September 2, 2014

Built for Efficiency, Not Speed: Scientists Discover New Propulsive Mechanism in Jellyfish

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Contact: Diana Kenney, Marine Biological Laboratory
508-289-7139; dkenney@mbl.edu

MBL, WOODS HOLE, Mass.—Jellyfish may not be fast, but they are one of the most efficient propulsors on the planet—which is why they can “bloom” and overrun an ecosystem by outcompeting much swifter predators like fish, Marine Biological Laboratory (MBL) visiting scientists report this week.

Moon jellyfish (aurelia aurita), processed data from high speed recordings showing the structure of the vortex. Courtesy of Brad Gemmell.

Moon jellyfish (Aurelia aurita), processed data from high speed recordings showing the structure of the vortex. Courtesy of Brad Gemmell.

“It seems counterintuitive,” says lead author Brad Gemmell, who worked at the MBL as a postdoctoral scientist with John Costello of Providence College and Sean Colin of Roger Williams University.

“Fish are visual hunters; they can detect prey from a distance and they have very good chemosensory capabilities,” he says. “Jellyfish, by comparison, are seemingly poor predators. They need direct contact with their prey through their tentacles, to feed, and they aren’t fast swimmers.”

So how can swarms of jellyfish get the ecological upper hand? These passive-seeming blobs, the scientists found, are energetically very efficient. “A jellyfish expends far less energy getting from point A to point B” than a fish does, Gemmell says. So for every gram of prey it eats, the jellyfish can put more of the energy it gains into growth and reproduction, rather than locomotion.

The key to the jellyfish’s efficiency, the paper reports, is a previously overlooked part of its locomotive cycle during the relaxation phase after the animal contracts. “The contraction phase is when the animal is moving the fastest,” Gemmell says. “That is when you see these vortex rings [in the water] being ejected behind the animal.” The relaxation phase was primarily thought to be a period of reset for the next contraction. However, the scientists discovered that, during relaxation, a second vortex ring comes up the underside of the animal and gives it a second locomotive boost.

“That secondary vortex ring contributes up to 30 percent of the total distance the animal travels with each contraction, which is significant,” Gemmell says. “It’s a neat little trick they use. The animal is paused; it’s not expending any additional energy, but it is still accelerating.”

They call this new locomotive mechanism “passive energy recapture.”

Gemmell, who just accepted a position with the University of Texas at Austin, Marine Science Institute in Port Aransas, plans to return to the MBL next summer to continue the collaboration. One goal is to take the underwater video system the team has developed to watch marine animals swimming and feeding from its current 2-D capabilities to 3-D, “which would be totally revolutionary,” Gemmell says.

This is a moon jellyfish swimming through seawater seeded with hollow glass spheres (10um diameter). The spheres are illuminated by a laser to show the fluid structures created by the swimming jellyfish and recorded by a high speed camera at 1000 frames per second [particle image velocimetry (PIV)]. Courtesy of Brad Gemmell.

This video is showing pressure around the jellyfish that was calculated from particle image velocimetry (PIV) data (the technique used in video above). High pressure underneath the animal creates a force that moves it forward. Courtesy of Brad Gemmell.

Citation:

Gemmell BJ, Costello JH, Colin SP, Stewart CJ, Daviri JO, Tafti D and Priya S. Passive energy recapture in jellyfish contributes to propulsive advantage over other metazoans. PNAS, published online October 7, 2013, doi: 10.1073/pnas.1306983110

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The Marine Biological Laboratory (MBL) is dedicated to scientific discovery and improving the human condition through research and education in biology, biomedicine, and environmental science. Founded in Woods Hole, Massachusetts, in 1888, the MBL is a private, nonprofit institution and an affiliate of the University of Chicago.