Filter-feeding in common bream (Abramis brama), white bream (Blicca bjoerkna) and roach (Rutilus rutilus) : structures, functions and ecological significance

In this thesis the retention mechanism of the branchial sieve of three sympatric cyprinid fish species, the common bream (Abramis brama), the white bream (Blicca bjoerkna) and the roach (Rutilus rutilus) , is studied. In eutrophic lakes zooplankton is an important food resource and common bream is dominant. Previous research indicated that common bream retains zooplankters in the medial channels on its gill arches. The mesh size of its branchial sieve can be reduced by rotating the lateral rakers into these channels. In this thesis it is shown that of the three species under study the channels of common bream are comparatively the widest. Because of its extra long and pointed lateral gill rakers common bream is most suited to reduce its channels, in the way described above. The retention ability of the fishes was calculated from the decline in density of zooplankton as a function of its size in experimental tanks with filter-feeding fish. These data were compared with the predictions from three retention models. In these predictions the influence of the shape of the zooplankton on retention was taken into account. This aspect was studied in a separate experiment with industrial sieves (with square meshes). The ratio of body width and depth of the zooplankton, proved to be a crucial size parameter for retention. It was concluded from the filter-feeding experiments that the channel model with adjustable mesh size can be applied to common bream. The same model can be applied to white bream, but without the possibility to adjust the mesh size. Roach probably retains zooplankton on its gill slits, according to the saw-tooth model. The presence of abductor muscles for the lateral rakers, which allows them to rotate, is a prerequisite for the application of the reducible-channel model. A detailed micro anatomical study showed that all lateral gill rakers of each gill arch of common bream have such muscles. In white bream and roach, however, these muscles are only present on the lateral gill rakers of the first gill arch. Therefore, the reducible- channel model cannot be applied to these species. During the uptake of zooplankton by suction feeding, the gill arches move along with the expanding head. These gill arch movements affect the relative position of the gill rakers on either side of each gill slit. The gill arch movements were studied in X-ray films of white bream and common bream. A novel, 3D method of analysis was developed to analyze these films quantitatively. The lateral gill rakers of both species proved to be long enough to reach across the gill slit (i.e. into the medial channels), even when the width of the gill slits was maximal. The sideward movement of the lateral gill rakers out of the centre of the medial channels was considerable. The adjustable branchial sieve is not rigid. Due to the mucus encapsulement of trapped particles, the conical shape of the lateral gill rakers and the possibility to rotate them sidewards, the reducible-channel mechanism still functions well. The filter-feeding effectiveness of the three species was quantified in terms of energy. Common bream proved to have a higher energy gain from filter-feeding than its relatives white bream and roach. This difference is probably a crucial factor explaining the dominance of common bream in eutrophic lakes. The present research corroborates the fundamental idea that there is a strong relation between the functional morphology and the ecological niche of a species. Furthermore, small morphological differences between related species (the presence of abductor muscles of the lateral gill rakers) can be used to explain and predict interspecific differences in the exploitation of food sources and the performance in the ecosystem.