Jellyfish are amazing creatures, travelling in massive blooms and pulsating mesmerically to drive themselves through the water. But how does that simple motion manage to push them through the water so quickly?
While the contraction of the jellyfish's bell is pretty, the act of expelling water in such a way doesn't actually provide that much thrust. But this image, which shows the pressure generated during the power stroke of a moon jellyfish, reveals that the motion actually creates a ring-shaped vortex of spinning water -- shown in red as an area of high pressure. The pocket of positive pressure pushes the body of the jellyfish and creates extra thrust.
In fact, calculations suggest that the creation of the vortex means that the moon jellyfish swims three time more efficiently than a salmon. Not bad for something so simple. [Proceedings of the National Academy of Sciences via New Scientist]