Jellyfish are passive creatures when it comes to feeding; they rely on water flow generated by their swimming movements to draw hapless prey towards their stinging tentacles. Intrigued by previous observations that jellyfish snack while swimming, John Dabiri at the California Institute of Technology hoped that scrutinizing the fluid dynamics of jellyfish swimming would shed some light on this interaction().
Dabiri explains that jellyfish swim by contracting and relaxing their soft umbrella-shaped bodies. During the power stroke, the animal collapses inwards and water is ejected from its subumbrellar cavity as a jet. The swirling fluid that forms in the animal's wake is called the starting vortex ring, which propels the animal forwards. But nobody had really considered what happens to water flow when the animal relaxes its body.
To see what happens during jellyfish recovery strokes, Dabiri needed water flow measurements during jellyfish swimming. So he was delighted when jellyfish behaviour experts Sean Colin and John Costello sent him video footage of swimming jellyfish and asked if he could use it to quantitatively reconstruct the flow field around the animals. Colin and Costello had videotaped the creatures in their home, a Croatian marine lake, so their footage revealed jellyfish behaviour in natural conditions. To visualise water flow during swimming, they had squirted fluorescent dye around the jellyfish. When Dabiri saw the fluorescent swirling vortices, he remembers being `amazed at how clearly you could see the flow. I immediately knew we could learn something from this one-of-a-kind data.'
Dabiri began to quantify the intricate flow patterns generated by the jellyfish. To measure how much fluid the animals eject during swimming, he needed to measure the inner surface of each animal's subumbrellar cavity,which is tricky, since these jellyfish are transparent. Undaunted, Dabiri constructed algorithms to determine the inner surface of each animal. Calculating this for each frame of video, he soon had a time frame of expelled water volume changes as the jellyfish swam. He noticed that vortex rings downstream of the jellyfish had a larger volume than when they were ejected from the animal. `As a vortex rotates, it grows by sucking in surrounding water,' Dabiri explains. The more momentum the flow in the animal's wake has,the more momentum the animal's body has in the opposite direction. As the starting vortex ring swells, it increases downstream fluid momentum, `which translates into increased forward thrust for the jellyfish' says Dabiri. He adds, `the existence of this mechanism for enhanced thrust challenges the common notion that swimming via jet propulsion is inherently inefficient.'
It turns out that jellyfish benefit from the starting vortex in other ways too. Dabiri was excited to see that, during the recovery stroke, jellyfish produce a stopping vortex that rotates in the opposite direction of the starting vortex. When these two vortices interact downstream, they form a complex superstructure. `The start vortex propagates away from the animal,'says Dabiri, `but when it bumps into the stop vortex, it slows down, giving the animals' tentacles more time to pick prey out of the swirling water.' By creating a vortex superstructure, jellyfish not only have a very efficient propulsion mechanism, but can also eat on the move.
