Researchers used an old theory on vibrating plane wings to study how fish swim so well. They were able to double the swimming efficiency of their robotic fish over a range of speeds.
Fish fins are extremely flexible yet also strong. A special segmented fin design is the key to this useful combination of properties and could inspire new morphing materials.
Even the best engineered filters get clogged eventually. Fish mouths have evolved structures that create unique fluid dynamics patterns that solve that problem.