Contrasting decay rates of freshwater bivalves' shells: Aquatic versus terrestrial habitats

Freshwater flow regimes are particularly vulnerable to global climate change with changes to the volume and regime of water contributing to global declines in freshwater biodiversity. Droughts or floods can cause massive mortalities of freshwater bivalves, facilitating the accumulation of shells in the aquatic but also in adjacent terrestrial habitats. In order to fully understand the long term impact of these massive mortality events, it is important to assess how bivalve shells persist in the environment. Given that, the present study aimed at studying the shell decays of four different bivalve species (Anodonta anatina, Corbicula fluminea, Potomida littoralis and Unio delphinus) in aquatic (i.e. river) versus terrestrial (i.e. sand soil) habitats. Shell decay rates were significantly different among species and habitats. In the aquatic habitat the shell decay rates varied among species, with the native species A. anatina, which have the largest and thinnest shell, showing the highest decay rate. Alternatively, in the terrestrial habitat the shell decay rates were more even among species and not related to a particular shell feature or morphology, with the native U. delphinus showing the fastest decay. The shell decay rates were 6 to 12 times higher in aquatic than in the terrestrial habitat. These results suggest that bivalve shells can persist for long periods of time on both habitats (but mainly in terrestrial), which may perhaps trigger significant changes on the ecosystem structure and functioning.