Scientists have painstakingly uncovered the mechanism behind the highly efficient swimming methods of jellyfish and lampreys. Their study, published in the journal Nature Communications, finds that instead of creating pockets of high pressure to be jetted away from, these creatures actually prefer to generate areas of low pressure to be sucked into.
Being able to conserve as much energy as possible while doing things provides creatures with a huge evolutionary advantage: They tend to need to consume less food. Swimming is no exception to this, and a marine environment’s most efficient swimmers aren’t necessarily the fastest in the ocean, but they use comparatively less energy for each underwater movement than others.
Jellyfish and lampreys (eel-like beasts) are free-swimming animals that move through the water column using wave-like body motions. This is known as undulatory locomotion; land-based snakes crawl in this way, rhythmically pushing against the ground to move themselves. It had been widely assumed based on direct observations that lampreys and jellyfish did a similar thing against the water, much in the way a human swimmer would.
Pushing the water away would temporarily compress the mass of water, creating a pocket of high pressure behind the swimmer. As this sets up two distinct regions of pressure – a high-pressure zone behind the swimmer and a low-pressure region roughly where the swimmer is – the swimmer is then pushed across this pressure gradient from the high to low pressure zones, which propels them forwards.
In order to precisely investigate the locomotion of lampreys and jellyfish, they were placed in large tanks of water along with tiny glass beads illuminated with lasers. As they swam through their environment, the beads would be moved from pockets of high pressure to regions of low pressure, meaning that the researchers could precisely track the pressure gradients around the creatures. The efficiency of the swimming could also be measured using high-speed cameras.
Screengrab of the eel-like lamprey (black outline) swimming in a water tank. Blue depicts suction (low pressure) and red denotes pushing (high pressure). Black lines and arrows indicate water flow directions around the lamprey. Credit: John O. Dabiri.
They found that when lampreys swim, they actually create low-pressure pockets inside each bend of their undulating body, which the water ahead of them rushes to fill in. The motion of the inflowing high-pressure water “sucks” the lamprey forwards. Jellyfish, despite being physically quite different, swim using the same motion, using low-pressure pockets beneath their “umbrella” to create an identical suction effect in order to swim.
As reported by the Guardian, the study’s co-author John Dabiri of the University of Stanford said: “By measuring for the first time the pressure that swimming animals exert on the surrounding water, we have shown that the mechanism of efficient swimming is much different from conventional wisdom.”
Of course, there are multiple ways to swim, and merely pushing against the water with mechanical (physical) actions is just one of them: Squid, for example, use jet propulsion to propel themselves along. Perhaps this newly discovered swimming technique will be co-opted by human engineering, just like jet propulsion was long ago in the design of submarines and aircraft.