Shape of Zooplankton and Retention in Filter-Feeding: A Quantitative Comparison 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 than 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 ratio than copepods (0.599 and 0.882, respectively). Therefore, Daphnia could pass through the square meshes diagonally. In filter-feeding experiments with common bream (Abramis brama), the smallest retained copepods correspondingly were about 35% less wide than the smallest retained Daphnia. White bream (Blicca bjoerkna) did not retain copepods smaller than Daphnia. In the reducible-channel model of filter-feeding, particles are retained in the channels between the medial gill rakers. The mesh size can be reduced by lowering the lateral rakers into these channels. We calculated that zooplankton depth is the critical size parameter in reduced channels and zooplankton width in unreduced channels. We found that white bream was feeding with unreduced channels and common bream with reduced channels. The depth/width ratio (35% lower in Daphnia than in copepods) therefore explains the difference in retention of copepods and Daphnia by common bream whereas no such difference was expected for white bream. The shape of zooplankton thus affects the trophic segregation and the exploitation of food resources by fish.