This study was conducted in the Itacuruçá mangrove, in the Sepetiba Bay, Rio de Janeiro, Brazil, with the objective of investigating the population structure of the species of Uca Leach, 1814. Sampling was carried out monthly from June, 2005 to May, 2006, during low tides. Crabs were captured manually by two people for a period of 15 min each. A total of 2580 crabs were obtained, of which 1465 were males and 1115 were females. The size of the fiddler crabs in the mangrove of Itacuruçá were the smallest reported so far in Brazilian mangroves. However, males attained a larger size than females. The size frequency distribution was unimodal for most of species, with the exception of U. thayeri Rathbun, 1900 and U. vocator (Herbst, 1804), which did not show a clearly-defined pattern. Males were most abundant among the largest size classes. The sex ratio differed significantly from 1:1 (male: female), most often with a higher frequency of males, except for U. thayeri and U. victoriana von Hagen, 1987, in which females were predominant.
Fiddler crab; frequency distribution; Sepetiba Bay; sex ratio; size
