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The Rio de la Plata. located between Uruguay and Argentina, generates a tremendous freshwater influx from the estuary into the coastal sea. It is thus suggested to constitute a biogeographic barrier for many taxa exhibiting a marine planktonic larval dispersal. However, evidence for corresponding intraspecific dispersal constraint has yet to be provided. We compared mitochondrial haplotypes of ten individuals for each of two populations of Uca (Leptuca) uruguayensis north and south of the estuary to examine the potential effect of the Rio de la Plata on the species' distribution. The populations are separated by nearly 2,000 km and were collected in São Paulo State (Brazil) and Mar Chiquita (Argentina). We found no significant indication for restricted gene flow between them, based on 823 basepairs of the mitochondrial cytochrome c oxidase subunit I gene. The structure of the corresponding haplotype network and the estimated nucleotide diversities, however, suggest that the Argentinean population is genetically more diverse than the one from Brazil. In order to establish possible significant differences in haplotype distribution, further research including more populations and larger sample sizes will be necessary.

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This effort was funded by the Louisiana Coastal Protection and Restoration Authority's (CPRA) Barrier Island Comprehensive Monitoring program. We would like to thank Syed Khalil (CPRA) for numerous helpful discussions about the habitat mapping methods and analysis for this project. We thank John Barras for his assistance with the development of coastal reach boundaries for this effort. We are grateful to Hana Thurman (Cherokee Nation Technologies) for her assistance with designing the cover graphic. Finally, we thank Syed Khalil, Glen Curole (CPRA), Brady Couvillion (U.S. Geological Survey; USGS), and Richard Day (USGS) for reviewing this report.