Locomotion through the seas can be arduous. Water is more viscous than air, and so underwater creatures must overcome strong frictional resistance as they swim.
To make things more difficult, liquid water provides nothing solid to push off against.
But lowly jellyfish, which have swum in the world’s oceans for half a billion years, have come up with an elegant, efficient means of propulsion.
A jellyfish swimming through laser sheet with tracer particles. Gemmell et al., 2020
Scientists have found that through their pulsing gelatinous undulations, at least one species of jellyfish creates vortices that rotate in opposite directions. Where flows of the two vortices meet, the collision creates a region when the water is stationary — in effect, creating a wall that the jellyfish use to push off.
With a simple body structure that is conveniently transparent, jellyfish “represent a really nice model to understand how animals interact with the water around them, to move very efficiently,” said Bradford J. Gemmell, a professor of integrative biology at the University of South Florida. “More efficiently than humans can create vehicles, for example.”