Jellyfish commute around oceans in cocoons of water
Locomotion through the seas can be arduous. Water is more viscous than air, 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 a half-billion years, have come up with an elegant, efficient means of propulsion.
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 Gemmell, at the University of South Florida. “More efficiently than humans can create vehicles, for example.”
In a paper published Wednesday in Proceedings of the Royal Society B, Gemmell and his colleagues described the new discovery
about jellyfish motion.
“This paper documents another in what’s a growing portfolio of approaches that these animals use to swim efficiently,” said John Dabiri, a professor of aeronautics and mechanical engineering at the Caltech.
Dabiri has collaborated with Gemmell in the past but was not involved with the current research.
Locomotion by landlubber animals like us is easy because the ground beneath us generally does not move.
“We push against that, and it doesn’t go anywhere,” Gemmell said. “So all that force gets transferred to our legs, your foot, and then you move forward.”
Push against water and it moves out of the way. How to get the water to stay still?
The counterspinning vortices employed by the jellyfish are a variation of ground effect.
“It’s been known for a very long time that there’s a well- documented boost in performance that you get when you swim or fly near a solid boundary,” he said.
That is because f low of a liquid slows down near a solid surface like the seafloor and is indeed at a stop right at its surface. So when something is sw imming near the bottom, the water cannot move out of the way as easily and that makes it a bit easier to propel oneself.
There are no walls or ground or other surfaces in the open ocean, so jellyfish create their own walls of water.