FILTER MESH SIZE OF CLADOCERANS PREDICTS RETENTION EFFICIENCY FOR BACTERIA

This study tested the hypothesis that filter mesh size can predict retention efficiency of very small particles by cladocerans. Morphology of the filtering apparatus and size distribution of the food particles were measured by scanning electron microscopy- From these morphological data the retention of food particles could be predicted for individual animals. Short-term, dual-label experiments were used to measure the uptake of small food particles by four species of Cladocera (Daphnia magna, Daphnia pulex, Simocephalus vetulus, and Diaphanosoma brachyurum). The green alp Scenedesmus acutus was paired with each of six classes of small particles; three size fractions of free-living planktonic bacteria, two cyanobacterial species of different sizes, and the green alp Stichococcus minutissima were fed. The clearance rates for these very small particles were divided by the clearance rate for Scenedesmus to calculate retention efficiencies. Retention efficiencies were between 33 and 154%. Cladoceran species with coarser filtering meshes showed lower retention efficiencies for bacteria and cyanobacteria. Within species, both predicted and observed retention efficiencies were greater for juveniles than for adults. Although collection mechanisms other than sieving probably operate, the size distribution of particles collected by cladocerans is closely predicted by the size-frequency distribution of meshes of the thoracic feeding limbs.