Distribution of juvenileUca pugnaxandU. pugilatoracross habitats in a South Carolina estuary, assessed by molecular techniques

Uca pugnaxandU. pugilatorare common fiddler crabs in salt marshes on the Atlantic coast of the United States. As adults,U. pugnaxfrequent muddier, vegetated (typicallySpartina alterniflora) substrate whileU. pugilatorusually occupy sandier, open habitats. It is unclear where juvenileU. pugnaxandU. pugilatorreside because the early crab stages of these species are difficult to identify by simple gross morphology. Using a novel restriction fragment length polymorphism (RFLP) protocol to distinguish postlarvalU. pugnaxandU. pugilator, we studied their distribution along a horizontal gradient in the North Inlet Estuary, South Carolina. We collected juvenile crabs along transects at 3 different sites that spannedS. alterniflora-covered mud and open sand habitats with adult populations ofU. pugnaxandU. pugilator, respectively. Over 75% of the juveniles collected wereU. pugnax, showing greater recruitment by this species.U. pugnaxjuveniles of all sizes preferred the same muddy habitat occupied by adults, but habitat preferences of juvenileU. pugilatorvaried by site. Generally,U. pugilatordisplayed a shift in distribution fromS. alternifloracover to sandier habitat during early juvenile stages. The younger stages may preferS. alterniflora-covered, muddier habitat because it provides better cover from predators, or so that they can avoid displacement by currents during high tides; alternatively, they may be able to feed better on muddy sediment.U. pugilatordevelops specialized mouthparts to scrape organic matter from larger sand grains, but these are not present in early juveniles nor inU. pugnaxjuveniles. Although young juvenileU. pugnaxstrongly favoredS. alternifloracover, older juveniles (those large enough to dig burrows for protection) were occasionally found in sandier habitat withU. pugilator.