Marine microorganisms live in a hydrodynamic world at low Reynolds numbers. In most cases inertia is therefore irrelevant, and friction plays a dominant role in Zooplankton's swimming and feeding. First, some of the deeply surprising effects of hydrodynamics at low Reynolds figures are explained, and then two current studies are presented on the swimming and feed uptake of water loppers and collar flagellates, both of which are carried out in collaboration between biologists and physicists at DTU.