Here’s how swamp grass shrimp reduce drag while swimming

Swamp shrimp (Palaemonetes vulgaris) are impressively fast and agile swimmers, as anyone who has seen them whiz over tide pools on the beach can attest. Nils Tack, a postdoctoral researcher at Brown University, studies the biomechanics and fluid dynamics of how these tiny creatures control performance. He presented his latest findings at a recent American Physical Society meeting on fluid dynamics in Indianapolis. Essentially, the shrimp uses its flexible and closely spaced legs to significantly reduce drag. The findings will help scientists design more efficient bioinspired robots for exploring and monitoring underwater environments.

Tack is a biologist by training and currently works in the lab of Monica Wilhelmus. Earlier this year, the group introduced RoboKrill, a small one-legged 3D-printed robot designed to mimic the leg movement of krill (Euphasia superba) so that it can move smoothly in underwater environments. Admittedly, the robot is significantly larger than the actual krill – about 10 times the size, in fact. But keeping and studying krill in the lab is a challenge. RoboKrill’s “leg” copied the structure of the krill swimmers with a pair of gear-driven appendages, and Wilhelmus et al. used high-speed imaging to measure the angle of its appendages as it moved through water. Not only did RoboKrill produce similar patterns to real krill, but it could also mimic the swimming dynamics of other organisms by modifying its appendages. They hope to one day use the robot to monitor swarms of krill in the wild.

Regarding the swamp grass shrimp’s swimming style, previous studies showed that the creatures were able to maximize forward thrust due to the stiffness and increased surface area of ​​its legs. That study essentially treated the legs (also called pleopods) as paddles or flat plates that pushed on water. But no one looked closely at how the legs flexed during recovery strokes. “It’s a very complex system,” Tack said during a briefing at the meeting. “We’re trying to approach [the topic] through two angles, looking at the fluid and looking at the mechanical properties of the legs.”

Specifically, Tack and his colleagues seeded the water with microscopic particles, which allowed them to track and calculate the speed and direction of flow characteristics, then used bright-field particle imaging velocimetry (PIV) to visualize fluid flow around the shrimp’s throbbing legs. They also studied the mechanical properties of the shrimp’s legs – no small feat since each leg is about the size of a grain of sand. “We basically pushed the legs with a known force to see how they bend,” Tack said.

This dual approach enabled the team to identify two main drag reduction mechanisms. First, for each Tack, they noticed a major difference in patterns between the thrust-producing power stroke and the recovery stroke. “We found that the legs are about twice as flexible during the recovery stroke and bend heavily,” he said. “They stay nearly horizontal to the direction they’re swimming.” The result is less direct interaction with the water and reduced wake (smaller vertebrae), unlike the power stroke, where the leg remains very stiff to maximize interaction with the water.

Second, the grouping of the pleopods during the recovery stroke was also found to be significant. “Whenever they return the legs to the original position, they keep them close together 100 percent of the time,” Tack said. This is made possible by the flexibility, which ensures a good seal between the shrimp’s legs. So instead of three legs moving separately, their legs essentially move as one, greatly reducing drag. “They’re beating their legs six times a second for hours on end, so that’s potentially a lot of energy that they’re not wasting,” Tack said. He and his colleagues will modify their grass shrimp-inspired robot design accordingly.

Frame image by Smithsonian Environmental Research Center/CC BY 2.0