Jellyfish are the ocean's most efficient swimmers - here's how they do it

Originally published on January 9, 2021.

Jellyfish may not be the fastest swimmers in the ocean, but they are remarkably efficient, using less energy for locomotion than any other sea creature.

Now a Canadian researcher and his team have learned how they manage to be so efficient. They exploit a phenomenon better known from aerodynamic studies of aircraft called "ground effect." but they do it without the benefit of ground.

Brad Gemmell, an  associate professor in the Department of Integrative Biology at the University of South Florida in Tampa, discovered they make their own hydrodynamic "ground effect" by generating opposing vortices of water below them. 

The swim cycle of the moon jellyfish captured with high speed cameras (Brad Gemmell)

Ground effect in the open water

Jellyfish propel themselves through the water by contracting their bell-shaped body, then propelling water away from the direction of travel at the end of each contraction, or swim cycle.

With each swim cycle, the jellyfish creates a doughnut shaped vortex in the water, a bit like a smoke ring. The jellyfish resets, contracts, and repeats the swim cycle, creating a second vortex that rotates in the opposite direction. This creates a virtual wall between the two opposite sign vortex rings, increasing the pressure in the water, and propelling the jellyfish as if it were pushing against solid ground.

Moon jellyfish making vortex rings to achieve ground effect (Brad Gemmell)

Swimming boost

Gemmell used high speed cameras in his study, to compare jellyfish performance with and without vortex rings. Jellyfish got a fifty per cent boost from the two vortex rings, compared to when they swim without making only a single vortex ring as they do when they take off from a standing start.