The fiddler craborcalling crab may be any of more than one hundred species of semiterrestrial marine crabs in the family Ocypodidae,[2] well known for their sexually dimorphic claws; the males' major claw is much larger than the minor claw, while the females' claws are both the same size.[3] A smaller number of ghost crab and mangrove crab species are also found in the family Ocypodidae. This entire group is composed of small crabs, the largest being slightly over two inches (5 cm) across. Fiddler crabs are found along sea beaches and brackish intertidal mud flats, lagoons, swamps, and various other types of brackish or salt-water wetlands.
Like all crabs, fiddler crabs shed their shells as they grow. If they have lost legs or claws during their present growth cycle, a new one will be present when they molt. If the large fiddle claw is lost, males will develop one on the same side after their next molt.[4] Newly molted crabs are very vulnerable because of their soft shells. They are reclusive and hide until the new shell hardens.
Male lemon-yellow clawed fiddler crab (Austruca perplexa), waving his big claw in display
Fiddler crabs communicate by a sequence of waves and gestures;[6] males have an oversized claw or chela; used in clashes of ritualised combat of courtship over a female and signal their intentions between conspecifics. The movement of the smaller claw from ground to mouth during feeding explains the crabs' common name; it looks as if the animal were playing the larger claw like a fiddle.
The crab's smaller claw picks up a chunk of sediment from the ground and brings it to the mouth, where its contents are sifted through (making the crab a detritivore). After anything edible is salvaged, be it algae, microbes, fungus, or other decaying detritus, the sediment is replaced in the form of a little ball. The presence of these sediment balls near the entrance to a burrow is a good indication of its occupation. Some experts believe that the feeding habits of fiddler crabs play a vital role in the preservation of wetland environments; by sifting through the sands, they aerate the substrate and prevent anaerobic conditions.[7]
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.[8] Females choose their mate based on claw size and also quality of the waving display.[9] 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.[10] 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.[11]
Fiddler Crabs Fighting in Belle Hall, Mount Pleasant, South Carolina on March 24, 2023
Male versus male competition also occurs as fighting with the major claws.[12] 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 Austruca 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.[13][14] 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.[15]
More than 100 species of fiddler crabs make up 11 of the 13 genera in the crab family Ocypodidae. These were formerly members of the genus Uca. In 2016, most of the subgenera of Uca were elevated to genus rank, and the fiddler crabs now occupy 11 genera making up the subfamilies Gelasiminae and Ucinae.[16][2][17]
The obvious asymmetry of male fiddler crabs make them useful figures in illustrating the non-orientability of certain geometric objects, like the Möbius strip shown here.
Fiddler crabs are occasionally kept as pets.[20] The fiddler crabs sold in pet stores generally come from brackish water lagoons. Because they live in lower salinity water, pet stores may call them fresh-water crabs, but they cannot survive indefinitely in fresh water.[20] Fiddler crabs have been known to attack small fish in captivity, as opposed to their natural feeding habits.[21]
^Levinton, J. S., Judge, M. L., and Kurdziel, J. P., 1995, Functional differences between the major and minor claws of fiddler crabs (Uca, family Ocypodidae, order Decapoda, Subphylum Crustacea): A result of selection or developmental constraint?: Journal of Experimental Marine Biology and Ecology, v. 193, p. 147-160.
^Pope, D. S., 2000, Testing function of fiddler crab claw waving by manipulating social context: Behavioral Ecology and Sociobiology, v. 47, p. 432-437.
^Perez, D. M., Rosenberg, M. S., and Pie, M. R., 2012, The evolution of waving displays in fiddler crabs (Uca spp., Crustacea: Ocypodidae): Biological Journal of the Linnean Society, v. 106, p. 307-315.
^Reaney, L. T., and Backwell, P. R. Y., 2007, Temporal constraints and female preference for burrow width in the fiddler crab, Uca mjoebergi: Behavioral Ecology and Sociobiology, v. 61, p. 1515-1521.
^Matsumasa, M., Murai, M., and Christy, J. H., 2013, A low-cost sexual ornament reliably signals male condition in the fiddler crab Uca beebei: Animal Behaviour, v. 85, p. 1335-1341.
^Callander, S., Kahn, A. T., Maricic, T., Jennions, M. D., and Backwell, P. R. Y., 2013, Weapons or mating signals? Claw shape and mate choice in a fiddler crab: Behavioral Ecology and Sociobiology, v. 67, p. 1163-1167.
^Swanson, B. O., George, M. N., Anderson, S. P., and Christy, J. H., 2013, Evolutionary variation in the mechanics of fiddler crab claws: Bmc Evolutionary Biology, v. 13.
Media related to Uca at Wikimedia Commons
