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Locomotion across the seas can be difficult. Water is more viscous than air, so underwater creatures have to overcome a strong resistance to friction when swimming.
To make matters more difficult, liquid water does not provide anything solid to push against.
But the humble jellyfish, which have been swimming in the world’s oceans for half a billion years, have developed an elegant and efficient means of propulsion.
Scientists have discovered that through their pulsating gelatinous ripples, at least one species of jellyfish creates vortices that spin in opposite directions. Where the flows of the two eddies meet, the collision creates a region when the water is stationary – in effect, creating a wall that the jellyfish use to push.
With a simple and transparent body structure, jellyfish “represent a very good model for understanding how animals interact with the water around them, to move very efficiently,” said Bradford J. Gemmell, professor of integrative biology at the ‘University of the South. Florida. “More efficiently than humans can create vehicles, for example.”
In an article published Wednesday in Proceedings of the Royal Society B, Dr Gemmell and colleagues described the new discovery on jellyfish movement.
“This article documents another in what is a growing portfolio of approaches these animals use to swim effectively,” said John O. Dabiri, 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 in current research.
Locomotion by land animals like us is easy, as the ground beneath us usually does not move. “We’re pushing against this, and it’s not going anywhere,” Dr Gemmell said. “So all that force is transferred to our legs, to your foot, and then you move forward.
Push against the water and it gets out of the way. How do you keep the water still?
The counter-rotating vortices employed by jellyfish are a variation of what is known as the ground effect. “It has been known for a very long time that there is a well-documented improvement in the performance you get when swimming or flying near a solid border,” said Dr Gemmell.
This is because the flow of a liquid slows down near a solid surface like the seabed, and is in effect stopping just at its surface. So when something is swimming close to the bottom, the water can’t move away that easily, which makes propelling a bit easier.
There are no walls, floors or other surfaces in the open sea, so jellyfish create their own water walls.
Scientists captured high-speed video of eight lunar jellyfish, Aurelia aurita, to study their swimming motion.
When a jellyfish completes one of its moves and relaxes, it spawns a donut-shaped ring of rotating liquid called the stop vortex, and the animal’s “bell” blobby part traps that 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 still ring of water only exists briefly, so it’s not as effective as a real wall. But it still helps the jellyfish. “The cool thing is that they are able to do it in open water,” said Dr Gemmell. “They don’t need to be near a solid surface to gain this advantage.”
Knowledge of efficient jellyfish swimming could inspire future underwater robots.
This is what Dr Dabiri has tried to build for years, but now he has taken a different approach: embedding microelectronics into living jellyfish to order them to go where scientists want them to go, potentially turning them into living robots. that could carry sensors to measure ocean conditions. “We stimulate their muscles to swim at a pace that we choose,” he says.
This research, reported last year in the journal Science Advances, is ethical, Dr. Dabiri said, because jellyfish do not have pain receptors or brains and do not exhibit a stress response.
“We were able to show that animals, in fact, we can make them swim more efficiently than they do in the wild,” he said.
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