Mathematics of Swimming: Why do the Limbs of Krill Move like a Wave?

Antarctic krill spend the majority of their life swimming across the ocean and use about 70% of their energy expenditure on swimming. These beautiful creatures, along with other long-tailed crustaceans (such as crayfish), swim with a remarkable stroke pattern known as the metachronal propulsion. Unlike rowing, in which the crew dip their oars into the water synchronously, the limbs in crustaceans move with a 20-25% phase difference, with the more posterior limb leading the cycle. This frequency-invariant coordinated motion resembles a traveling wave. Why and how do crustaceans do this? The "why" is a fluid mechanical question, whereas the "how" is a neurophysiological one, and both questions are mathematical ones!