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Fifteen species of fiddler crabs are reported for eastern North America between Massachusetts and Quintana Roo, Mexico. Thirteen occur in the United States and 11 in Mexico, with eight in common to the two countries. Of 13 species in the Gulf of Mexico, five are endemic and a sixth is restricted largely to the peninsulas of Florida and Yucatan. The status of U. rapax in the northern Gulf remains to be resolved. Range limits of most species approximate one or the other of two sets of intersecting thermal and geological boundaries that subdivide the Gulf of Mexico along north‐south and east‐west axes. Species belonging to subgenus Minuca tend to replace one another at the thermally‐controlled Carolinian‐Caribbean marine biotic boundary across the Florida peninsula and northern Gulf. However, only U. minax of all the North American fiddler crabs exhibits the classical disjunct Carolinian distribution, and this appears basically to reflect the discontinuous distribution of temperate salt marshes that are the habitat of the species. Distributions of species belonging to subgenus Celuca adhere for the most part to the subdivision of the Gulf into western terrigenous and eastern carbonate sedimentary provinces. The northern transition occurs in the vicinity of Apalachee Bay and the southern at Laguna de Terminos. A third distributional pattern is shown by U. subcylindrica, a specialized endemic species of the hypersaline Laguna Madre system of the western Gulf. The level of endemism in the fiddler crabs is relatively high in comparison with that of other marine groups within the Gulf of Mexico. This may be a consequence of the adaptations of fiddler crabs as specialized deposit feeders to regional differences in climatic and edaphic characteristics of a marginally marine upper shore habitat. The distributional patterns of the endemics could prove useful in reconstructing palaeoecological events of evolutionary significance within the Gulf of Mexico.

Comparatively large ova, low fecundity and low per capita egg production are adaptations to terrestrial habits in Uca snhcylindrica. An unusual morphology of the genitalia appears to have co-evolved with the necessity for a lecithotrophic egg. Isolated populations are apparently sustained by rapid larval development. The crab stage population has a bimodal size-frequency distribution with an intermediate sex ratio pattern. Initially the sexes are equal, but males are more common in the larger size classes. Survival into the crab stage is relatively low. There is an increased rate of mortality with size. Only 30% of the individuals grow to modal carapace size; even fewer appear to participate in reproduction.

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Species with larger geographic distributions are more likely to encounter a greater variety of environmental conditions and barriers to gene flow than geographically-restricted species. Thus, even closely-related species with similar life-history strategies might vary in degree and geographic structure of variation if they differ in geographic range size. In the present study, we investigated this using samples collected across the geographic ranges of eight species of fiddler crabs (Crustacea: Uca) from the Atlantic and Gulf coasts of North America. Morphological variation in the carapace was assessed using geometric morphometric analysis of 945 specimens. Although the eight Uca species exhibit different degrees of intraspecific variation, widespread species do not necessarily exhibit more intraspecific or geographic variation in carapace morphology. Instead, species with more intraspecific variation show stronger morphological divergence among populations. This morphological divergence is partly a result of allometric growth coupled with differences in maximum body size among populations. On average, 10% of total within-species variation is attributable to allometry. Possible drivers of the remaining morphological differences among populations include gene flow mediated by ocean currents and plastic responses to various environmental stimuli, with isolation-by-distance playing a less important role. The results obtained indicate that morphological divergence among populations can occur over shorter distances than expected based on dispersal potential.