The filtration apparatus of Cladocera: Filter mesh-sizes and their implications on food selectivity

The filtering apparatus of eleven Cladoceran species was studied. The distances between the setulae, which act as filters, were measured. Among adult individuals, they vary from 0.2 μm in Diaphanosoma brachyurum to 4.7 μm in Sida crystallina. Species can be grouped according to the mesh-sizes, as "fine mesh filter-feeders": Diaphanosoma brachyurum, Ceriodaphnia quadrangula, Chydorus sphaericus, Daphnia cucullata and Daphnia magna; "medium mesh filter-feeders": Daphnia galeata, D. hyalina. D. pulicaria, Bosmina coregoni, and "coarse mesh filter-feeders": Holopedium gibberum and Sida crystallina. In Daphnia hyalina, the distances between setulae increase from 0.3-0.4 μm in small juveniles, to 0.8-2.0 μm in adults. In Daphnia magna, the mesh-size of the filter does not increase significantly with growth. There is good evidence that the relative abundance of the filter-feeding types varies with the trophic state of the lake. In oligotrophic lakes the "coarse mesh filter-feeders" usually dominate throughout the year. The seasonal succession of zooplankton species in eutrophic lakes can be interpreted as a succession of feeding types; during winter "coarse mesh filter-feeders" dominate, while "fine mesh filter-feeders" are most abundant during summer phytoplankton blooms. Our results support the hypothesis that the species composition of filter-feeding zooplankton is strongly influenced by the amount of suspended bacteria which are available as food only for filter-feeding species with fine meshes.