The motion of jellyfish and other sea creatures might mix the oceans just as much as the winds and tides do, according to a study published in Nature. The study’s findings provide support for a theory called Darwin drift, which was developed by Charles Galton Darwin (the grandson of the Darwin). The theory holds that a body moving through water brings along some of the wet stuff.
To test the theory of Darwin drift, researchers first modeled the motion of swimming organisms in a lab, using liquids of various viscosities, or levels of internal resistance. They found that bodies drag more liquid along with them when the liquid is thicker. This effect can be significant; in fact, when marine plankton-sized objects moved a couple of body lengths forward in the most viscous liquids, they carried with them up to four times their volume in liquid. Next the researchers monitored jellyfish as they swam through clouds of dye in a lake on the Pacific island of Palau. A trail of dye followed each animal, as Darwin’s mechanism would predict. Using a laser-equipped camera, the team then measured the dye’s movement and the stirring of suspended particles in the animal’s wake [Nature News]. The scientists found that the mechanism proposed by Galton Darwin provided for 90 percent of the mixing between the water and the dye.
In fact, based on a calculation using the estimated total energy of marine life, the scientists found that swimming causes about the same amount of mixing as do winds or tides. The findings suggest ocean swimmers can move water over long distances and that they could help run the vertical currents that push nutrients around between the sea floor and surface waters [New Scientist].
How exactly does this mechanism work? The act of swimming literally pulls the liquid along with the moving body. “As a body moves in a fluid, a high-pressure field is created in front of the body, and a low-pressure field behind. Because fluid moves from high to low pressure, the fluid that’s adjacent to the rear of the body moves along with it,” said [co-author Kakani] Katija. “You get a permanent displacement of the water” [Wired.com]. If the study’s findings pan out it could affect oceanic climate models, which don’t currently take this so-called biogenic mixing into account.
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Photo courtesy of K.Katija/J.Dabiri