Why does a crayfish move its walking legs in a rhythmic back and forth motion when it is lying with one side of its carapace near the surface of the water?

A crayfish moves its walking legs in a rhythmic back-and-forth motion to facilitate respiration and maintain water flow over its gills. This movement helps to create currents that draw oxygen-rich water into the branchial chamber, allowing for efficient gas exchange. Additionally, this motion can assist in stabilizing the crayfish while it rests on the substrate or surface.