How the moon jellyfish won

A paper published this week in Proceedings on the National Academy of Sciences reports that jellyfish are unusually efficient swimmers, allowing them to divert their energy into growth and reproduction

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    Moon jellyfish are prodigious colonizers. A new paper helps explain how they do it.
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When an army of moon jellyfish invaded a Swedish nuclear reactor’s pipes last week, no one was surprised; jellyfish pull stunts like this all the time.

Moon jellyfish, much like humans, are prodigious colonizers. Under circumstances that are still unclear – climate change may be related – moon jellyfish will invade and overwhelm an ecosystem. There, they’ll engage in a pitched battle with the region’s fish and humans alike.

And the moon jellyfish will win. It will grow bigger and bigger, and it will reproduce and reproduce some more. It will do these things faster than will fish, and it will hobble local fisheries. As it continues to grow, it will become an expensive annoyance for costal tourist attractions – “swimming with jellyfish” is not a popular advertising slogan – as well as for shoreline power plants whose underwater pipes are vulnerable to clogging.

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But how does the moon jellyfish win? How does a primitive animal that looks like little more than an errant blob of gelatin manage to outdo us?

Well, it’s because moon jellyfish don’t have to try very hard to beat us. A paper published this week in Proceedings on the National Academy of Sciences reports that jellyfish are unusually efficient swimmers. Since jellyfish expend little energy in propelling themselves through the water, the bulk of their energy can be redirected into growth and reproduction, the authors say. This means that the slow and steady jellyfish, Aesop's tortoise of the ocean, is primed to win in the end.

“This can explain the counter-intuitive find that jellyfish can bloom and outcompete more advanced predators,” says Brad Gemmell, a researcher at the Marine Biological Laboratory at Woods Hole, Massachusetts and the lead author on the paper.

The word “jellyfish” refers to the some 10,000 species with the cnidarian phylum, as well as some 100 species within the ctenophores phylum. Some mollusks, worms, and vertebrates are also sea-faring, gelatinous beings and can be called “jellyfish.”

The moon jellyfish, Aurelia aurita, is in the cnidarian phylum and belongs to perhaps the most studied jellyfish genus, Aurelia. The species is fantastically far-flung around the world, found everywhere from the topics to the sub-arctic. For these reasons, the moon jellyfish is perhaps the quintessential jellyfish: a squishy, globular parachute, daintily adorned with some swishy streamers. It is often distinguished from other species by its four gonads arranged like flower petals and visible through its top; from an aerial view, the moon jellyfish looks like a see-through sand dollar.

Nothing about the moon jellyfish is anthropomorphic. It does not have a heart, and it does not have a brain. Besides, it doesn’t appear to have anywhere to put and arrange these things: the jellyfish is some 98 percent water. Perhaps most baffling to the human mind is that the moon jellyfish appears unflappable. Slipping through a vast water world, drenched in pastel light, the serene jellyfish seems to have no worries at all. No wonder that Ray Bradbury, in his 1951 short story “Fire Balloons,” made the humans that had forfeited needing or wanting or feeling anything at all floating blue spheres. Floating orbs, it seems, are about as far from human as it’s possible for a mobile organism in the animal kingdom to get.

All this would seem to set up the jellyfish as one of the most hapless beings on Earth, an ancient animal left behind as other animals around it evolved to get respiratory, circulatory, and excretory systems. But the jellyfish’s simple design (bare but boneless) has in recent years been fingered as the clue to its millions of years of success in the planet’s competitive oceans. Now, it unfolds that this simplicity makes the moon jellyfish highly efficient swimmers.

That moon jellyfish are efficient swimmers, let alone superlatively so, is counterintuitive. That’s because moon jellyfish are plankton and all plankton share one trait: they are bad swimmers. Plankton, including jellies, can swim neither quickly nor strongly enough to fight a current. They are drifters – or, “wanderers,” after the Greek word from which their name derives.

So, if jellyfish are slow and weak swimmers, it would seem to make sense that the animals are also inefficient ones. After all, moon jellyfish are less than 1 percent muscle, while salmon are more than 50 percent muscle.

The moon jellyfish’s swimming efficiency had previously been measured using what is known as the Froude number (Ef), the metric that was first conceived to quantify the propulsive power of ships. Efs quantify how much power is lost during locomotion versus how much is in fact used.  In Efs, the moon jellyfish is not terribly efficient: it displays Ef values of just 0.09–0.53. Meanwhile, the salmon pumps along at an Ef value of about 0.8, making salmon about 900 times more efficient swimmers than jellyfish.

But in this latest research, the authors measured the moon jellyfish’s efficiency using what is called net cost of transport (COT) analysis, which measures how much energy is takes to move a mass a certain distance. By this measurement, the moon jellyfish is some three times more efficient in their swimming than are salmon.

The genius is in the second phase of a moon jellyfish’s “stroke,” to put it in anthropomorphic terms. A moon jellyfish’s “swimming” is a series of contraction and relaxation phases. In the contraction phase, the jellyfish uses its muscles to squeeze itself up, storing energy in its tissue much as energy is stored in a pulled rubber band. In the relaxation phase, the jellyfish releases its muscles to reach peak drag. The first phase saps up energy, while the second phases requires none.

But it now turns out that the jellyfish gets an energetic freebie in this second phase. When the jellyfish releases its muscles, a water vortex develops behind it, propelling it through the water. This vortex reduces the moon jellyfish’s cost of transport by almost half, the researchers found.

“The jellyfish gets a second boost of force from this relaxation phase using no additional energy,” said Gemmell.

Gemmell says that highly efficient swimming could explain how jellyfish grow and reproduce so fast: little of their energy has to be diverted to locomotion, and it can be in large part marshaled toward growth and reproduction.

“This mechanism of swimming provides a competitive advantage to the jelly,” he said.

The research is part of a broader research inquiry, funded by the Office of Naval Research, into how marine animal’s morphologies can be re-purposed for naval craft design.

“It’s well known that animals move more efficiently through water than any man made machine does,” says Gemmell.

The jellyfish’s swimming technique would not be particularly useful in a vessel designed to cross the Atlantic, he said – not unless the ship’s passengers were entirely absent a desire to actually get there, or to get there within a reasonable time frame. But the jellyfish’s strategic swimming would be useful in an oceanic monitoring device designed to hold its position in the water for a long time and use minimal energy to do so, he said.

His team is now investigating how aquatic animals’ flexible bodies factor into their underwater prowess, he said.

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