Unravelling the diversity of limnic molluscs in the Parnaíba River Basin, Brazil: can ecoregions influence species composition?

Molluscs are among the most threatened freshwater organisms in the world; yet, their occurrence and distribution in most ecosystems on Earth remain unknown. This study aimed to survey molluscs in the Parnaíba River Basin, Piauí, Northeastern Brazil, with the goal of understanding the variation in species richness among rivers and the role of ecoregions in their composition. Molluscs associated with macrophytes and sediments were collected from the upstream, middle, and downstream of each of the main rivers in the Basin. Species richness was compared between the rivers and the ecoregions in which they are located. Our study is the first conducted in the Northeast region at the river basin level. It reveals the diversity and complexity of freshwater mollusc assemblages, challenging the idea of low species richness attributed to the Brazilian Northeast and semiarid regions, documenting 10 genera and 12 species of molluscs in the Parnaíba River for the first time. Although no endemic species have been reported for the basin, the species unique to each ecoregion reflect the environmental complexity of these areas. The increasing environmental degradation affecting the region makes it essential to carry out further studies to document the diversity of molluscs in the basin area.