ADAPTATION CHRONICLES: Jellyfish build water-walls to swim around the ocean

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. 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.”

In a paper published in Proceedings of the Royal Society B, Dr. 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 O. Dabiri, a professor of aeronautics and mechanical engineering at the California Institute of Technology. Dr. Dabiri has collaborated with Dr. 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,” Dr. 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 counter-spinning vortices employed by the jellyfish are a variation of something known as the 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,” Dr. Gemmell said.

That is because flow of a liquid slows down near a solid surface like the sea-floor, and is indeed at a stop right at its surface. So when something is swimming 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. The scientists captured high-speed video of eight moon jellyfish, Aurelia aurita, to investigate their swimming motion. As a jellyfish completes one of its strokes and relaxes, it generates a doughnut-shape ring of rotating liquid called the stopping vortex, and the blobby “bell” portion of the animal traps this vortex. As the bell contracts, it creates a second ring of liquid, the starting vortex, rotating in the opposite direction. As the jellyfish rises in the water, the starting and stopping vortices meet, producing a virtual wall that aids propulsion.

This ring of non-moving water exists only briefly so it is not as effective as a real wall. But it still helps the jellyfish. “The cool thing is, they’re able to do this in open water,” Dr. Gemmell said. “They don’t need to be anywhere near solid surface to get this benefit.” The knowledge of how jellyfish swim efficiently could inspire future underwater robots.