Fiddler Crab Uca pugnax

Uca pugnax, the Atlantic Mud Fiddler Crab, is native to the tidal mudflats and saltmarshes of the eastern United States from Massachusetts to Florida. It is one of the most instantly recognisable crustaceans in the world, with males bearing an enormously enlarged major claw — the 'fiddle' — used in courtship displays and territorial combat.

In the paludarium, U. pugnax requires a biotope-style tidal zone setup: a sandy or muddy substrate shoreline that slopes gradually into brackish water, allowing the crabs to burrow above the waterline. The water should be brackish with a salinity similar to estuarine conditions. Tidal fluctuation is not strictly necessary but is beneficial if achievable.

Fiddler crabs are social and do best in groups of mixed sexes. Males will wave and spar constantly, providing endlessly entertaining behaviour. They are substrate feeders, sorting organic matter and microorganisms from sand and mud using their smaller claw as a feeding instrument. They are sensitive to pollution and chemical contamination, so clean, well-filtered water with regular partial changes is essential.