Moving through the sea can be difficult. Water is more viscous than air, so underwater creatures must overcome strong frictional resistance while swimming.
To make things more difficult, liquid water offers no solids to press against.
But low-lying jellyfish, which have been swimming in the world’s oceans for half a billion years, have devised an elegant, efficient propellant.
Scientists have found that at least one species of tortoise by their pulsating gelatinous undulations creates vortices that rotate in opposite directions. Where flows of the two vortices meet, the collision creates an area when the water is stagnant. This creates a wall that the jellyfish use to push down.
Bradford J. Gemmell, a professor of integrative biology at the University of South Africa, has a simple body structure that is comfortably transparent, a very good model for understanding how animals deal with the water around them, to many to move efficiently. Florida. “For example, more efficient than humans can create vehicles.”
In a paper published Wednesday in Proceedings of the Royal Society B, dr. Gemmell and his colleagues describe the new discovery about tormentor movement.
“This article documents another in a growing portfolio of approaches that these animals use to swim efficiently,” said John O. Dabiri, a professor of aerospace and mechanical engineering at the California Institute of Technology. Dr. Dabiri has met with Dr. Gemmell collaborated but was not involved in the current research.
Movement by animals that are like land animals like us is easy because the ground beneath us usually does not move. “We are pushing against it, and it is not going anywhere,” said Dr. Gemmell said. “So all this power is transferred to our legs, your foot and then you move forward.”
Press against water and move out of the way. How do you get the water quiet?
The vortices used by the jellyfish are a variation of something known as the soil effect. “It has long been known that there is a well-documented performance improvement you get when you swim or fly near a fixed limit,” said Dr. Gemmell said.
This is because liquid flow along the solid bottom slows down like the seabed, and indeed stops on its surface. So if something swims near the bottom, the water can not move out of the way so easily, and this makes it a little easier to propel yourself.
There are no walls or soil or other surfaces in the open ocean, so jellyfish create their own walls of water.
The scientists took a quick video of eight-month-old jellyfish, Aurelia aurita, to investigate their swimming motion.
While a jellyfish completes and relaxes one of its strokes, it generates a ring of the rotating fluid with a donut called the stop vertebra, and the blobby ‘bell’ portion of the animal traps this vertebra. As the clock contracts, it creates a second ring of fluid, the starting vortex, which rotates in the opposite direction. As the jellyfish rises into the water, the starting and stopping vertebrae meet, creating a virtual wall that aids propulsion.
This ring of non-moving water only exists briefly and is therefore not as effective as a real wall. But it still helps the jellyfish. “The coolest thing is that they can do it in open water,” said dr. Gemmell said. “They do not have to be close to the solid surface to achieve this advantage.”
The knowledge of how jellyfish swim efficiently can inspire future underwater robots.
This is what dr. Dabiri has been trying to build for years, but now he has taken a different approach: he has embedded microelectronics in live jellyfish to command them to go where scientists want, which could potentially turn them into live robots that can carry sensors to measure the conditions of the ocean. “We stimulate their muscles to swim at a pace we decide,” he said.
The research, which was reported last year in the journal Science Advances, is ethical, said dr. Dabiri said, because jellyfish do not possess pain receptors or brains and do not have a stress response.
“We were able to show that the animals can make them swim more efficiently than they do in the wild,” he said.