Uca pugnaxAtlantic marsh fiddler

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Geographic Range

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

Physical Description

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, et al., September 1989)

  • Sexual Dimorphism
  • male larger
  • Range length
    13 to 23 mm
    0.51 to 0.91 in
  • Average length
    16 mm
    0.63 in


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, et al., September 1989)


Uca pugnax reproduce sexually, forming "lek-like" communal breeding assemblages. These leks are gatherings where the males of the species display, "fiddling" with their major cheliped, to attract females to their burrows. (Croll and McClintock, 2000; Grimes, et al., September 1989)

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. Female U. pugnax then follows the male to the burrow to copulate. Unlike many other crabs, copulation takes place when the exoskeleton of the female is in the hardened state.

After mating, female U. pugnax release the fertilized eggs onto their abdominal flap in a spongy cluster. Eggs hatch and are released after several months. The number of progeny range from 4,500 to 23,700 eggs. Female U. pugnax release the hatched larvae on the nocturnal high tide. (Grimes, et al., September 1989; Priest III, Fall 2000)

  • Breeding season
    Lower lat - April to Sept; Upper lat - July to August
  • Range number of offspring
    4,500 to 23,700
  • Average number of offspring
  • Average gestation period
    2 hours
  • Average age at sexual or reproductive maturity (female)
    1 years
  • Average age at sexual or reproductive maturity (male)
    1 years

After mating, female U. pugnax release the fertilized eggs onto their abdominal flap in a spongy cluster. Eggs hatch and are released after several months. Female U. pugnax release the hatched larvae on the nocturnal high tide. (Grimes, et al., September 1989; Priest III, Fall 2000)

  • Parental Investment
  • pre-fertilization
    • provisioning
  • pre-hatching/birth
    • protecting
      • female
  • pre-independence
    • protecting
      • female


The expected lifespan in the wild is one to 1.5 years. (Grimes, et al., September 1989)

  • Typical lifespan
    Status: wild
    1 to 1.5 years


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, et al., September 1989)

Communication and Perception

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, et al., September 1989)

Food Habits

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, et al., September 1989; Ringold, 1979)


Uca pugnax retreat to their burrows when threatened. (Grimes, et al., September 1989)

Ecosystem Roles

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, et al., September 1989; Hoffman, et al., 1984)

Economic Importance for Humans: Positive

Uca pugnax do not directly benefit humans.

Conservation Status

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


Renee Sherman Mulcrone (editor).

Carol Vinton (author), Hood College, Maureen Foley (editor), Hood College.



living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map


uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

brackish water

areas with salty water, usually in coastal marshes and estuaries.


uses smells or other chemicals to communicate


the nearshore aquatic habitats near a coast, or shoreline.


an animal that mainly eats decomposed plants and/or animals


particles of organic material from dead and decomposing organisms. Detritus is the result of the activity of decomposers (organisms that decompose organic material).


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature


an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.


union of egg and spermatozoan


a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.


An animal that eats mainly plants or parts of plants.


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.


marshes are wetland areas often dominated by grasses and reeds.


A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.


having the capacity to move from one place to another.


an animal that mainly eats fungus

native range

the area in which the animal is naturally found, the region in which it is endemic.


generally wanders from place to place, usually within a well-defined range.


reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.


chemicals released into air or water that are detected by and responded to by other animals of the same species


photosynthetic or plant constituent of plankton; mainly unicellular algae. (Compare to zooplankton.)


an animal that mainly eats plankton


having more than one female as a mate at one time

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season


remains in the same area


reproduction that includes combining the genetic contribution of two individuals, a male and a female

soil aeration

digs and breaks up soil so air and water can get in


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement


movements of a hard surface that are produced by animals as signals to others


uses sight to communicate


Bertness, M., T. Miller. 1984. The distribution and dynamics of *Uca pugnax* burrows in a New England salt marsh.. Journal of Experimental Marine Biology and Ecology, 83 (3): 211-237.

Burger, J., M. Gochfield. 1992. Effects of an oil spill on emergence and mortality in fiddler crabs (*Uca pugnax*). Environmental Monitoring and Assessment, 22 (2): 107-115.

Croll, G., J. McClintock. 2000. An evaluation of lekking behavior in the fiddler crab *Uca* sp.. Journal of Experimental Marine Biology and Ecology, 254 (1): 109-121.

Grimes, B., M. Huish, J. Kerby. September 1989. Species profiles: life histories and environmental requirements of coastal fishes and invertebrates (Mid-Atlantic) -- Atlantic marsh fiddler.. U.S. Fish Wildl. Serv. Biol./U.S. Army Corps of Engineers, Rep. 82 (11.114): 1-18.

Hoffman, J., J. Katz, M. Bertness. 1984. Fiddler crab deposit-feeding and meiofaunal abundance in salt marsh habitats. Journal of Experimental Marine Biology and Ecology, 82: 161-174.

Priest III, W. Fall 2000. Wetland denizens, Fiddler crab, *Uca* species. The Virginia Wetlands Report, 15 (No. 3).

Ringold, P. 1979. Burrowing, root mat density, and the distribution of fiddler crabs in the eastern United States. Journal of Experimental Marine Biology and Ecology, 36: 11-21.