Jellyfish, as their name implies, aren’t exactly the hulks of the sea. Despite their relative lack of muscles, however, they still appear to glide effortlessly through dense and churning waters. The trick? Their bodies get an extra push from pressure that builds up in the water around them as they move. High-speed video of moon jellyfish (Aurelia aurita), reported online today in the Proceedings of the National Academy of Sciences, reveals that the animals don’t just speed up when their bell-shaped bodies are contracting, as others had observed, but also for a brief time after they have relaxed. Up to 32% of the jellyfish’s movement can be attributed to this phase of motion. To figure out how this happens, the researchers measured the pressure around a jellyfish’s body at each point in a thrust. As water rushes into the bell of the jellyfish after a muscle-driven contraction, they found, a large region of positive pressure forms in a ring shape under the jellyfish bell. The resulting motion of the water is enough to give the jellyfish an extra boost of propulsion. The finding helps explain how jellyfish, some species of which are considered invasive species, have adapted to compete with other, more muscle-filled fish to take over an ecosystem and compete for resources.