The lion’s mane jellyfish is the world’s largest jellyfish and, as John Dabiri from Stanford University can attest, its tentacles pack a very painful sting. During a photoshoot for Popular Science, a photographer convinced Dabiri to hold a small individual in his gloved hands, while sitting in shallow water and clad only in swimming trunks. “The tentacles were dripping on my legs and thighs and I got dozens of stings, mostly on my crotch,” he says. “After that, I’ve learned to say no to photographers.”

Both the setting and the stings were unusual for Dabiri. He’s not a marine biologist but a mechanical engineer, and he studies jellyfish while they’re in aquarium tanks and he’s on dry land. His discoveries have cast these gelatinous animals as the most efficient swimmers in the sea, as stealthy predators whose command of water currents can disrupt entire ecosystems, and as sources of inspiration for medical devices. Just this year, he made a truly surprising discovery that upturns our understanding of how animals swim, with important implications for the design of underwater vehicles.

Inheriting a curiosity for all things mechanical from his engineer dad, Dabiri was that kid—the one who would dismantle his Christmas toys to see how they worked. After graduating from Princeton, he fully expected to return to his hometown of Toledo, Ohio to be an auto mechanic, until one of his professors convinced him to try a life of research. “I just didn’t realize I could do work like this and have someone pay you for it,” he says.

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.

Meanwhile, he has been developing better ways of studying the forces produced by swimming animals. Now, instead of colored dyes, he seeds aquarium tanks with tiny glass beads, which he illuminates with green lasers. The lasers track the movements of the beads as they flow around a swimming animal, and an algorithm uses those patterns to deduce how the creature changes the water pressure around it. When Dabiri first used these methods on jellyfish in 2012, he noticed something very strange.

You’d expect jellyfish to create areas of high pressure behind them, as they propel themselves along. They certainly do, but Dabiri saw that their undulating bells also create huge areas of low pressure in front of them. And these low-pressure areas accounted for a greater proportion of their forward thrust. The jellyfish weren’t primarily pushing themselves through the water, but pulling themselves forwards. They swam by sucking.

“I spent months trying to fix the code and the new methods; it’s fun to come up with the contrarian unexpected result, but you don't want to be wrong,” says Dabiri. “And after a while, I became confident that the code was correct and started thinking that maybe our basic understanding of how these animals move wasn't.”

Dabiri’s postdoc Brad Gemmell found the blood-sucking lamprey moves in the same way. Unlike the moon jellyfish, it swims by undulating its sinuous, eel-like body. As it does so, it causes the water inside each bend to swirl, creating pockets of low pressure much like the eye of a hurricane or tornado. And these pockets suck the lamprey forward.

Gemmell checked this by surgically altering lampreys so they couldn’t produce their normal undulating wriggles. Instead they swam by flicking the tips of their tails like the kicking motions of a human swimmer. They couldn’t produce the low-pressure whirlpools that suck them forward, so they swam more slowly and less efficiently, even though they were still pushing back against the water behind them.

Lampreys and jellyfish are very different animals, separated by almost a billion years of evolution. If they’re swimming in the same way, chances are other creatures are too—at least those that prize efficiency over speed.  To work out how prevalent swimming-by-suction is, Dabiri’s team have created a portable version of their laser equipment, which divers can use to study wild fish in their natural environments. They call it SCUVA, substituting the B for “breathing” with a V for “velocimetry.” “We’re using it to understand human swimming and dolphin swimming,” he says. “Others are using it to understand flow in corals or kelp forests.”

Engineers should take note, too. Most of our underwater vehicles work by using propellers to generate high pressure—the intuitive way. “Maybe that approach is backwards,” says Dabiri. “These animals say that there’s a completely different mechanism that we might try to emulate. It’s like if we found out tomorrow that the Golden Gate Bridge was supported from the bottom rather than held up from the top. We’d be surprised, and it would change how we designed our bridges.”

How might we design vehicles to suck rather than push? “It’s hard to imagine that a  small underwater vehicle would undulate like a lamprey, but what if I put ports along the vehicle’s side that could suck in fluids? Or vacuum pumps to lower the pressure locally? We've only had a short time to consider the options, so I don't have a lot of answers.”

And while he’s developing ways of building better swimming robots, he has a more immediate goal: Learn to swim himself. During fieldwork trips, while his colleagues are collecting animals and deploying SCUVA in the water, Dabiri plays the deckhand. When he won a MacArthur Fellowship five years ago, he swore he’d use some of the money to take lessons, but none of them stuck. “Despite making a lot of progress in understanding how animals swim, to this day, in 6 feet and 3 inches of water, I’d drown,” he says.