Most aquatic creatures achieve motility through the dynamic interaction of their flexible body with the surrounding medium. This flexibility is used to provide a spectrum of active and passive control, allowing the creature to sometimes prescribe its shape changes and at other times extract energy from the fluid. This mix is particularly important in the moderate Reynolds number regime, in which wake vortices play an important energetic role. A well-devised control strategy for a bio-inspired vehicle should – perhaps must – exploit such flexion and energy exchange; as yet, we lack sufficient understanding to develop such a strategy. In this work, we present two canonical problems that distill fundamental modes of fluid/flexible body mechanics in biological systems, which are analyzed using high-fidelity numerical simulation. The first system consists of an articulated three-link swimmer considered in free-swimming. The second system involves an articulated jellyfish, in which the active/passive flexibility mix is explored by designation of the individual hinges.