Unionid growth patterns in rivers of differing riparian vegetation

1. Growth rates were examined as potential controlling agents of unionid di stributions. 2. Analyses of long-term growth in unionid shells revealed two distinct pat terns. Species which dominate rivers with forested riparian vegetation (e.g . Elliptio dilatata) show slow growth throughout life, whereas species whic h dominate grassy rivers (e.g. Pyganodom 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 abu ndant in grassy habitats, demonstrates a single growth pattern in both basi n types. Ln contrast, Lasmigona complanata (Barnes, 1823), which occurs equ ally in both basin types, demonstrates two growth patterns which differ dep ending upon which river is inhabited. In forested rivers, L. complanata gro w 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 th is pattern of growth for specialist species.