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Fiddler crabs are small, semi-terrestrial crabs in the genus Uca that are distinctive in the extreme size difference between the left and right claws in males. They are most closely related to the ghost crabs (genus Ocypode). As of 2007, there were 97 recognized species/subspecies of fiddler crabs. Crane (1975) produced a landmark work on fiddler crab biology and systematics which is widely available online. Crane recognized 62 species, most of which are still recognized as valid, although many of her subspecies are now generally treated as full species and several additional species have been described subsequent to her publication. These taxonomic changes were detailed by Rosenberg (2001) and are summarized by him at http://www.fiddlercrab.info/uca_systematics.html.
All fiddler crabs are largely diurnal, active at low tide, and gregarious. They feed by filtering bits of organic matter from the shore surface. The small claw, which is specialized for lifting mud or sand, rhythmically brings pinches to the mouthparts. Here food is separated from the mineral matter which, accumulating outside, is periodically wiped off and dropped to form a growing line of pellets.
Male fiddler crabs wave their "major claw" (the large claw) to attract females and repel male rivals. Males may also produce sounds by stridulation and substrate-thumping and even build structures near their burrows to help attract mates. Mating generally occurs once the male has led the female into his burrow (Brusca and Brusca 2003). A variety of videos showing fiddler crabs feeding, displaying, and mating can be viewed at http://www.fiddlercrab.info/uca_video.html.
Fertilized female fiddler crabs carry hundreds or thousands of eggs beneath the abdomen (females in this condition are sometimes known as "sponge" crabs). When the eggs are ready to hatch, the female enters the water and the eggs release microscopic free-swimming larvae. These early stage larvae are known as zoeae or zoeas (singular: zoea). The larvae live in the open water as part of the plankton. As they grow, they pass through a number of molt stages. Older post-larval crabs are known as megalopa (plural: megalopae, megalopa, or megalopas). At the end of the final larval stage, the larvae molt into immature crabs. The amount of time spent as a swimming larva (i.e, from hatching to true crab stage) varies among species, but ranges from a few weeks to a few months. The crabs return to land and begin to grow. Juvenile male and female crabs look alike. As they grown larger and mature into adults, the secondary sexual characteristics (such as the asymmetric claws) begin to develop. Adult crabs mate and the cycle starts over again.
The approximate geographic distributions of the various fiddler crab species of the world (based largely on Crane 1975 but updated with new information up to around 2005) are available at http://www.fiddlercrab.info/uca_maps.html. A comprehensive list of fiddler crab literature references through around 2006 is available from http://www.fiddlercrab.info/.
Fiddler crabs have been the subject of many investigations of behavior, especially with respect to sexual selection and the function of the male's enlarged claw . Summaries of a sampling of such research can be found on the Science News website.
(Crane 1975; Rosenberg 2001; www.fiddlercrab.info)