In the popular imagination, jellyfish are just blobs—listless drifting things, without eyes, ears, or even a brain for figuring out how to get from one place to another. Scientists have long argued against this misguided notion. They say the familiar medusa-style (or bell-shaped) jellyfish are highly effective at getting where they need to go, employing both jet propulsion and a rowing motion. They travel efficiently enough, in fact, that jellyfish often outcompete the fish that appear to be their bigger, faster, smarter rivals.
So how do they do it? The secret to jellyfish locomotion, according to a new study in the Proceedings of the National Academy of Sciences, isn’t about how hard the jellyfish works. It’s about how it relaxes.
Until now, scientists understood jellyfish movement this way: When a jellyfish contracts, it shoots out water from within the bell. At the same time, the outer edges of the jelly flap and push water away, much as each oar on a boat spins off a vortex in its wake. The motion of contracting also causes a rubbery disk called the mesoglea in the middle of the jellyfish to bend down at its outer edges. Then, when the jellyfish relaxes, the mesoglea springs back out again, filling the bell with water for the next burst of speed.
Even to scientists, though, this explanation of how a jellyfish gets around has never been entirely satisfying. Not to put too fine a point on it, but a jellyfish is mostly gelatinous goo. Sea snot, even. Only about one percent of its mass is muscle, compared to more than 50 percent in the average fish. Moreover, jellyfish muscle is only one cell layer thick.
“It’s always been sort of counterintuitive,” says Brad Gemmell of the Marine Biological Laboratory at Woods Hole, Massachusetts. “Fish are highly advanced predators with great visual and chemosensory abilities.” They can spot an energy-rich food source at a distance and chase it down. A jellyfish, meanwhile, can eat only what it happens to bump into.
And yet a vast bloom of jellyfish can suddenly appear in a habitat and gobble up all the available food, including fish eggs and the fish themselves. In one particularly ghastly case off the coast of Ireland, a jellyfish flotilla 10 square miles in area swarmed over an organic fish farm, killing 100,000 salmon worth more than $2 million.
And jellyfish blooms have become far more common in recent years, probably because of increasing ocean acidification. They've clogged intake lines and shut down a nuclear power plant in Sweden last week, and they’ve driven swimmers out of the water from Florida to Italy. They’ve also slowed the recovery of commercial fisheries by outcompeting cod and other fish for prey.
Biologists had customarily focused on the way the jellyfish contracts its muscles to get around. There didn't seem to be much happening during the relaxation part of the cycle. But when Gemmell and his co-authors took a closer look at two common species, including moon jellies, they discovered that jellyfish are taking advantage of a hidden form of locomotion: Because jellyfish have that familiar radial shape, each contraction creates a donut-shaped vortex inside the bell. That vortex draws in water and pushes the jellyfish forward as it is basically coasting, with no muscle movement whatsoever.
This energy-efficient power source accounts for about 30 percent of forward motion, according to the new study. Combine it with the passive energy storage and recovery that comes from the springiness of the mesoglea, and muscle movement occurs just 20 percent of the time during the jellyfish swimming cycle.
That’s far more efficient than any fish. The only limit has to do with size: Because jellyfish muscles are only one cell layer thick, they become less effective as a jellyfish gets bigger. Even so, it takes a fish weighing more than 220 pounds to begin to match the energy efficiency of a jellyfish.
Gemmell says the new study, part of a larger U.S. Navy project on nontraditional forms of locomotion, could ultimately lead to high-efficiency, low speed vehicles—for instance, oceanic measuring devices meant to maintain their position in the water column unattended for months or years at a time.
For now, though, it’s enough to understand how one important animal group succeeds with almost no effort—and to know that strong and sophisticated do not always triumph over small, simple, and slow.