Jelly propulsion: studies of medusan motion reveal secrets of the Earth's first muscle-powered swimmers

Science News,  Feb 23, 2008  by Rachel Ehrenberg

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From the Jetsons to James Bond, flying via jet pack has become an icon of the futuristic way to travel. But jet propulsion is actually older than the Flintstones. It's a standard means of locomotion for jellyfish, the earliest animals to swim the seas using muscles. Jellies have been jet-propelling for at least 550 million years, yet only recently have scientists begun to understand how the challenges of moving in fluid have shaped jellyfish evolution.

Jellyfish invented muscle-powered movement, a feat that allowed them to diversify into a number of ecological nooks and crannies. But jelly muscles are relatively meager and the jet-pack method of motion requires serious strength. That has presented a mystery about how some species of jellyfish can get so big. New studies have begun to explain how enormous gelatinous creatures muster the strength to swim. The answers may lead to novel designs for underwater vehicles and are prompting scientists to rethink how to harness energy from wind currents.

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If you've seen a jellyfish washed up on the beach, its brawn probably wasn't the first thing that struck you. Their bell-shaped bodies are mostly gelatinous goo, surrounded by a network of nerves and a paper-thin layer of tissue. But on the interior wall of the bell is a layer of muscle. Contracting this muscle ejects water from the opening at the base of the bell, propelling the animal on its path.

"There's probably no source of locomotion that's easier to evolve--it's a pipe with a muscle around it" says biomechanics expert Steven Vogel of Duke University in Durham, N.C.

THE JET SET In fact, jet propulsion appears again and again in animal evolution, Vogel says. Dragonfly larvae make use of an anal jet, and some squid can blast themselves to speeds of 25 miles an hour. But while the jet pack allows for a speedy escape, it is inefficient energetically, releasing a lot of kinetic energy into the water that can't be recovered, says John Dabiri, an expert in fluid dynamics at the California Institute of Technology in Pasadena. He points to more efficient swimmers such as dolphins or tuna, which glide through the water without a lot of disturbance.

And jet propulsion is not the best strategy for bigger beasts. A large jellyfish must expel a large volume of water behind it to move forward. Such an expulsion requires brute strength.

Jellyfish don't have those muscular capabilities. The muscle that lines their interiors is a mere one cell-layer thick. Making it bigger would take more than calisthenics--it would take a circulatory system that could supply those muscles with oxygen and nutrients.

"As you get bigger, you have less and less wiggle room evolutionarily," says Vogel. "Jet propulsion is fabulous when you are a micron in size and fabulously bad when you are big."

Yet jellyfish do get big--some, such as the well-named giant jellyfish (Nemopilema nomurai), can grow to almost 8 feet across and weigh in at 400 pounds. But when Dabiri modeled the forces required for jet propulsion and did the math, the numbers said that jellyfish much bigger than a softball shouldn't even exist.

Then Dabiri took closer notice of a relationship between the size of a jellyfish and the shape of its bell. The smaller jellyfish tend to look like thimbles or little rockets, their bells always taller than wide. The larger jellies had bells shaped more like UFOs--wider than they were tall. To investigate, he ordered some crystal jellies, Aequorea victoria, little thimble-shaped creatures small enough to swim comfortably in a petri dish. As a jellyfish explored its surroundings, Dabiri's colleagues Sean Colin and John Costello squirted a bit of harmless fluorescent dye behind the animal, to better see the water's motion. The small, thimble-shaped jelly zipped around jet-pack style, and the dye revealed the lost kinetic energy swirling in its wake.

IN THE SLOW LANE Then the research team filmed some broad, UFO-shaped jellies known as moon jellyfish, or Aurelia aurita, in shallow waters of the Adriatic Sea and in a saltwater lake on the Adriatic island of Mljet. Again, the scientists used dye to visualize the animals' wakes. The researchers immediately noticed that these jellies didn't zip to and fro, but meandered, using a leisurely half-jet, half-paddle approach. Like their rocket-shaped relatives, these broader, flatter jellies moved by contracting their meager muscles, squeezing water from their bells into a swirling vortex behind them. But when a moon jellyfish relaxed, postsqueeze, and water rushed in to refill its bell, the dye revealed a second vortex forming at the bell's edge. Dabiri realized that this second vortex was swirling in the opposite direction of that of the first, like water swirling inward at the edge of a bowl pushed down into a basin of water. The collision of these opposing, swirling masses of water was providing enough thrust to propel the moon jellyfish forward.