THE IMPACT OF RIVER IMPOUNDMENT AND OTHER HUMAN ACTIVITY ON THE SHELL SHAPE OF A NORTHERN NEW YORK FRESH WATER BIVALVE
In this study we compare shell morphologies of unionids from two rivers located in St. Lawrence County, northern New York. We collected over 420 specimens of the unionid Elliptio complanata from 11 localities along the Raquette River, and 8 localities along the Oswegatchie River. The sampled reach of the Raquette River includes eight dams and runs through a more developed area than the Oswegatchie River, which is disrupted by only three dams in the study area. We hypothesize that the damming and anthropogenic land use of the riparian zone impacts the unionid shell morphology and, thus, we should see a difference in shell shape in samples from the two rivers.
Shell outlines were captured with 60 semi-landmarks. The fitted landmark data was analyzed with a principal component analysis and PC scores were compared with Mann Whitney U tests; all localities include more than 25 specimens.
Overall, 74% of the variation can be explained by PC1 expressed by a straightening of the posterior portion of the hinge line and posterior-ventral lengthening of the shell. A shift from the average shell outline to a more oval shape with a straight ventral and dorsal margin (PC2) represents 12.78% of the variation.
Shell outlines of samples from the Raquette River are significantly different than the material from the freer flowing Oswegatchie River (p=1.998E-25). Interestingly, samples from the less impounded river show more variation in shell shape than the Raquette River material. Frequent impoundments and urbanization of the riparian zones lead to a loss of habitats and this might be a cause for the observed homogenization of shell shape in the examined unionids from the Raquette River.