The fiddler crab gets his name from the male of the species. He has one large cheliped (or claw - some are right-handed, some left) which looks like a fiddle and a second smaller claw which resembles a bow. He waves them in the air to attract his mate. The male fiddler crabs have a ritualized combat of arm-waving, bobbing of the body and standing upright, but their combat rarely gets physical - other than some harmless wrestling to show off to the girl crabs. Female crabs have two small claws which serve much better for gathering food.
Those large claws are a tempting treat for predators. So the male fiddler crabs have a unique talent - they can drop their claws! The predator gets a snack and the fiddler crab gets to run away and live another day. Interestingly, the missing large claw gets replaced by a small claw, while the small claw grows into a large claw. That sort of gives a new meaning to the word, "ambidextrous".
There are several species of fiddler crabs in the marsh. They contribute to the heath of the marsh in several ways. They are a major food source to the blue crab, rails, egrets, herons, and raccoons. They also help to aerate the marsh. The dig burrows that are about a half an inch wide and about a foot deep. Their burrows provide protection from predators, the sun, and high tide.
During high tide, they plug the holes of their tiny homes with a ball of mud, trapping air inside the burrow. Surprisingly, all crabs have gills, but land crabs have adapted to remove oxygen from air instead of water. Their gills, however, must still stay wet.
Scientists have attempted to estimate the number of fiddler crabs in an acre of marsh grass. Their estimates range from 500,000 to one million fiddler crabs per acre. They survive off of the detritus in the marsh.
Female fiddler crabs carry their eggs under their belly until they are ready to hatch. The mass of eggs is known as a sponge. When they hatch most are eaten by fish or plankton. As they grow, they will molt about 5 times in their life-time, which lasts about one to one and a half years.