Unionid growth patterns in rivers of differing riparian vegetation

1. Growth rates were examined as potential controlling agents of unionid distributions.
2. Analyses of long-term growth in unionid shells revealed two distinct patterns. Species which dominate rivers with forested riparian vegetation (e.g. Elliptio dilatata ) show slow growth throughout life, whereas species which dominate grassy rivers (e.g. Pyganodon grandis ) show rapid growth during early life and achieve a smaller maximum size at a younger age.
3.  Pyganodon grandis , which occurs in both river types but is much more abundant in grassy habitats, demonstrates a single growth pattern in both basin types. In contrast, Lasmigona complanata (Barnes, 1823), which occurs equally in both basin types, demonstrates two growth patterns which differ depending upon which river is inhabited. In forested rivers, L . complanata grow slow and steady, whereas in grassy rivers, these unionids grow faster and achieve maximum size earlier.
4. Short-term transplant studies of P . grandis and E . dilatata confirmed this pattern of growth for specialist species.