CHANGES IN SHELL SIZE AND SHAPE OF NATIVE FRESHWATER MUSSELS ASSOCIATED WITH THE NORTH AMERICAN INVASION OF ASIAN CLAMS (CORBICULA FLUMINEA)

The Asian Clam (Corbicula fluminea) is suspected of being a factor in the decline of native freshwater mussels (Unionida) since its arrival in North America in the early- to mid-1900s. One proposed mechanism is food competition, but few biological monitoring data exist contemporaneous with the initial invasion. We used historical mussel specimens from the Ohio State University Museum of Biological Diversity to evaluate changes in mussel shell size and shape during and after Asian Clam invasion. We focused on Big Darby Creek, Ohio, which experienced a mussel decline since the 1960s but continues to support a rich mussel fauna. Asian Clams colonized the creek by 1986, and the creek has experienced neither major changes in temperature and precipitation nor other anthropogenic stressors clearly associated with the mussel decline. We obtained shells of adult mussels from three time periods: 1982, 1996-2000, and 2015, and for three species: Eurynia dilatata, Lampsilis siliquoidea, Ptychobranchus fasciolaris. We measured shell length, width, height, and mass of each specimen. To control for potential differences in age structure among the samples, we calculated three standardized variables for each specimen: width/length (W/L), mass/mean length (M/L), height/length (H/L). Mean W/L, H/L, M/L of E. dilatata decreased across the timeseries. P. fasciolaris showed no changes in the H/L variable over time, while W/L and M/L decreased between 1996-2000 and 2015. L. siliquoidea showed no significant changes in any variable over time. Our results show that, during and following the invasion of Corbicula, changes in mussel shell size and shape were variable and species-specific. We do not know if or how shell morphology may be related to potential food competition with Asian Clams or any other anthropogenic stressor. However, historical shells in museum collections represent a valuable resource for examining a wide array of questions related to mussel conservation and ecology.