Latitude and environmental pressure: their impact on carapace morphology in three species of fiddler crabs (genus Uca) across the Atlantic Ocean

Among semi-terrestrial fiddler crabs, three species from the subgenus Uca (sensu stricto) appear to be basal in the phylogeny of the genus: Uca major (Herbst 1782-1804), Uca maracoani (Latreille 1802-1803) and Uca tangeri (Eydoux 1835). These species evolved vicariantly as tectonic plates separated to form the Atlantic Ocean. Currently, the three species, found in Africa, South America, and the Caribbean, exhibit a high affinity for their respective tectonic plates. Interand intra-specific variation in carapace shape was assessed using geometric morphometrics to analyze 12 surface landmarks in the three species. In 314 female specimens, surficial coordinates were superimposed, standardized, and subjected to canonical variance (CVA) and principal component (PCA) analysis, after which the three species form distinct clusters. Within each species, there are clear differences among populations from northern to southern latitudes. In Uca maracoani, there is a significant correlation between carapace shape and certain environmental factors, such as biotope, salinity, and substrate. As it has not been demonstrated that there is extensive, structured geographic variation in the population genetics for Uca, this polymorphism is likely a result of environmental pressures, i.e., ecophenotypy.