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.
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.Read New Scientist, The Royal Society and Dr. Brad Gemmell