MEPS 288:211-220 (2005)  -  doi:10.3354/meps288211

ABSTRACT: Uca pugnax and U. pugilator are common fiddler crabs in salt marshes on the Atlantic coast of the United States. As adults, U. pugnax frequent muddier, vegetated (typically Spartina alterniflora) substrate while U. pugilator usually occupy sandier, open habitats. It is unclear where juvenile U. pugnax and U. pugilator reside 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 postlarval U. pugnax and U. 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 spanned S. alterniflora-covered mud and open sand habitats with adult populations of U. pugnax and U. pugilator, respectively. Over 75% of the juveniles collected were U. pugnax, showing greater recruitment by this species. U. pugnax juveniles of all sizes preferred the same muddy habitat occupied by adults, but habitat preferences of juvenile U. pugilator varied by site. Generally, U. pugilator displayed a shift in distribution from S. alterniflora cover to sandier habitat during early juvenile stages. The younger stages may prefer S. 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. pugilator develops specialized mouthparts to scrape organic matter from larger sand grains, but these are not present in early juveniles nor in U. pugnax juveniles. Although young juvenile U. pugnax strongly favored S. alterniflora cover, older juveniles (those large enough to dig burrows for protection) were occasionally found in sandier habitat with U. pugilator.

KEY WORDS: Uca pugnax · U. pugilator · Postlarval settlement · Restriction fragment length polymorphism · RFLP