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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Dabiri crunched the numbers again, incorporating bell dimensions and the force of the second vortex into his equations. His new model, published with Colin and Costello in the June 2007 Journal of Experimental Biology, suggests that broad jellies, no matter how big, should be able to generate enough force to swim, albeit via a gentle, slow paddle, not a jet. And because of the superior elasticity of a jelly's gooey cellular matrix, the critter doesn't use extra energy to generate the second vortex. It's like a spring that's been compressed and wants to recoil, says Dabiri. "The relaxation phase is essentially for free."

Dabiri is impressed by the fancy footwork of these broad jellies and by how they've managed with the hand (or tentacles) that they've been dealt.

"We think of them as blobs on the beach that don't have the capabilities of complex swimmers," Dabiri says. In fact, the signature move of the broader jellies, the jet-paddle, is sophisticated enough to inspire Dabiri to rethink the constraints faced by underwater vehicles. His graduate student Lydia Trevino is working on modifying propellers in such a way that they could generate enough force to move an otherwise cumbersome machine more efficiently in the fluid environment of the sea.

While the two swimming styles of jellyfish appear to allow for the breadth of sizes seen in jellies today, scientists such as Allen Collins of the National Oceanic and Atmospheric Administration seem more struck by the fact that Dabiri's equations predict the limits on jelly bell shapes that are manifest in nature.

"They can't seem to get beyond what is theoretically possible," says Collins, who is also curator of the Smithsonian Institution's jellyfish and glass sponge collections at the National Museum of Natural History.

IN THE SWIM Before choosing betwixt jet and paddle, jellies had to become free-floating beasts, a first for their lineage. Jellyfish belong to a larger group of animals known as Cnidarians, united by their ability to make stinging, poisonous barbs, a feat they presumably inherited from a common, ancient ancestor (knidi is Greek for "stinging nettle"). Corals and anemones are part of this group, as are critters known as sea fans and sea pens. Like jellyfish, most Cnidarians have a tubular body with a mouth on one end surrounded by tentacles. But many of these creatures are anchored to sand or rock. They can't move, by jet or by paddle.

Young jellies are also limited in terms of purposeful movement. They begin life as small larvae dispersed by currents and eventually settle on the bottom of the sea. The majority then grow into polyps, small finger- or pear-shaped lumps. Some species have polyps that can crawl around a bit, but mostly they stay put, waiting for something tasty to stumble into their tentacles. This was life in the 'burbs for Cnidarians, until the day, roughly 550 million years ago, that a polyp ancestor of today's jellies grew a little bud that broke off and got into the swim of things. Called medusans, these free jellies are the adult jellyfish that marinelife fans know and love (or fear). Almost all of today's jellies still begin as larvae, become polyps, and eventually medusans, free to roam the seas.