Although fiddler crabs are territorial, the species is quite social and lives in large groups. Abundance estimates are limited to U. rapax in the IRL, but populations in some Brazilian mangrove forests reach about 20 individuals per square meter (Koch et al. 2005).Molting & Limb Regeneration: Like other arthropods, fiddler crabs must molt in order to grow larger. This process, known as "ecdysis", occurs most frequently in fast-growing juveniles and slows during adulthood (da Silva Castiglioni et al. 2007). During ecdysis, the hard exoskeleton is shed in one piece, exposing the new, soft underlying skeleton. Water is pumped into the body to expand the size of the new exoskeleton before it hardens (eg. Guyselman 1953). Molting is not only used for growth, but also to regenerate missing limbs. During combat or to escape predators, fiddler crabs autotomize or cast off limbs at a predetermined point (Weis 1977), usually at the base of all walking legs (Hopkins 2001). New limbs grow in a folded position within a layer of the cuticle, unfolding and expanding during the molting process. Ecdysis is triggered and accelerated by multiple autonomy and removal of the eyestalks (Abramowitz & Abramowitz 1940, Hopkins 1982). Molting under these circumstances may not result in growth, and the overall size of the crab may even decrease as energy is used to regenerate several missing limbs (Hopkins 1982). A single molt in some individuals is often enough to completely regenerate a missing limb (Hopkins 2001), but other crabs may require several molts before an appendage is restored to its original size. Several factors affect the frequency and success of molting and limb regeneration, including food availability, temperature and pollution. For example, the presence of methylmercury in polluted waters can partially or fully inhibit regeneration of limbs in both temperate and tropical fiddler crabs (Weis 1977). When compared to U. pugilator and U. thayeri, the mudflat fiddler regenerated limbs and hardened its carapace more quickly after ecdysis when inhabiting waters with heavy metal pollutants.
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