWEEN INDUSTRIAL SIEVES AND THE BRANCHIAL SIEVES OF COMMON BREAM (ABRAMIS-BRAMA) AND WHITE BREAM (BLICCA-BJOERKNA)

Industrial sieves retained all cycloid copepods with a width larger th an their mesh size, but Daphnia, with a width up to 1.4 times the mesh size, still passed through them. Daphnia have a lower depth/width rat io than copepods (0.599 and 0.882, respectively). Therefore, Daphnia c ould pass through the square meshes diagonally. In filter-feeding expe riments with common bream (Abramis brama), the smallest retained copep ods correspondingly were about 35% less wide than the smallest retaine d Daphnia. White bream (Blicca bjoerkna) did not retain copepods small er than Daphnia. In the reducible-channel model of filter-feeding, par ticles are retained in the channels between the medial gill rakers. Th e mesh size can be reduced by lowering the lateral rakers into these c hannels. We calculated that zooplankton depth is the critical size par ameter in reduced channels and zooplankton width in unreduced channels . We found that white bream was feeding with unreduced channels and co mmon bream with reduced channels. The depth/width ratio (35% lower in Daphnia than in copepods) therefore explains the difference in retenti on of copepods and Daphnia by common bream whereas no such difference was expected for white bream. The shape of zooplankton thus affects th e trophic segregation and the exploitation of food resources by fish.