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Jellyfish push off a pocket of water under their bell to swim faster

Moon jellyfish create a pocket of water under their bell as they squeeze their bodies to swim

ALEXANDER SEMENOV/SCIENCE PHOTO LIBRARY

Even though they lack fins and their bodies are gelatinous, jellyfish are some of the most energy-efficient swimmers. They achieve this by making use of their bells to create a wall of water to push off so they can propel themselves more quickly.

When flying or swimming, an animal can get a boost from the ground effect, in which drag is reduced and lift increased as they approach a surface. The effect also comes into play in aeroplanes. But moon jellies (Aurelia aurita) spend most of their time in open water, so they don’t have surfaces to push against. Instead, they create high pressure pocket of water beneath their bell that gives them a boost.

Brad Gemmell at the University of South Florida, and his colleagues recorded the movements of eight moon jellyfish swimming freely in a tank. To track the eddies of water created as the jellyfish swam, they shone a laser into the tank to illuminate microscopic glass beads that moved with the flowing water.

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They found that as the jellyfish squeezed their bodies to swim, they generated two pairs of vortices. As a jellyfish extends and contracts its bell, it creates two vortices at the edge of the bell that move water away from the animal’s body. Another two vortices flowing in the opposite direction are then created underneath the bell, pushing water towards the jelly’s tentacles.

There, the water collides, creating a pocket of high pressure that makes a kind of wall of water, which serves a similar purpose to a real surface in allowing the ground effect to take place.

Previous work has shown that this “stopping vortex” beneath the jellyfish lets the animal recapture energy and move 30 per cent further than it would have done otherwise.

Gemmell thinks that this finding could be useful for creating more efficient vehicles by inspiring new designs that employ this vortex effect ‒ to allow submarines to move more quickly through open water, for example.

It wouldn’t be surprising if other animals like fish also created and used vortices for this purpose, Gemmell says, but the simplicity and transparency of jellyfish makes them ideal for studying their motions.

Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2020.2494

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