Fiddler crabs include approximately 100 species of semi-terrestrial marine crabs within the genus Uca, and six species recognized in the Western Indian Ocean. They are a medium sized crab, with their carapace (protective shell) reaching up to 35 millimeters in width. Males are distinguished by their large chela (claw), which they wave to establish their territories and attract females, while the female’s claws are the same in size. Like all crabs, fiddler crabs molt: they shed their shell as they grow. If they lose a leg or a claw, they will regrow a new one during the moulting period. When they lose their large fiddle, the males will develop one on the opposite side. Fiddler crabs have a constant circadian rhythm (daily cycle), that mimicks the ebb and flow of the tides; they tend to turn dark in colour during the day and light at night.
Fiddler crabs are often found in or around mangroves or in brackish inter-tidal mud flats. They dig burrows where they forage for organic matter, algae, microbes and fungus. These crabs use their smaller claws to pick up sediment from the ground and bring it to their mouth where they sift through its contents for food. After every chunk of sediment is sifted through, the rejected sediment is formed into small round pellets. It is believed that this behaviour plays an important role in keeping wetland and mangrove ecosystems healthy, as it helps aerate the soil and prevent anaerobic (no oxygen) conditions.
Fiddler crabs usually live no longer than two years. During courtship, males perform a waving display with their major claw. The size of the claw and quality of display determines the choice of males by females. The size of the males claw indicates to the female the size of the burrow which would influence incubation temperature and the healthiness and vigorousness of the males, and also suggest that this would help produce viable offspring. Females will naturally lay their eggs in the burrow of their mate. After a two-week gestation period in the burrows, the female crabs will release their eggs in the receding tide. The resulting larvae will remain planktonic for another two weeks.
The most pressing threat faced by fiddler crabs is habitat destruction and loss, although this type of crab is not at risk of extinction.
The fiddler crab gets its name from the movement of the small claw during feeding, which makes it look like the animal is playing the fiddle with its small claw on its larger claw.