Morphometric analysis of Carychium exile and Carychium exiguum in the Great Lakes region of North America

Statistical analyses were conducted on shell heights and widths of 5177 Carychium exile and 1178 Carychium exiguum individuals from 141 locations across a 1300 x 1000 km region centered on the North American Great Lakes. Both within co-occurrence sites, and between all sampled populations, the shell dimensions of these taxa were bimodally-distributed and statistically distinct. Thus, C. exile and C. exiguum are clearly separated and represent different species. Continuous latitudinal variation was observed in the shell size of both species, with largest individuals occurring in the north. In C. exile, these larger forms have often been referred to as C. e. canadense. Scanning Electron Micrographs of individuals from eight populations ranging from southern Illinois to northern Michigan to southern Ontario demonstrate that internal lamellae shape is too variable both within and between populations to be taxonomically useful. Analysis of the within population proportion of C. e. exile, C. e. canadense, and intermediate size classes demonstrate that most support individuals representing all three classes. Thus, the division of C. exile into distinct subspecies does not appear warranted..