Ciliates are ubiquitous in the marine environment and important consumers of phytoplankton and flagellates. The feeding on suspended food particles in ciliates is complex and relies typically on coordinated motion in bands of transversal rows of cilia known as membranelles. Many ciliates are upstream collectors that use a single membranelle band to generate feeding flow, retain food particles, and transport them to the mouth region. To explore a representative upstream collector, we consider the ciliate Euplotes vannus. We determine the clearance rate using particle tracking and estimate the flow-generating force by fitting a point force model to the observed flow. To obtain a mechanistic understanding of feeding flow and particle retention, we use high-speed video microscopy. The cilia move parallel to the membranelle band towards the mouth region in the power strokes, and a metachronal wave propagates away from the mouth region parallel to the band and outwards along the membranelles. A gap, therefore, opens between neighboring membranelles from the inner side of the band where the mouth region is located, and while food particles are retained, water is drawn in and pushed outwards across the membranelle band as the gap closes from the inner side. We suggest a model that rationalizes our observations and quantifies the mechanism by which the membranelle band can both generate feeding flow and retain food particles. The model compares favorably with our observations and suggests a trade-off between the clearance rate and the membranelle gap that determines the lower cutoff in the prey size spectrum. We quantify the trade-off and discuss the ecological significance of our findings.