Species composition and monthly variation of the Molluscan fauna in the freshwater subtidal area of the River Minho estuary

Despite their high ecological and environmental importance, little attention has been devoted to the study of freshwater tidal estuarine areas. Information about the biodiversity of these ecosystems remains scarce and very fragmented. In this study, the molluscan fauna of three sites located in the freshwater subtidal area of the River Minho estuary (NW of Portugal) was surveyed monthly between January and December 2005. The molluscan structure showed significant differences between sites and months of the year. A total of 14,684 specimens from 15 different species were identified. Abundance ranged from 304 to 3500 ind./m2, with an annual mean of 1632 ind./m2. Biomass ranged from 23.4 to 425.4 g AFDW/m2, with an annual mean of 167.7 g AFDW/m2. The non-indigenous invasive species Corbicula fluminea (Mu¨ller, 1774) had a clear predominance in the total abundance and biomass gathered. The multivariate analysis used revealed a community with three distinct groups, principally related to sediment characteristics. Due to great abundance and biomass recorded, C. fluminea is a potential key species in this estuarine ecosystem and its possible biological and environmental impacts need urgent investigation.