This work explores the utility of the finite-time Lyapunov exponent (FTLE) field forrevealing flow structures in low Reynolds number biological locomotion. Previous studies ofhigh Reynolds number unsteady flows have demonstrated that ridges of the FTLEfield coincide with transport barriers within the flow, which are not shown by amore classical quantity such as vorticity. In low Reynolds number locomotion (O(1)–O(100)), in which viscous diffusion rapidly smears the vorticity in the wake, the FTLEfield has the potential to add new insight to locomotion mechanics. The targetof study is an articulated two-dimensional model for jellyfish-like locomotion,with swimming Reynolds number of order 1. The self-propulsion of the model isnumerically simulated with a viscous vortex particle method, using kinematicsadapted from previous experimental measurements on a live medusan swimmer. Theroles of the ridges of the computed forward- and backward-time FTLE fields areclarified by tracking clusters of particles both backward and forward in time. It isshown that a series of ridges in front of the jellyfish in the forward-time FTLEfield transport slender fingers of fluid toward the lip of the bell orifice, which arepulled once per contraction cycle into the wake of the jellyfish, where the fluidremains partitioned. A strong ridge in the backward-time FTLE field reveals apersistent barrier between fluid inside and outside the subumbrellar cavity. Thesystem is also analyzed in a body-fixed frame subject to a steady free stream,and the FTLE field is used to highlight differences in these frames of reference.