COMPARATIVE MICROANATOMY OF THE BRANCHIAL SIEVE IN 3 SYMPATRIC CYPRINID SPECIES, RELATED TO FILTER-FEEDING MECHANISMS

According to the reducible-channel model of filter-feeding (Hoogenboez em et al., '91), small food particles are retained in the channels bet ween the medial gill rakers, while the mesh size can be reduced by low ering the lateral gill rakers into these channels. This movement requi res that all lateral gill rakers have a m. abductor branchiospinalis ( MAB). MAB runs from the radii branchiales to the raker feet. It is pre sent on the lateral side of all four gill arches of the cyprinids Abra mis brama and Cyprinus carpio but only on the first arch of Blicca bjo erkna, Rutilus rutilus, Ctenopharyngodon idella, Aspius aspius, and Sc ardinius erythrophthalmus. Therefore, the latter species do not fulfil l the structural requirement for the reducible-channel model, whereas A. brama and C. carpio do. Laboratory and field data confirm that A. b rama and C. carpio can reduce their mesh size according to this model and are the better filter-feeders. The seven cyprinid species studied show the same principal microanatomy of their branchial sieve. M. abdu ctor filamenti is a sheet of muscle fibers between the lateral radii b ranchiales and the ceratobranchial bone. M. branchialis superficialis is a specialized region of the subepithelial muscle fiber network, wit h origins along both sides of the ceratobranchial bone. The lateral gi ll rakers of the first gill arch differ conspicuously from all other r akers. They are longer and flattened, and they are tilted anteriorly. They probably form a sieve across the wide slit between the first gill arch and the operculum. The most revealing anatomical feature is the presence of MABs on gill arches 1-4. It might be a suitable bio-assay for identifying the better facultative filter-feeders among cyprinids. (C) 1994 Wiley-Liss, Inc.