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Plankton Become Stealthy When They Swim Breaststroke

Plankton and other tiny swimmers need to move around in order to find food and reproduce, but if they splash too much, they could easily become food themselves. According to new research by scientists in Denmark, this dilemma is solved by adopting the breaststroke, a naturally stealthy mode of swimming that can allow plankton to pass unnoticed by larger predators.
A planktonic copepod under a microscope. Credit: Chris Moody via flickr

Plankton are diverse set of microscopic organisms that inhabit oceans, lakes, and ponds in vast quantities. Their motion is largely at the mercy of fluid currents, but many types of plankton are also capable of swimming using various appendages such as antenna or flagella. On these small scales, predators sense their planktonic prey via fluid disturbances rather than by sight and thus quiet swimming is essential.

To measure the stealth of small swimmers, scientists measure how quickly fluid disturbances drop off with distance away from the swimmers—bacteria like E. Coli cause disturbances which decay as an inverse square law with distance, but last year an international group of researchers found that some types of plankton create disturbances that decay away much more quickly as an inverse cube law with distance. These plankton are especially quiet swimmers unlikely to be detected by predators that rely on water disturbances to find their food. At the time, the team noticed that the quiet swimmers favored a breaststroke style but they did not have a theory to explain why.

Various types of plankton swim with a breaststroke.
Credit: Andersen et al. 2015. Panel (a) credit: Knut Drescher  
Motivated by these observations, two of the researchers, Navish Wadhwa and Thomas Kiørboe, worked with their colleague, Anders Andersen, at the Centre for  Ocean Life at the Technical University of Denmark, to create a simple mathematical model of swimming plankton. Their model confirms that the breaststroke is indeed a stealthy way of swimming compared to other styles.

"Before doing this work we had not realized that by appropriately arranging its propulsion apparatus, a small swimmer can significantly reduce its fluid disturbance," said Andersen, lead author on the new paper which describes the theory. "That this could possibly be realized by a large and important group of organisms like breaststroke swimmers came as a surprise."

The model focuses on three forces—two equal forces from the "arms" of the plankton pointing away from the direction of travel, and a third force caused by the body of the plankton, pointing in the direction of travel. From these forces, the team modeled the motion of the water in all directions and found that it matched last year's observations of how disturbances decay around breaststroke-swimming plankton.

The team are now applying their model to plankton with different arrangements of flagella to see if stealth-mode swimming is a common survival technique.

Unfortunately this research doesn't mean you'll now be master of "Marco, Polo". Andersen said that just because breaststroke swimming allows plankton to pass unnoticed doesn't mean that the same physics applies to humans. Different types of fluid dynamics dominate on human scales, including inertia and turbulence, while water viscosity dominates on plankton scales.

For plankton to be stealthy they require just the right combination of breaststroke swimming style and water viscosity in order to minimize the fluid disturbances which could mean the difference between finding food and becoming food.

This research has been accepted for publication in the journal Physical Review E.

By Tamela Maciel, also known as "pendulum"


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