Distribution of Fiddler Crabs in Georgia Salt Marshes

Abstract

The study of the distribution of animals in nature is especially interesting when several species of one genus occupy the same general area. Along the east coast of the United States there are three species of fiddler crabs of the genus Uca which live in the salt marshes. All three are found in the marshes surrounding Sapelo Island, Georgia, and this paper deals with experiments concerning their distribution in these marshes. I deal with the problem of why the crabs live in certain habitats within their range and not in others, not what determines or has determined their geographical range. The three species of crabs are: Uca mtinax, the largest, generally associated with brackish water; CUca ptigilator, commonly known as the sand fiddler crab, and Uca. pitgnax, found generally throughout the salt marsh. Components of the environment which might be important in determining the distribution of these animals are substratum, food, salinity, exposure or tides, and other animals. The effects of these factors can be evaluated by answering the questions: W~hat is the substratum choice of the animals when other factors are constant? Does the presence of other species of fiddler crabs affect this choice? Can the three species survive in all types of salt marsh or are they restricted to certain types ? What are the salinity and temperature tolerances and salinity preferences of the species ? \Vlhat do the crabs feed on and how is this food distributed in the marsh? Adult animals were used in most of the experiments. It is true that choice of area for settling by the megalops larvae is important in the distribution of the adults; nevertheless, adult fiddler crabs can and do move considerable distances over the marsh and could easily move to an environment different from that chosen by the larvae.