Researchers examined the leg movements of shrimp while swimming and found that reducing drag is an important factor in their movement.
Introduction
Have you ever tried to walk through a swimming pool or a lake? It can be hard. Small animals like shrimp and krill can have a hard time moving around in the water. The water can seem sticky to the small animals that live in it, causing a lot of friction. Luckily, many animals have traits that can help them move more easily through water.
Some animals perform metachronal swimming. This is when an animal beats their legs one after another through the water. Sometimes the legs even come together and act as a group. It is a coordinated movement that produces forward thrust. And it works better than if the legs beat on their own.
Leg flexibility also helps animals move in the water. If a leg is stiff on the power stroke, it can generate a lot of thrust. But if it is stiff during the recovery stroke (getting back to the beginning), it can produce a lot of drag. There is less resistance from the water if the leg bends. Think about moving through a waist-deep pool. If you keep your legs straight, you can’t move very fast. And it takes a lot of energy. But if you bend your knees to walk through the water, you can move more easily.
Researchers know a lot about how swimming style and leg flexibility can increase thrust. But we don’t know a lot about how they reduce drag. We had several questions. How much does leg flexibility contribute to reduced drag? Does metachronal swimming reduce drag? By how much? This type of information could help us design new underwater robots in the future.
Methods
Leg Flexibility
We took legs from recently deceased grass shrimp. We scanned the legs to figure out how curved they were. Then we mounted the legs to a support that didn’t move. We used a pin to push on each side of the leg. Then we calculated how much the leg bent in each direction.
Metachronal Swimming
We looked at live grass shrimp in an aquarium. We observed their recovery stroke. We timed how long each leg was coordinated with or touching the other legs. We compared this to the length of the recovery stroke. We also looked at water flow around the shrimp’s legs. We did this by putting very small particles in the water. Then we illuminated the particles with a thin sheet of laser light. We used a very fast camera to see how the particles moved around the legs.
Robotic Shrimp
We used our measurements from live animals to design a robotic shrimp (Fig. 1). We made it using 3D printing, plastic heat shaping, and electronics. Our robot was 20 times bigger than a real grass shrimp. We put our robot in a fluid that mimicked the conditions that our live grass shrimp lived in. We looked at the robot swimming with different stroke patterns. We also looked at water flow around the legs. Finally we calculated real-time thrust and drag measurements using sensors on the robot.
Results
We found several interesting results:
We saw that the back face of the grass shrimp’s legs was curved. The legs were up to 1.7 times stiffer when we pushed on the back (similar to the power stroke) compared to the front (similar to the recovery stroke). We found that this reduced drag in individual robotic legs by up to 75%.
In the aquarium, we observed the middle three legs (out of 5) come together and move as a group. In our robot model, we found that metachronal swimming increased total thrust by about 30%. This was compared to independent leg swimming (Fig. 2).
We saw that water flow around the shrimp’s legs created vortices. These are the small wakes the legs create as they move through the water. Vortices can increase drag if the other legs try to swim through them. We found that metachronal swimming had the fewest interactions with vortices.
Discussion
Our results suggest that moving through water isn’t just about thrust. It’s also about drag. Bending can reduce the area of the leg moving through water. This reduces drag during the recovery stroke. Bending also allows the legs to group together. The first leg in a group acts as a shield, protecting the legs behind it from the effects of water. This then reduces drag and there are fewer vortex interactions.
Reducing drag helps animals move faster and further through the water. They might be able to escape from predators better. It could also help them catch food. Shrimp, comb jellies, bristle worms, and copepods are all metachronal swimmers. It is a common way to swim. This suggests it evolved many times because it was so helpful.
We should consider metachronal swimming when we design underwater vehicles. Many designs focus on maximizing thrust. What if we could reduce drag instead? Could we make underwater vehicles that use less energy? Hopefully, we will find out!
Conclusion
Our research shows that movement can be complex. And it is specific to the environment an animal lives in. These traits have evolved over millions of years. And we are just learning how some of them work!
The next time you go to the zoo or a park, take a look at all the different ways animals move. Do they swim? Fly? Or maybe they slither along the ground? Then look at the traits that allow them to move that way. Do they have wings, or a tail, or sticky skin? Can you figure out what advantages those traits give the animal? How do they help it move and survive?
