This paper comprises: 1) A note on Systematists of fiddler crabs from the Bay of Rio de Janeiro, with its new species Uca olympioi, Uca salsisitus, and a new variety U. pugnax var. brasiliensis. 2) Female Uca characterization is easier and infuse certainness when we inquire her third maxillipod. 3) Redescriptions are made for type-locality animals at Rio de Janeiro and nearness. 4) Male characterization become possible through its maxillipod if we lose its quelipod. 5) A historical referendary originating into indigenous dialect « Guarany » about the name Uca and brasilian nicknames of the fiddler crabs. 6) A few Uca species from the Rio de Janeiro determinative keys asked after main distinctions among others species described over the world in paragraph « diagnose diferencial ». 7) A collected material catalogue and some mensurement tables.