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A new study crowns the jellyfish as an unlikely champion in the marathon of life.
Elite long-distance runners like the winner of this past weekend’s New York City Marathon, Geoffrey Mutai, are known to spend less energy running than their less accomplished competitors. But as efficient as today’s top marathoners are, they have nothing on the jellyfish.
Until recently, the translucent tentacled marine animals were not particularly revered for their swimming prowess—even by scientists who like to be quantitative about these sorts of things. An old mode of calculating swimming efficiency for ship performance, the Froude number, gave the jellyfish poor marks (0.09-0.53) compared to its ocean-bound brethren, bony fish (approximately 0.8). But this measure doesn’t factor in energy demands, and therefore says little about how costly movement is for a given animal.
So scientists at the Marine Biological Laboratory in Woods Hole, Massachusetts, Virginia Tech, and the California Institute of Technology used a more comprehensive measure, the net cost of transport, or COT (defined as the amount of energy per kilogram of body weight required to move one meter). Publishing the results in the journal PNAS last month, they found that the moon jellyfish (Aurelia aurita) is in fact remarkably efficient, expending less energy to move than all other animals previously tested—including runners, fliers, and well-regarded swimmers such as the salmon (as you might expect, the latter is the closer competition).
How is the jellyfish able to move farther without using as much energy? The secret has to do with the mesmerizing undulation that makes the jellyfish such a reliable aquarium crowd-pleaser. Jellyfish use a single layer of muscle cells to contract their main body section, or bell, to push water out, and propel them in the opposite direction. Using field measurements and computers to model the fluid dynamics, researchers found that because of the way the bell naturally refills with water, jellyfish get an added burst of acceleration, allowing them to travel 30% farther—all without having to flex more muscles. This aspect of jellyfish movement, termed ‘passive energy recapture,’ improves the cost of transport by about 50%, making the jellyfish Earth’s most efficient animal.
For ecologists, this finding makes sense because jellyfish are highly successful in the ocean, often rapidly taking over an area and outcompeting other species. And while humans—even the likes of Geoffrey Mutai—are no competition for jellyfish, we may be able to borrow their tactics when designing low-energy aquatic vehicles. The engineers at Virginia Tech have already built prototypes of a robotic jellyfish powered by just a small battery, which the Navy plans to use for surveillance and scientific research.