Food selectivity and feeding behaviour in omnivorous filter-feeding ciliates: A case study for Stylonychia

The trophic role of ciliates in microbial food webs has often been consider ed to be either bacterivore, herbivore or predatory. In general, single spe cies have been assigned to only one trophic level, although early publicati ons have already pointed to omnivorous feeding in many different groups of ciliates. However, the knowledge on the quantitative aspects of ciliate foo d preferences is scarce up to now The aim of this study was to quantify foo d selectivity with the help of different fluorescently labeled potential pr ey organisms offered to the omnivorous filter-feeding ciliate Stylonychia m ytilus as a model organism. Different fluorescently labeled bacteria (FLB) and algae (FLA), and inert fluorescent particles (FP) were offered in feedi ng experiments and evaluated with the help of epifluorescence microscopy. I n addition, different live-stained fluorescent ciliates (FLCi) were offered as living prey. A vital staining method for obtaining FLCi is described. T he results of this study showed a selective ingestion of different ciliates as well as a clear selection between different algae. Bacteria and FP were ingested, but negatively selected. There was a variability in the food pre ference regarding different-sized cells by Stylonychia mytilus indicating o ntogenetic differences in feeding habits. Young daughter cells were not abl e to ingest the largest prey items. Feeding rates of Stylonychia were highe st during the first twenty minutes of the experiments and reached a plateau after 60 min. Capture efficiency for ciliate prey was 70% at the beginning and declined to a value of about 30% after 30 min. Such behavioural patter ns may significantly overestimate feeding rates of ciliates determined by s hort-term experiments. Food concentration was shown to have strong effects on ciliate ingestion rates leading to a clearly detectable functional respo nse. Various factors influencing the feeding behaviour of filter-feeding ci liates and their significance in grazing experiments are discussed.