The relative importance of substratum characteristics and recruitment in determining the spatial distribution of the fiddler crab Uca uruguayensis Nobili

Abstract

The roles of sediment characteristics and the pattern of recruitment in influencing the abundance of the fiddler crab Uca uruguayensis on Argentinean mudflats were evaluated. The density of adult crabs showed a patchy distribution related to the sediment thickness (depth at which a layer of fossil shells are buried), but the density of juvenile crabs was not coupled with the density of adult crabs. In a field experiment, fossil shells were removed and the density of crabs significantly increased, which demonstrates that the presence of the layer of shells is a structure that may hinder the establishment of burrows. The density of crabs was related to sediment thickness, sediment torque and organic matter content. The importance of each of these variables was different for adult and juvenile crabs, indicating that the distribution of adult crabs may be caused by mechanisms affecting adult crabs themselves and is not established by the recruitment pattern. Moreover, in a field experiment, the density of juveniles decreased when adult crabs were added, and increased when adult crabs were removed. The morphology of burrows was related to sediment characteristics. Burrows were deeper, longer and more voluminous when sediment thickness was high. The volume of burrows decreased with increasing sediment torque. These results suggest that the morphology of burrows is related to the space available and the ease with which sediment it can be excavated. However, an important amount of variability remained unexplained, suggesting the presence of additional environmental variables or behavioural plasticity not considered by this study. Together, these results demonstrate that the spatial heterogeneity in the environmental factors can be translated to a spatial heterogeneity in the distribution of fiddler crabs.