A fiddler crab, also called a calling crab, is a land crab that lives in mud or sand flats near the beach. The reason this crab is called a fiddler crab is because it holds its large cheliped in front of its body and moves it back and forth as if it is playing a fiddle.

Fiddler crabs come out at low tide to forage on the silt in the sand. As they do this they ball the sand up and push it out of the holes they dig as they work their way into the ground. The best time to find them is at low tide when the water moves out and large amounts of sand are exposed.

Males fiddler crabs have one very large claw. Female fiddler crabs are smaller and only have small claws, so it's easy to tell them apart from the males. The fiddler crabs that live in the US can grow to be as big as 1 ½ inches. Fiddler crabs also have gills, like a fish, so they can live underwater.

Fiddler crabs live rather brief lives of no more than two years (up to three years in captivity). Male fiddler crabs use the major claw to perform a waving display as a form of female courtship. Females choose their mate based on claw size and also quality of the waving display. In many fiddler crab species, the female occupies the burrow of their mate while she lays her clutch of eggs.

Research shows that the male major claw size is also correlated with burrow width; the width of the burrow influences incubation temperature. Therefore, the female will choose a male mate whose claw size indicates the best burrow environment for her clutch of eggs. The waving display is also thought to indicate to females the overall healthiness of the male; a more vigorous display is more difficult to do and thus requires the male to be in prime health condition, which suggests that the male will help produce viable offspring.

Male versus male competition also occurs as fighting with the major claws. If a male loses his larger claw, the smaller one will begin to grow larger and the lost claw will regenerate into a new (small) claw. For at least some species of fiddler crabs, however, the small claw remains small, while the larger claw regenerates over a period of several molts, being about half its former size after the first molt. The female fiddler carries her eggs in a mass on the underside of her body. She remains in her burrow during a two-week gestation period, after which she ventures out to release her eggs into the receding tide. The larvae remain planktonic for a further two weeks.

Fiddler crabs such as Uca mjoebergi have been shown to bluff about their fighting ability. Upon regrowing a lost claw, a crab will occasionally regrow a weaker claw that nevertheless intimidates crabs with smaller but stronger claws. This is an example of dishonest signalling.

The dual functionality of the major claw of fiddler crabs has presented an evolutionary conundrum in that the claw mechanics best suited for fighting do not match up with the mechanics best suited for a waving display.