GENETIC DIFFERENTIATION IN TRANS-FLORIDIAN SPECIES COMPLEXES OF SESARMA AND UCA (DECAPODA, BRACHYURA)

Most nearshore brachyuran crabs of the northern Gulf of Mexico have been treated as conspecifics of those in warm-temperate Carolinian waters on the southeastern Atlantic coast of North America. However, historical physiographic constraints appear to have periodically restricted gene flow between northern Gulf populations and sibling Atlantic coast populations, and contemporary disjuncture of ranges often persists across south Florida. The present examination of intertidal complexes has centered on grapsid crabs presently assigned to Sesarma reticulatum and ocypodid crabs assigned to Uca minax, 2 species in which we have observed marked variations in coloration over their distributional range. Genetic differentiation between populations has been assayed by allozyme electrophoresis, and resultant data have been evaluated with F-statistics and cluster analysis of genetic distance. Allozyme divergence between Gulf and Atlantic populations of the S. reticulatum complex is at levels previously reported for speciated populations, while that between trans-Floridian populations of U. minax is much less pronounced. In both species, minimal divergence can be measured between 2 widely separated Atlantic coast sample localities that were compared, while more complex grades of differentiation are evident between sample localities compared within the northern Gulf of Mexico. Trans-Floridian divergence of populations for both of these species is compatible with models for periods of contact and subsequent isolation of Gulf and Atlantic stocks during and since peak glacial advances in North America. Less conspicuous patterns of genetic differentiation between sample localities within the northern Gulf of Mexico may reflect a history of glacial and postglacial alluvial events which resulted in contemporary physiography of nor-them Gulf estuaries.