The fluid dynamics and functional diversity of the flagella of free-living flagellates.

Flagellates, unicellular organisms equipped with one or a few flagella, are phylogenetically and functionally hugely diverse. Yet, most studies have focused on a few model organisms and on the role of the flagellum in propulsion, ignoring the fundamental role of the flagellum in foraging. The number and position of flagella vary between species; the flagella may be naked or equipped with vanes or hairs; the kinematics and wave patterns vary and may be planar or three-dimensional; and the flagellum may extend from the surface of the cell or lie within a groove on the cell surface. All these features impact the fluid dynamics and functioning of the flagellum. Here we explore some of this functional diversity with a focus on the fluid dynamics of phagotrophic foraging. Finally, we identify gaps in our knowledge of flagellar functioning in this diverse and ecologically significant group of organisms.This article is part of the theme issue 'Biological fluid dynamics: emerging directions'.