Getting around the seas can be arduous. Water is more viscous than air, so underwater creatures have to overcome strong frictional resistance when swimming.
To make matters more difficult, liquid water doesn’t provide anything solid to repel against.
However, low jellyfish, which have been swimming in the world’s oceans for half a billion years, have come up with an elegant and efficient means of propulsion.
Scientists have found that at least one species of jellyfish, through its pulsating gelatinous waves, creates eddies that rotate in opposite directions. Where the currents of the two eddies meet, the collision creates a region in which the water resides – creating a wall that the jellyfish use to repel.
With a simple body structure that is conveniently transparent, jellyfish “make a really nice model for understanding how animals interact with the water around them and move very efficiently,” said Bradford J. Gemmell, Professor of Integrative Biology from the University of South Florida. “For example, vehicles can be manufactured more efficiently than humans.”
In an article published Wednesday in Proceedings of the Royal Society B, Dr. Gemmell and his colleagues made the new discovery of jellyfish movement.
“This paper documents another in a growing portfolio of approaches these animals are using to swim efficiently,” said John O. Dabiri, professor of aerospace and engineering at the California Institute of Technology. Dr. Dabiri has worked with Dr. Gemmell collaborated but was not involved in the current research.
Landlubbers like us are easy to get around because the ground below us generally does not move. “We’re pushing against it, and it’s going nowhere,” said Dr. Gemmell. “So all of this force is transferred to our legs, your foot, and then you move forward.”
Push against water and it moves out of the way. How do you get the water to stay still?
The opposing eddies of the jellyfish are a variation of the so-called ground effect. “It has long been known that there is a well-documented increase in performance you get when you swim or fly near a fixed limit,” said Dr. Gemmell.
This is because the flow of liquid near a solid surface like the ocean floor slows down and actually comes to a standstill right on its surface. So when something is floating near the bottom, the water doesn’t get out of the way as easily, and that makes it a bit easier to propel itself.
There are no walls, floors, or other surfaces in the open ocean, so jellyfish form their own walls of water.
The scientists recorded a high-speed video of eight moon jellyfish, Aurelia aurita, to study their swimming motion.
When a jellyfish completes one of its strokes and relaxes, it creates an annular ring in the shape of a donut called a stop vortex, and the animal’s dumb “bell” part captures that vortex. When the bell contracts, a second ring of liquid is created, the starting vortex, which rotates in the opposite direction. When the jellyfish rises in the water, the start and stop vortices meet and form a virtual wall that supports the drive.
This ring of immobile water is short-lived and therefore not as effective as a real wall. But it still helps the jellyfish. “The cool thing is you can do this in open water,” said Dr. Gemmell. “You don’t have to be near a solid surface to get this benefit.”
Knowing how jellyfish swim efficiently could inspire future underwater robots.
Dr. Dabiri tried for years to build, but now he’s taken a different route: embedding microelectronics in live jellyfish to command them to go where scientists want, and possibly turn them into living robots that carry sensors to measure ocean conditions could. “We stimulate your muscles to swim at a pace we set,” he said.
This research, published in Science Advances last year, is ethical, said Dr. Dabiri because jellyfish do not have pain receptors or a brain and do not show a stress response.
“We were able to show that the animals can actually swim more efficiently than in nature,” he said.
Comments are closed.