Invasive mollusks Tarebia granifera Lamarck, 1822 and Corbicula fluminea Müller, 1774 in the Tuxpam and Tecolutla rivers, Mexico: Spatial and seasonal distribution patterns

The Tuxpam and Tecolutla rivers in the Gulf of Mexico, are located in the Mesoamerican biodiversity hotspot and support different human activities: crude oil extraction, agriculture and livestock, that provoke environmental disturbances. Aquatic mollusks, physicochemical water characteristics and substrate type were examined along the main watercourse of these rivers during three periods of one year: the wet season, dry season, and during the northern winds season, when hurricanes are more frequent. In both rivers, physicochemical water characteristics, types of substratum and mollusk fauna demonstrated environmental gradients and differentiate two river zones: freshwater and estuarine. Differences were also found between the dry season, with higher inorganic salts content, and the wet and northern winds and hurricane seasons, when inorganic and organic nutrient inputs occurred. The mollusk fauna is composed of nine and eleven taxa in the Tecolutla and Tuxpam rivers, respectively. In both rivers, the introduced gastropod Tarebia granifera is the dominant species in the freshwater zone with regard to population density and the area it covers within the ecosystem, followed by the introduced bivalve Corbicula fluminea. Native mollusk species were confined to point-locations and attained very low densities. The gastropod Neritina virginea and the bivalve Brachidontes exustus were dominant in the estuarine zone of both rivers. Mollusk population densities declined during the wet, northern winds and hurricane seasons, while in the dry season both alien species reached higher densities, which could indicate that alien mollusks are removed by effect of climatic events. T. granifera and C. fluminea exhibit traits characteristic of invasive species and pose a risk to native mollusk biodiversity in these rivers.