Studies of medusan motion reveal secrets of the Earth's first muscle-powered swimmers
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.
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.
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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.
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 (knidï 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.
It’s likely that the first free-floating jellies were the only swimmers in the ancient seas, says Collins. There would have been algae and coral larvae and such floating around, and eventually ancient versions of lobsters and other marine arthropods. But the highways were basically clear. No sharks. No fish. Certainly no people. The jellies had the pool to themselves.
Jet vs. Paddle
But what stroke the earliest jellyfish used isn’t as clear. When Dabiri and his colleagues realized that the same swimming styles cropped up in distinct groups of jellies, the researchers wondered whether the first ancient swimming jelly blasted from place to place via jet pack or gently paddled around. So the researchers looked up the most recent version of the jellyfish family tree. (The tree was generated using molecular data by Collins and colleagues published in Systematic Biology in 2006.)
When Dabiri’s team plotted swimming strategies onto the tree, it appeared that both swimming styles have been invented again and again in jellyfish evolution. But Collins cautions that jellyfish are understudied beasts. Without surveying all of the species in every group it is difficult to say if jets or paddles emerged first. Scientists often look to the fossil record for answers to what-came-first kinds of questions. And while some fossilized jellies have been found, the record remains murky.
It is clear that some groups tend to favor one mode of motion. Among the box jellies (Cubozoans), which are known for their fierce venom and distinct cube shape, bell size has been restricted and many of these jellies are small, jet-propelled species. The hydrozoans, a sister group of the box jellies, show more variation. Hydrozoans called Trachymedusae have diminutive bells and belong to the jet set. Other hydrozoans called siphonophores include species like the Portuguese man-o-war that may grow up to several feet long, but are actually colonies made up of many smaller bells chained together. While technically too large to jet, siphonophores pull off jet propulsion through the coordinated thrusts of the individual bells.
The leisurely paddle propulsion also appears more than once in the greater jellyfish family tree, and different groups have made use of various body parts to enhance the paddlelike edges of their bells. Thimble-shaped hydrozoans have a velum, a sort of muscular shelf at the inner edge of the bell, that boosts propulsive power by providing a stiff collar through which to blast the water. The larger, flatter paddling hydrozoans known as Narcomedusans sport a tweaked velum—a flapping paddlelike appendage—that helps generate the second vortex.
Some of the wispy creatures’ body plans fall between the extremes, or switch as teens, going from UFO-shaped juveniles to rocket-shaped adults. But it appears that it isn’t advantageous to take the middle road. Examining dining preferences hints at why, say Dabiri and his colleagues in an upcoming issue of Invertebrate Biology.
Hunters and gatherers
Jet-propellers tend to be what ecologists call ambush predators—they lie in wait for a small creature to swim by, then ensnare it in a stinging mass of tentacles. Like Agent 007, most of these jellies appear to employ the jet pack to escape from an enemy rather than to attack. On the other hand, what’s known of the paddling jellyfish suggests that they are largely cruising foragers—they amble along, capturing soft-bodied, slow-moving prey such as drifting eggs or tadpole-like creatures.
Of course, jellies may have done it first, but most animals have since figured out how to generate force by contracting muscles, points out Edwin DeMont of St. Francis Xavier University in Antigonish, Nova Scotia. But many creatures use two muscles where jellies use one. Human biceps and triceps, for example, pair up so that when one contracts, the other pulls back to rest. The equivalent in jellies is the springy, postsqueeze expansion of their goo.
“They can’t increase that rate—it is passive,” says DeMont. “They’ve had to capture the fluid processes in the environment.”
From Dabiri’s perspective, the ability to harness these fluid processes is one of the marvels of these graceful ghosts of the sea. He hopes to do something similar with air currents. Inspired by the flow dynamics employed by the jet-paddling jellies, he has begun investigating how to capture the energy of winds whipping through a city. Because this wind can quickly change direction and strength as it slides down buildings, turns corners, or blasts down streets, taking advantage of it requires thinking more like a jelly than a tuna. Dabiri recently received funding from the National Science Foundation to explore the energy conversion that happens when eddies and vortices are generated by animals like jellyfish.
“Whether water or air,” Dabiri says, “it all comes back to the same equations.”