An adult male specimen of Uca rapax (Crustacea, Decapoda, Ocypodidae), containing bilaterally hypertrophied chelipeds, was found in the mangrove area of the Rio Grande do Norte state, north-eastern Brazil. The chelipeds are subequal in size and similar to the major cheliped of normal male specimens of Uca rapax. This paper is the first record of the aforementioned anomaly for the species from the Brazilian coastline.

To assess and analyse accurately their geographical distributions, fiddler crabs (Uca) from the Atlantic coast of Brazil were studied using field collections, museum specimens and literature reports. More than 7000 specimens of ten Uca species were collected across five coastal biomes from 63 sites on the Atlantic coast of South America between the Brazilian States of Amapá and Santa Catarina in 2009 and 2010. Based on our analyses of mouthparts and habitat osmolality (=salinity), each species seems to prefer a particular niche defined by a specific array of physical factors. The species' distributions form four geographical patterns along the South Atlantic coast; however, the different species do not occur uniformly within each biome. Most common are U. (M.) rapax, U. (L.) leptodactyla, U. (B.) thayeri and U. (U.) maracoani with the relative abundance of each varying among the biomes according to habitat availability. Owing to their high constituent diversity, two geographical areas are candidates for biological conservation efforts. Considering fossil records, molecular clocks, oceanic hydrology and plate tectonics over the last 65 million years, the biogeographical patterns of the extant species can be used to reconstruct an evolutionary scenario for Uca in the tropical and temperate zones of the western Atlantic Ocean.

In this study, four species of fiddler crabs (Uca) at Inhaca Island, Mozambique, were studied: their fecundity determined and egg loss between the first and last embryonic development stage evaluated. Ovigerous female crabs were collected throughout February to March (wet season) of 2006, during day-time at low tide, in Ponta Rasa (U. annulipes and U. inversa) and Saco (U. chlorophthalmus and U. vocans) mangroves. Average brood size increased significantly with female and species size, ranging from 932 (CW = 8.51 mm) in U. annulipes, to 18,057 (CW = 23.5 mm) in U. vocans. The number of embryos decreased significantly over the incubation period for each species. Brood loss from embryonic stage I to stage IV for U. annulipes, U. inversa, U. chlorophthalmus and U. vocans was 12.3%, 14%, 43.2% and 23.6%, respectively (P < 0.05). Positive allometry was recorded for all species in the early development stage, however, due to the higher relative brood loss in larger females (U. vocans), when comparing the number of embryos in late developmental stage, a negative allometry was possible to observe. The average volume of embryos at the same stage differed among species although significant differences were found between embryonic stages for all species. Embryo volume increased 96.1%, 93.3%, 92.9% and 96.3% in, respectively, U. annulipes, U. inversa, U. chlorophthalmus and U. vocans (P < 0.05). Differences in brood loss were attributed to habitat substrate and structure, stress factors and behavioural aspects.

Dual stable isotope analyses (δ13C and δ15N) of fiddler crabs from a forest-fringed, land-locked lagoon in Puerto Rico indicated the differential assimilation of material from ingested sediments. Fiddler crabs preferentially selected for nitrogen-fixing benthic microalgae (cyanobacteria) over vascular plant detritus. These results question the assumption that mangrove detritus is always the principal source of energy to estuarine consumers. Previous research from this lagoon as well as from Amazonia suggests that the magnitude of land-water ecotonal coupling may be low for these particular tropical systems where benthic algal productivity is presumably high.