Research biases are common and constant issues in biological research, with major consequences forbiodiversity conservation. Freshwaters represent one of the most threatened ecosystems worldwide, butknowledge of freshwater biota is unevenly distributed due to bias toward marine and terrestrial groups.Therefore, detecting biases and associated knowledge gaps is crucial to steer future research effort andto guide applicable conservation policies for freshwater ecosystems. In this study, we investigated theexistence of biases and gaps in knowledge about the biodiversity of invertebrate fauna (zooplankton,mollusks and freshwater crabs) in the hydrographic regions of Brazil. We searched for all studies pub-lished in this country on zooplankton, mollusks and freshwater crabs, in the Web of Science database.Subsequently, the number of papers was classified by taxonomic group and hydrographic region to detectresearch biases. Each report within a paper between a given taxonomic group and a hydrographic regionwas termed as a case. We also recorded human population density for each hydrographic region. Amongthe taxonomic groups, zooplankton was the most studied taxon, followed by mollusks and crabs, and itwas also dominant across the hydrographic regions. The hydrographic region of Paraná comprised thelargest number of cases for the three invertebrate groups. We detected a disproportionately low increaseof number of cases in relation to human population density in the hydrographic regions. The identificationof the major gaps reported here limits our ability to draw scenarios for the conservation of hydrographicregions and their megadiverse biota in Brazil.
