March of the Fiddler Crabs

We took a walk in the salt marsh by Lieutenant Island near Wellfleet. The tide was low, leaving pools where we could see hundreds of the Fiddler Crabs coming out of their little holes in the salt marsh. They are very sensitive to sound and vibration and ran away when we approached. Why do they march in a line?

We did a little research and found out that you can easily identify males and females by their claws. Males have a greatly enlarged claw, while females do not. These large claws are used in contests for breeding burrows and to attract females. When a mating claw is lost, it is replaced with a small, feeding claw while the remaining claw expands into a new mating claw.