Post-flood strandings of unionid mussels

I studied widespread strandings of unionid mussels in normally terrestrial habitats, which were inundated during Mississippi River flood of 1993. Stranded mussels included three species - Anodonta grandis, Potamilus ohiensis, and Leptodea fragilis. Anodonta grandis and P. ohiensis became stranded more often than would be expected if all species from possible source areas had equal chances to become stranded. Furthermore, stranded A. grandis had smaller shell lengths than those from the nearest source area. This suggested that the strandings were not random in relation to species or to size cohorts of one particular species. Specific gravity was determined for 15 species of unionid mussels. The three species found stranded had the lowest specific gravities among these 15 species. Low specific gravity, which could predispose species to flood transport, could explain the observed strandings.