Hairy legs make you swim better – if you’re a shrimp

Having hairy legs may make shrimp better swimmers.

“Can hair help you swim better? Swimmers will say no, but shrimp would say yes,” said Sara Oliveira Santos from Brown University in Rhode Island at a fluid dynamics conference this week. She and her colleagues studied how shrimp and shrimp-inspired robots swim to determine whether it is advantageous for them to be hairy.

Shrimp use a special swimming technique called metachronal swimming to easily move through water. They beat their swimming appendages called swimmerets in just the right sequence to create a wave that travels from their tails to their heads and makes drag forces propel them forward instead of slowing them down.

For human swimmers, hairiness makes swimming harder exactly because it increases drag, but many shrimp have swimmerets covered in thin hair-like structures called setae and even smaller setules. To determine whether this hairiness makes metachronal swimming easier, the researchers first attached a leg from a dissected shrimp to a mechanical joint, immersed it in water and filmed how it interacted with a jet of dyed fluid. Their recordings showed that under certain conditions, very little liquid got through the hair-like structures and the appendage moved like an efficient paddle.

They also built several scaled-up robotic swimming appendages, some with many artificial setae and setules and some without any. The researchers moved these through a fluid filled with tiny particles that light up when illuminated. Recordings of these experiments showed how vortices formed and moved around each robotic appendage.

Based on an analysis of this vortex motion, the team concluded that hairier appendages experienced less stress on their joints than the fully smooth ones which means that having hairs could be making it easier for the animals to swim.

Oliveira Santos presented the work at the Meeting of the American Physical Society Division of Fluid Dynamics in Washington DC on 19 November.

Now, the team are working on further quantifying the size of the advantage shrimps’ hairy appendages give them, such as how much it increases their speed.

Topics:

• fluid dynamics/
• marine life