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 were examined electrophoretically and morphologically. One form has a nacre with highlights (the white form) and the other form has a deep nacre (the purple form). The two forms have fixed allelic differences at six of 26 genetic loci and are remarkable in that both forms are homozygous at every locus. The two 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 two North American species; however, conflicting morphological, reproductive and distributional data prohibit assigning either of these names (or any others) to the two introduced species until revisionary work is carried out on the genus Corbicula throughout its native range.