Macroinvertebrates as biomarkers are discussed, with emphasis on decapod crustaceans of infraorder Brachyura (crabs), and the efficiency of these organisms to reflect the conservation status of natural environments, with emphasis on estuaries and mangrove habitats. The significant increase of the human population and the intensification of anthropic activities have damaged and impacted many ecosystems in our planet, generating a diversity of xenobiotics. Some species of macroinvertebrates have been used as bioindicators, reflecting the presence of xenobiotics and their harmful effects in the environment. As an example, the mangrove crab Ucides cordatus has been recently used to monitor the conservation status of mangrove ecosystems. Owing to a wide interaction of this crab with different environmental matrices, the use of biomarker protocols, through hemolymph samples, allows evaluation of the effect of stressors promoted by a set of pollutants. Physiological damage has been revealed by Neutral Red Retention Time (NRRT) and also by micro or macro DNA damage, using the Comet (CO) and micronucleus assays (MN‰), respectively. These biomarkers are discussed, assessing their applicability and efficiency for a range of crabs.