Uca pugnax
Atlantic marsh fiddler

Neartic: Uca pugnax lives predominately along the Mid-Atlantic coast of the United States. (Grimes, Huish, and Kerby, September 1989)

Uca pugnax lives in saltmarshes along the Mid-Atlantic coast. Marsh fiddler crabs construct burrows that are utilized for mating, rest, and "hibernation" during the winter. The burrows of U. pugnax also serve as refuge from predators, heat, and incoming tides. Burrows are approximately 1.3 cm wide and are between 30.5 cm and 92 cm deep. Commonly located in sandy and muddy substrates, burrows may end in a small room or be connected with other burrows. During high tide, U. pugnax plug their burrows with mud. Often these burrows are found near hard structural elements or grass stems in areas of intermediate root mat density. (Bertness and Miller, 1984; Grimes, Huish, and Kerby, September 1989)

The sexually dimorphic Atlantic marsh fiddlers can be found to be up to 16 mm long and 26 mm wide. The average male carapace is 15 mm long and 23 mm wide, while the average female carapace is 13 mm long and 18 mm wide. The males are dark olive to almost black in color with a royal blue spot on the center of its carpace. Female U. pugnax are of similar color as the males, but do not have the blue spot. Either one of the male's chelea may grow until it is half of the crabs body weight, while the claws of the female are isomorphic. The chelea are brownish yellow at the base with white fingertips in both sexes. Uca pugnax have slender eyestalks and dark banded walking legs. The dorsal carapace of U. pugnax is flattened rather than convex as in other similar species of crabs. (Grimes, Huish, and Kerby, September 1989)

After hatching U. pugnax larvae progress through five stages of zoea (lasting anywhere from one week to a month) and one megalops stage (lasting 4 days to a month). Megalops further metamorphose into the first juvenile crab stage (lasting a few days). At this stage in its life U. pugnax attaches to a hard substrate. The second and third stage juvenile crab stages last for a combined total of approximately 10 days. Atlantic marsh fiddlers are mature after one year. Uca pugnax molts one to two times per year.

The enlarged claw of the male Atlantic marsh fiddlers is a result of sexual selection. This claw, or chelea, enlarges to almost half the male's body weight. The larger chelea is both an advantage and hindrance. In displays of aggression the male with the larger chelea is usually the victor, but the male with the larger claw is at a disadvantage in burrow construction and foraging. (Bertness and Miller, 1984; Grimes, Huish, and Kerby, September 1989)

The expected lifespan in the wild is one to 1.5 years. (Grimes, Huish, and Kerby, September 1989)

Adult U. pugnax defend their burrows and challenge other smaller crabs in an attempt to take over the smaller crab's burrow. Uca pugnax respond defensively when probed by tilting their bodies upward, extending their legs, raising their chelea, and moving away. Challenges between U. pugnax males rarely result in injury. (Burger and Gochfield, 1992; Grimes, Huish, and Kerby, September 1989)

Uca pugnax communicate visually and audibly. Male U. pugnax produce visual and acoustical displays to attract their mates. Precopulatory male fiddler crabs wave their major cheliped in a circular motion to generate one form of these acoustical signals. The male marsh fiddler crabs also vibrate the bottom sediment by stamping their walking legs. (Grimes, Huish, and Kerby, September 1989)

Uca pugnax emerge from their burrows at low tides to feed. Uca pugnax are filter feeders, feeding on detritus. This species feeds by scooping mud into their mouths and filtering out the detritus. The detritus is separated from the mud by water pumped through their gills. The digestible material is separated from the undigestible material within the gut, and the undigestable material is deposited as fecal pellets. Uca pugnax takes in approximately 0.4 g of material in six hours.

Foods eaten have been estimated to be 33% diatoms, 25% fungi, 20% vascular plants, 20% unknown material. (Grimes, Huish, and Kerby, September 1989; Ringold, 1979)

Uca pugnax retreat to their burrows when threatened. (Grimes, Huish, and Kerby, September 1989)

Uca pugnax affects nutrient cycles and energy flow throughout the salt marsh. Atlantic marsh fiddlers' burrows increase soil drainage, increase the amount of nutrients available in the soil, and allow plants to penetrate further into the soil. The bioturbation of the species stimulates algal growth and oxygenates the sediment. The undigestable fecal pellets released also contribute organic nitrogen to the soil. (Grimes, Huish, and Kerby, September 1989; Hoffman, Katz, and Bertness, 1984)

Uca pugnax do not directly benefit humans.

There is currently no conservation data available for U. pugnax.