Here’s how swamp grass shrimp reduces friction while swimming.

marsh grass shrimp (Palaemonetes vulgaris) are impressively fast and agile swimmers, as anyone who has seen them run in 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 manage this feat. He presented his latest findings at a recent American Physical Society meeting on fluid dynamics in Indianapolis. Essentially, shrimp use their flexible and closely spaced legs to significantly reduce drag. The findings will help scientists design more efficient bio-inspired robots to explore and monitor underwater environments.

Tack is a trained biologist and currently works in Monica Wilhelmus’ lab. 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. Granted, the robot is significantly larger than real krill – actually about 10 times larger. However, keeping and examining krill in the laboratory is difficult. RoboKrill’s “leg” copied the structure of 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 the water. Not only did RoboKrill produce models similar to real krill, it could also mimic the swimming dynamics of other organisms by adjusting the appendages. They hope to one day use the robot to track krill swarms in the wild.

Regarding the swimming style of the marsh grass shrimp, previous research has shown that the creatures are able to maximize their forward thrust, thanks to the stiffness of their legs and increased surface area. This research mainly considered the legs (aka pleopods) as water-repellent paddles or flat plates. But no one has taken a close look at how the legs bend during recovery strokes. “It’s a very complex system,” Tack said during a briefing at the meeting. “We’re trying to approach [the topic] from two angles, looking at the liquid and looking at the mechanical properties of the legs.”

Specifically, Tack and colleagues seeded water with microscopic particles that allowed them to track and calculate the velocity and direction of flow properties, and used bright-field particle imaging velocity measurement (PIV) to visualize fluid flow around the shrimp’s beating legs. They also studied the mechanical properties of shrimp legs – no easy feat as each leg is roughly 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 allowed the team to identify two key mechanisms that reduce friction. First, they noticed a big difference in the patterns between the propulsion-generating power pulse and the rebound pulse, according to Tack. “We found that during the recovery stroke, the legs were about twice as flexible and bent heavily,” he said. “They stay almost horizontal relative to the direction in which they’re swimming.” The result is less direct interaction with the water and a reduced trail (smaller eddies), as opposed to power stroke, where the leg stays too stiff to maximize interaction with the water.

Second, the grouping of pleopods during the recovery shot also turned out to be important. “When they return the legs to their original position, they keep them close together 100 percent of the time,” Tack said. This is achieved thanks to the flexibility that creates a tight seal between the shrimp’s legs. So instead of three legs moving separately, it essentially acts as one leg, significantly reducing drag. “They beat their legs six times a second, for hours, so there’s a lot of energy they’re not potentially wasting,” Tack said. He and his colleagues will tailor their grass shrimp-inspired robot designs accordingly.

The listing image was made by the Smithsonian Environmental Research Center/CC BY 2.0.