Morphological and Electrophoretic Evidence for Two Species of Corbicula (Bivalvia: Corbiculidae) in North America

Abstract

Two syntopic color forms of the Asiatic clam genus Corbicula which have become established in the lower Brazos River system in Texas [USA] were examined electrophoretically and morphologically. One form has a white nacre with purple highlights (the white form) and the other form has a deep purple nacre (the purple form). The 2 forms have fixed allelic differences at 6 of 26 genetic loci and are remarkable in that both forms are homozygous at every locus. The 2 forms also differ in number of annuli and in shape of the shell (as determined by principal components analysis). Differences in ecological niche preference and in seasonal enzyme production are noted. In view of these data, a single-species concept proposed recently by Britton and Morton (1979) for North American populations of Corbicula cannot be justified. Two names (C. fluminea and C. fluminalis) that are applied to widely distributed Asian species may be applicable to the 2 North American species, but conflicting morphological, reproductive and distributional data prohibit assigning either of these names (or any others) to the 2 introduced species until revisionary work is carried out on the genus Corbicula thoughout its native range.