Abstract Fiddler crabs are important components of intertidal benthic macrofauna in estuarine habitats. However, these invertebrates often inhabit areas under anthropogenic-driven changes. In this study, we evaluated the habitat quality and the population biology of the fiddler crab Uca maracoani (Latreille, 1802) in a mangrove area under anthropogenic influence, on the southern coast of São Paulo state, Brazil. Sediment samples for ecotoxicological analyses and data on the population biology of U. maracoani were collected monthly between October 2008 and October 2009. Contamination was observed in sediment samples from all months, indicating that benthic organisms inhabiting the study area are continuously exposed to toxicity. The sex ratio did not deviate from 1:1 for the whole population of U. maracoani, within any given month (except in October 2008, when males outnumbered females). Adult males and females showed a similar size, while juvenile females were larger than juvenile males, probably reflecting that females attained sexual maturity at a larger size than males. For both juvenile and adult stages, the major cheliped propodus length and height in males and the abdomen width in females showed positive allometry. The size-frequency distribution of the studied population varied from unimodal to bimodal. Juveniles were recorded in all months, suggesting the studied population has continuous recruitment. In contrast, ovigerous females were not found in the study area for several months. Although U. maracoani is continuously exposed to contamination present in the sediment, it presents a well-established population and seems to persist in the study area.

Stalked barnacles Octolasmis lowei Darwin, 1851 are frequently found attached to decapod crustaceans. Their epibiotic association depends on many factors, which are mainly related to characteristics of the host's biology. This study evaluated the infestation and distribution of stalked barnacles in the branchial chambers of crabs, and analyzed the data with respect to the host's sex, maturity stage, molt cycle and size. The crab species Arenaeus cribrarius Lamarck, 1818, Callinectes danae Smith, 1869, Callinectes ornatus Ordway, 1863, Hepatus pudibundus Herbst, 1785, Libinia ferreirae Brito Capello, 1871, and Persephona punctata Linnaeus, 1758 were sampled and found to be infested by O. lowei. No juvenile crabs were infested. The prevalence of infestation by O. lowei was significantly different among C. danae, C. ornatus, and H. pudibundus males and females. All infested hosts were in the intermolt period. The mean size of infested crabs was larger than that observed for non-infested individuals. Internally, stalked barnacles were concentrated on the central gills or walls and floor of branchial chambers, suggesting that these gills provide more favorable conditions for the settlement and development of these epibionts. These results highlight the relationship between epibiont infestation and host biology, as well as the role of decapod crustaceans as a suitable substrate for the development of stalked barnacle O. lowei.