RANGE EXTENSION OF THE GIANT FLOATER (PYGANODON GRANDIS) INTO THE MAINSTREAM RIO GRANDE IN THE SAN LUIS VALLEY, COLORADO.

Giant floater (Pyganodon grandis) is native in Colorado to the eastern plains of the South Platte, Republican, and Arkansas River basins, but has never been reported from the Rio Grande basin in Colorado. Here we report for the first time, the presence of a naturally reproducing, giant floater population in the mainstem Rio Grande reach extending at least 24.6 km upstream into Colorado from the Colorado/New Mexico border. This floater population is not native to the Rio Grande basin in Colorado and is one of the few giant floater populations known to be extant in Colorado. Other native Mollusca found with the giant floater in this reach of the mainstem Rio Grande included the creeping ancylid (Ferrissia rivularis), and the striated fingernail clam (Sphaerium striatinum). Five species of pea clam were also collected in substrate dominated by sand and silt, including the ubiquitous pea clam (Euglesa casertana), ridged-back pea clam (E. compressa), triangular pea clam (E. variabilis), Adam pea clam (E. adamsi), and questionably the Walker pea clam (E. walkeri); the last two of which have not been reported in Colorado in prior surveys. A need exists to effectively, and routinely, sample and monitor all mollusk populations in Colorado. [ABSTRACT FROM AUTHOR]