Dabiri loved rockets and aircraft, but during his Ph.D. at Caltech, his advisor pointed out that animals use jet propulsion too. Perhaps he might like to study some? Dabiri was dubious: As a kid, he went to a small Baptist school that disputed evolution, and he thought of biology as a turgid exercise in memorizing species names. Still, he humored his advisor and paid a visit to the Aquarium of the Pacific in Long Beach. There, he became enchanted by jellyfish.

“With an engineer's hubris, I thought they’d be simple,” he recalls. They jet forwards by contracting their umbrella-shaped bells and pushing water towards their tentacles. They were, quite literally, not rocket science. He soon realized that he was wrong.

Teaming up with Sean Colin and Jack Costello, Dabiri would inject dye into the water around a swimming moon jellyfish, and watch how the curlicues of color were shaped by the animal’s pulsating bell. They soon realised that the creature was creating rings of water behind it. These “vortex rings” are like donuts that continuously roll into themselves, not unlike the smoke rings that some people can blow from their mouths. They travel down towards the tentacles, adding to the forces produced by the jellyfish’s pulsating bell, and providing more thrust for no extra energy.

The team later showed that the moon jellyfish actually produces two vortex rings for every beat of its bell. While the first one travels backwards, a second one rolls back into the bell itself, speeding up as it goes, and sucking water into the center of the jellyfish. This allows the animal to recapture some of the energy it spends on each swimming “stroke,” and pick up speed even when it’s making no effort. For that reason, the moon jellyfish is the most efficient swimmer in the ocean.

Vortex rings, according to Dabiri’s team, are a “unifying principle in biological propulsion,” produced by many species of jellyfish, and jetting squid. “Whenever the water’s at rest and is suddenly accelerated, you get these vortex ring structures,” says Dabiri. “The jellyfish squeezes its body and propels a jet of water. The smoker exhales suddenly with particular shapes of their lips or tongue. It all boils down to the fluid dynamics. The same equations govern how a fluid responds to forces, whether you’re talking about air, water, or blood.”

Yes, blood. When the heart pumps blood between its two left chambers, the liquid jets through a small hole and forms vortex rings. In a jellyfish, the shape, size, and speed of these rings tells you about the species that produce them and how efficiently they travel. In the heart, those same traits can reveal how efficiently the chambers are pumping, and the health of the heart. With his mentor Morteza Gharib, Dabiri showed that weaker vortex rings, detectable through non-invasive scans, could be used to detect heart problems. He now wants to develop a small, simple way of detecting these rings without a bulky medical scanner.