Jellyfish have been proven to be the most energy efficient swimmers of all animals. They move through the water by radially expanding and contracting their bell-shaped bodies to push water behind them. They pause between the contraction and expansion to create two vortex rings.
Muscles are used for the contraction of the body, which sheds the first vortex and pushes the animal forward, but the mesoglea is so elastic that the expansion is powered exclusively by relaxing the bell, which releases the energy stored from the contraction. By doing so, the second vortex ring rolls under it and begins to spin faster. This sucks in water which refills the bell and is pushed up against the centre of the body, giving it a secondary and "free" boost forward.
The mechanism, called passive energy recapture, only works at low speeds and relatively small body sizes, allowing the animal to travel 30 percent farther on each swimming cycle. Jellyfish achieved a 48 percent lower cost of transport (the amount of food and oxygen consumed, versus energy spent in movement) than other animals in similar studies.