When a jellyfish swims, its pulsing body gets an extra push from spinning water in its wake (pictured, red).

Brad Gemmell at the Marine Biological Laboratory in Woods Hole, Massachusetts, and his colleagues studied how water flows around pulsing or paralysed moon jellyfish (Aurelia aurita). The animal's bell-shaped body produces vortex rings of water as it pulsates.

Credit: BRAD GEMMELL

As the bell flattens and expands, one of the rings rolls inside the bell and sucks in more water. That pushes the jellyfish forward without any extra force from the creature's muscles. This makes the simple predator one of the most energy-efficient swimmers on the planet, the authors say.

Proc. Natl Acad. Sci USA http://doi.org/n7k (2013)