Variations in shell form in the gastropod genus Diodora

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Virginia Polytechnic Institute and State University


Recent specimens of the genus Diodora from the Smithsonian Institution Mollusc Collection were examined for the neontological portion of this study. Species included were D. cayenensis, D. listeri, D. sayi, and D. tanneri. Fifty eight measurements were made on each specimen and recorded with latitude and water depth at each collection site. Fifty specimens were selected as reference specimens, photographed, their perimeters digitized, and ornamentation and foramen shape evaluated qualitatively.

Principal Components Analysis shows that subtidal specimens are generally flatter, have a less constricted foramen and a greater number of ribs than intertidal specimens. Specimens from higher latitudes have a less constricted foramen and an increased number of ribs. In the subtidal sample, larger specimens characterize higher latitudes.

Analysis performed on all specimens of D. cayenensis, reveals 1.) a decrease in mean shell size and relative shell height in individuals collected from below the intertidal zone, 2.) shells from the province north of Cape Hatteras, N.C. have significantly more ribs than those of the same species from further south, and 3.) Gulf specimens are taller and relatively heavier for their size than those found elsewhere.

More than 500 fossil Diodorids from Smithsonian Institution collections were measured and analyzed as were the recent specimens. Shell morphology varies latitudinally like those of recent subtidal Diodorids: higher latitude specimens have more ribs and a less constricted foramen. Geologic age has little or no correlation to form.